View source for Hydrogen

{{About|the chemical element}}
{{pp-vandalism|small=yes}}
{{Use dmy dates|date=March 2022}}
{{Infobox hydrogen}}
'''Hydrogen''' is a [[chemical element]]; it has [[chemical symbol|symbol]] '''H''' and [[atomic number]] 1. It is the lightest element and, at [[standard conditions]], is a [[gas]] of [[diatomic molecule]]s with the [[chemical formula|formula]] {{chem2|H2}}, sometimes called '''dihydrogen'''<!--[[dihydrogen]] is a redirect to hydrogen also called '''diprotium'''-->,<ref>{{cite web |title=Dihydrogen |url=http://www.usm.maine.edu/~newton/Chy251_253/Lectures/LewisStructures/Dihydrogen.html |archive-url=https://web.archive.org/web/20090213174645/http://usm.maine.edu/~newton/Chy251_253/Lectures/LewisStructures/Dihydrogen.html |archive-date=13 February 2009 |access-date=6 April 2009 |work=O{{=}}CHem Directory |publisher=[[University of Southern Maine]] |df=dmy-all}}</ref> but more commonly called '''hydrogen gas''', '''molecular hydrogen''' or simply hydrogen. It is colorless, odorless,<ref>{{Cite web|title=Hydrogen|url=https://www.britannica.com/science/hydrogen|url-status=live|access-date=25 December 2021|website=[[Encyclopædia Britannica]]|language=en|archive-date=24 December 2021|archive-url=https://web.archive.org/web/20211224165150/https://www.britannica.com/science/hydrogen}}</ref> non-toxic, and highly [[combustible]]. Constituting about 75% of all [[baryon|normal]] [[matter]], hydrogen is the [[abundance of the chemical elements|most abundant]] chemical element in the [[universe]].<ref>{{cite web
|last=Boyd
|first=Padi
|title=What is the chemical composition of stars?
|url=https://imagine.gsfc.nasa.gov/ask_astro/stars.html#961112a
|publisher=[[NASA]]
|date=19 July 2014
|access-date=5 February 2008
|archive-url=https://web.archive.org/web/20150115074556/http://imagine.gsfc.nasa.gov/ask_astro/stars.html#961112a
|archive-date=15 January 2015
|url-status=live
}}</ref><ref group=note>However, most of the universe's mass is not in the form of baryons or chemical elements. See [[dark matter]] and [[dark energy]].</ref> [[Stars]], including the [[Sun]], mainly consist of hydrogen in a [[plasma state]], while on Earth, hydrogen is found in [[water]], [[organic compounds]], as [[Natural hydrogen|dihydrogen]], and in other [[molecular form]]s. The most common [[isotope of hydrogen]] (protium, {{sup|1}}H) consists of one [[proton]], one [[electron]], and no [[neutron]]s.

In the [[early universe]], the formation of hydrogen's protons occurred in the first second after the [[Big Bang]]; neutral hydrogen atoms only formed about 370,000 years later during the [[Recombination (cosmology)|recombination epoch]] as the universe cooled and plasma had cooled enough for electrons to remain bound to protons.<ref>{{cite journal |last=Tanabashi |first=M. |display-authors=etal |year=2018 |journal=[[Physical Review D]] |volume=98 |issue=3 |via=[[Particle Data Group]] at [[Lawrence Berkeley National Laboratory]] |url=http://pdg.lbl.gov/2018/reviews/rpp2018-rev-bbang-cosmology.pdf |page=358 |quote=Chapter 21.4.1 - This occurred when the age of the Universe was about 370,000 years. |title=Big-Bang Cosmology |url-status=live |archive-url=https://web.archive.org/web/20210629034426/https://pdg.lbl.gov/2018/reviews/rpp2018-rev-bbang-cosmology.pdf |archive-date=29 June 2021 |doi=10.1103/PhysRevD.98.030001 |doi-access=free }} (Revised September 2017) by [[Keith Olive|Keith A. Olive]] and [[John A. Peacock]].</ref> Hydrogen, typically [[nonmetallic]] except under [[High pressure|extreme pressure]], readily forms [[covalent bonds]] with most nonmetals, contributing to the formation of compounds like water and various organic substances. Its role is crucial in [[acid-base reactions]], which mainly involve proton exchange among soluble molecules. In [[ionic compounds]], hydrogen can take the form of either a negatively charged [[anion]], where it is known as [[hydride]], or as a positively charged [[cation]], H{{sup|+}}. The cation, [[deuterium|usually]] just a proton (symbol '''p'''), exhibits specific behavior in [[aqueous solution]]s and in [[ionic compound]]s involves [[Electric-field screening|screening]] of its [[electric charge]] by surrounding [[Chemical polarity|polar]] molecules or anions. Hydrogen's unique position as the only neutral atom for which the [[Schrödinger equation]] can be directly solved, has significantly contributed to the foundational principles of [[quantum mechanics]] through the exploration of its energetics and [[chemical bonding]].<ref name="Laursen04">{{cite web|last1=Laursen|first1=S.|last2=Chang|first2=J.|last3=Medlin|first3=W.|last4=Gürmen|first4=N.|last5=Fogler|first5=H. S.|title=An extremely brief introduction to computational quantum chemistry|url=http://www.umich.edu/~elements/5e/web_mod/quantum/introduction_3.htm|website=Molecular Modeling in Chemical Engineering|publisher=University of Michigan|access-date=4 May 2015|date=27 July 2004|archive-url=https://web.archive.org/web/20150520061846/http://www.umich.edu/~elements/5e/web_mod/quantum/introduction_3.htm|archive-date=20 May 2015|url-status=live}}</ref>

Hydrogen gas was first produced artificially in the early 16th century by reacting acids with metals. [[Henry Cavendish]], in 1766–81, identified hydrogen gas as a distinct substance<ref>{{Cite episode
|title = Discovering the Elements
|url = http://www.bbc.co.uk/programmes/b00q2mk5
|series = Chemistry: A Volatile History
|credits = Presenter: Professor Jim Al-Khalili
|network = [[BBC]]
|station = [[BBC Four]]
|air-date = 21 January 2010
|minutes = 25:40
|access-date = 9 February 2010
|archive-url = https://web.archive.org/web/20100125010949/http://www.bbc.co.uk/programmes/b00q2mk5
|archive-date = 25 January 2010
|url-status = live
}}</ref> and discovered its property of producing water when burned; hence its name means "water-former" in Greek.

Most [[hydrogen production]] occurs through [[steam reforming]] of [[natural gas]]; a smaller portion comes from energy-intensive methods such as the [[electrolysis of water]].<ref name="Dincer-2015">{{Cite journal|last1=Dincer|first1=Ibrahim|last2=Acar|first2=Canan|date=14 September 2015|title=Review and evaluation of hydrogen production methods for better sustainability|url=https://www.sciencedirect.com/science/article/pii/S0360319914034119|journal=International Journal of Hydrogen Energy|language=en|volume=40|issue=34|pages=11094–11111|doi=10.1016/j.ijhydene.2014.12.035|bibcode=2015IJHE...4011094D |issn=0360-3199|access-date=4 February 2022|archive-date=15 February 2022|archive-url=https://web.archive.org/web/20220215183915/https://www.sciencedirect.com/science/article/abs/pii/S0360319914034119|url-status=live}}</ref><ref>{{cite web
|title=Hydrogen Basics&nbsp;– Production
|url=http://www.fsec.ucf.edu/en/consumer/hydrogen/basics/production.htm
|publisher=[[Florida Solar Energy Center]]
|date=2007
|access-date=5 February 2008
|archive-url=https://web.archive.org/web/20080218210526/http://www.fsec.ucf.edu/en/consumer/hydrogen/basics/production.htm
|archive-date=18 February 2008
}}</ref> Its main industrial uses include [[fossil fuel]] processing, such as [[hydrocracking]], and [[ammonia production]], with emerging uses in [[fuel cell]]s for electricity generation and as a heat source.<ref name="Lewis-2021">{{Cite journal |last=Lewis |first=Alastair C. |date=10 June 2021 |title=Optimising air quality co-benefits in a hydrogen economy: a case for hydrogen-specific standards for NO x emissions |journal=Environmental Science: Atmospheres |language=en |volume=1 |issue=5 |pages=201–207 |doi=10.1039/D1EA00037C|s2cid=236732702 |doi-access=free }}{{Creative Commons text attribution notice|cc=by3|url=|authors=|vrt=|from this source=yes}}</ref> When used in fuel cells, hydrogen's only emission at point of use is water vapor,<ref name="Lewis-2021" /> though combustion can produce [[nitrogen oxides]].<ref name="Lewis-2021" /> Hydrogen's interaction with metals may cause [[Hydrogen embrittlement|embrittlement]].<ref name="Rogers 1999 1057–1064">{{cite journal |last=Rogers|first=H. C. |title=Hydrogen Embrittlement of Metals |journal=[[Science (journal)|Science]] |volume=159|issue=3819|pages=1057–1064 |date=1999 |doi=10.1126/science.159.3819.1057 |pmid=17775040 |bibcode=1968Sci...159.1057R |s2cid=19429952}}</ref>

{{Toclimit}}

== Properties ==
=== Combustion ===
[[File:19. Експлозија на смеса од водород и воздух.webm|thumb|Combustion of hydrogen with the oxygen in the air. When the bottom cap is removed, allowing air to enter at the bottom, the hydrogen in the container rises out of top and burns as it mixes with the air.]]
[[File:Shuttle Main Engine Test Firing cropped edited and reduced.jpg|thumb|[[Space Shuttle Main Engine]] burning hydrogen with oxygen, produces a nearly invisible flame at full thrust.|alt=A black inverted funnel with blue glow emerging from its opening.]]

Hydrogen gas is highly flammable:
:{{chem2|2 H2(g) + O2(g) → 2 H2O(l)}} (572&nbsp;kJ/2 mol = 286&nbsp;kJ/mol = 141.865 MJ/kg)<ref group="note">286&nbsp;kJ/mol: energy per mole of the combustible material (molecular hydrogen).</ref>
[[Enthalpy of combustion]]: −286&nbsp;kJ/mol.<ref>{{cite book
|author=Committee on Alternatives and Strategies for Future Hydrogen Production and Use
|date=2004
|title=The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs
|page=240
|publisher=[[National Academies Press]]
|isbn=978-0-309-09163-3
|url=https://books.google.com/books?id=ugniowznToAC&pg=PA240
|access-date=3 September 2020
|archive-date=29 January 2021
|archive-url=https://web.archive.org/web/20210129015745/https://books.google.com/books?id=ugniowznToAC&pg=PA240
|url-status=live
}}</ref>

Hydrogen gas forms explosive mixtures with air in concentrations from 4–74%<ref>{{cite journal
|last1=Carcassi|first1=M. N.
|last2=Fineschi|first2=F.
|title=Deflagrations of H<sub>2</sub>–air and CH<sub>4</sub>–air lean mixtures in a vented multi-compartment environment
|journal=Energy
|volume=30|issue=8|pages=1439–1451
|date=2005
|doi=10.1016/j.energy.2004.02.012
|bibcode=2005Ene....30.1439C
}}</ref> and with chlorine at 5–95%. The hydrogen [[autoignition temperature]], the temperature of spontaneous ignition in air, is {{convert|500|C|F}}.<ref>{{cite book
|url=https://books.google.com/books?id=-CRRJBVv5d0C&pg=PA402
|page=402
|title=A Comprehensive Guide to the Hazardous Properties of Chemical Substances
|publisher=Wiley-Interscience
|isbn=978-0-471-71458-3
|date=2007
|last=Patnaik
|first=P.
|access-date=3 September 2020
|archive-date=26 January 2021
|archive-url=https://web.archive.org/web/20210126131413/https://books.google.com/books?id=-CRRJBVv5d0C&pg=PA402
|url-status=live
}}</ref>

==== Flame ====
Pure [[Oxyhydrogen|hydrogen-oxygen]] flames emit [[ultraviolet]] light and with high oxygen mix are nearly invisible to the naked eye, as illustrated by the faint plume of the [[Space Shuttle Main Engine]], compared to the highly visible plume of a [[Space Shuttle Solid Rocket Booster]], which uses an [[Ammonium perchlorate composite propellant|ammonium perchlorate composite]]. The detection of a burning hydrogen leak, may require a [[flame detector]]; such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames.<ref>{{cite journal|title=Visible emission of hydrogen flames|last1=Schefer|first1=E. W.|last2=Kulatilaka|first2=W. D.|last3=Patterson|first3=B. D.|last4=Settersten|first4=T. B.|date=June 2009|journal=Combustion and Flame|volume=156|issue=6|pages=1234–1241|doi=10.1016/j.combustflame.2009.01.011|bibcode=2009CoFl..156.1234S |url=https://zenodo.org/record/1258847|access-date=30 June 2019|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015717/https://zenodo.org/record/1258847|url-status=live}}</ref> The [[Hindenburg disaster|destruction of the ''Hindenburg'' airship]] was a notorious example of hydrogen combustion and the cause is still debated. The visible flames in the photographs were the result of carbon compounds in the airship skin burning.<ref>{{Cite web|title=Myths about the Hindenburg Crash|url=https://www.airships.net/hindenburg/disaster/myths/|access-date=29 March 2021|website=Airships.net|language=en-US|archive-date=20 April 2021|archive-url=https://web.archive.org/web/20210420055020/https://www.airships.net/hindenburg/disaster/myths/|url-status=live}}</ref>

==== Reactants ====
{{chem2|H2}} is unreactive compared to diatomic elements such as [[halogens]] or oxygen. The thermodynamic basis of this low reactivity is the very strong H–H bond, with a [[bond dissociation energy]] of 435.7 kJ/mol.<ref>{{RubberBible87th}}</ref> The kinetic basis of the low reactivity is the nonpolar nature of {{chem2|H2}} and its weak polarizability. It spontaneously reacts with [[chlorine]] and [[fluorine]] to form [[hydrogen chloride]] and [[hydrogen fluoride]], respectively.<ref>{{cite book
|last=Clayton|first=D. D.
|title=Handbook of Isotopes in the Cosmos: Hydrogen to Gallium
|date=2003
|publisher=[[Cambridge University Press]]
|isbn=978-0-521-82381-4
}}</ref> The reactivity of {{chem2|H2}} is strongly affected by the presence of metal catalysts. Thus, while mixtures of {{chem2|H2}} with {{chem2|O2}} or air combust readily when heated to at least 500°C by a spark or flame, they do not react at room temperature in the absence of a catalyst.

=== Electron energy levels ===
{{Main|Hydrogen atom}}
[[File:hydrogen atom.svg|thumb|A depiction of a hydrogen atom with size of central proton shown, and the atomic diameter shown as about twice the [[Bohr model]] radius (image not to scale)|alt=Drawing of a light-gray large sphere with a cut off quarter and a black small sphere and numbers 1.7×10{{sup|−5}} illustrating their relative diameters.]]
The [[ground state]] [[energy level]] of the electron in a hydrogen atom is −13.6&nbsp;[[Electronvolt|eV]],<ref>{{cite web|author1=NAAP Labs|title=Energy Levels|url=http://astro.unl.edu/naap/hydrogen/levels.html|publisher=University of Nebraska Lincoln|access-date=20 May 2015|date=2009|archive-url=https://web.archive.org/web/20150511120536/http://astro.unl.edu/naap/hydrogen/levels.html|archive-date=11 May 2015|url-status=live}}</ref> equivalent to an [[ultraviolet]] [[photon]] of roughly 91&nbsp;nm wavelength.<ref>{{cite web|url=http://www.wolframalpha.com/input/?i=photon+wavelength+13.6+ev|title=photon wavelength 13.6 eV|access-date=20 May 2015|date=20 May 2015|work=Wolfram Alpha|archive-url=https://web.archive.org/web/20160512221720/http://www.wolframalpha.com/input/?i=photon+wavelength+13.6+ev|archive-date=12 May 2016|url-status=live}}</ref>

The energy levels of hydrogen can be calculated fairly accurately using the [[Bohr model]] of the atom, in which the electron "orbits" the proton, like how Earth orbits the Sun. However, the electron and proton are held together by electrostatic attraction, while planets and celestial objects are held by [[gravity]]. Due to the discretization of [[angular momentum]] postulated in early [[quantum mechanics]] by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.<ref>{{cite web
|last=Stern
|first=D. P.
|date=16 May 2005
|url=http://www.iki.rssi.ru/mirrors/stern/stargaze/Q5.htm
|title=The Atomic Nucleus and Bohr's Early Model of the Atom
|publisher=NASA Goddard Space Flight Center (mirror)
|access-date=20 December 2007
|archive-url=https://web.archive.org/web/20081017073826/http://www.iki.rssi.ru/mirrors/stern/stargaze/Q5.htm
|archive-date=17 October 2008
}}</ref>

A more accurate description of the hydrogen atom comes from a quantum analysis that uses the [[Schrödinger equation]], [[Dirac equation]] or [[Richard Feynman|Feynman]] [[path integral formulation]] to calculate the [[probability amplitude|probability density]] of the electron around the proton.<ref>{{cite web| last=Stern| first=D. P.| date=13 February 2005| url=http://www-spof.gsfc.nasa.gov/stargaze/Q7.htm| title=Wave Mechanics| publisher=NASA Goddard Space Flight Center| access-date=16 April 2008| archive-url=https://web.archive.org/web/20080513195241/http://www-spof.gsfc.nasa.gov/stargaze/Q7.htm| archive-date=13 May 2008| url-status=live}}</ref> The most complex formulas include the small effects of [[special relativity]] and [[vacuum polarization]]. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum—illustrating how the "planetary orbit" differs from electron motion.

=== Spin isomers ===
{{Main|Spin isomers of hydrogen}}
Molecular {{chem2|H2}} exists as two [[spin isomer]]s, i.e. compounds that differ only in the [[Spin (physics)|spin states]] of their nuclei.<ref name="uigi">{{cite web|author=Staff|date=2003|url=http://www.uigi.com/hydrogen.html|title=Hydrogen (H<sub>2</sub>) Properties, Uses, Applications: Hydrogen Gas and Liquid Hydrogen|publisher=Universal Industrial Gases, Inc.|access-date=5 February 2008|archive-url=https://web.archive.org/web/20080219073329/http://www.uigi.com/hydrogen.html|archive-date=19 February 2008|url-status=live}}</ref> In the '''orthohydrogen''' form, the spins of the two nuclei are parallel, forming a spin [[triplet state]] having a [[Spin quantum number#Total spin of an atom or molecule|total molecular spin]] <math>S = 1</math>; in the '''parahydrogen''' form the spins are antiparallel and form a spin [[singlet state]] having spin <math>S = 0</math>. The equilibrium ratio of ortho- to para-hydrogen depends on temperature. At room temperature or warmer, equilibrium hydrogen gas contains about 25% of the para form and 75% of the ortho form.<ref name="Green2012">{{cite journal |last1=Green |first1=Richard A. |display-authors=etal |title=The theory and practice of hyperpolarization in magnetic resonance using ''para''hydrogen |journal=Prog. Nucl. Magn. Reson. Spectrosc. |date=2012 |volume=67 |pages=1–48 |doi=10.1016/j.pnmrs.2012.03.001 |pmid=23101588 |bibcode=2012PNMRS..67....1G |url=https://www.sciencedirect.com/science/article/abs/pii/S0079656512000477 |access-date=28 August 2021 |archive-date=28 August 2021 |archive-url=https://web.archive.org/web/20210828222611/https://www.sciencedirect.com/science/article/abs/pii/S0079656512000477 |url-status=live }}</ref> The ortho form is an [[excited state]], having higher energy than the para form by 1.455&nbsp;kJ/mol,<ref name="PlanckInstitut">{{cite web |url=https://www.mpibpc.mpg.de/146336/para-Wasserstoff |language=de |website=Max-Planck-Institut für Biophysikalische Chemie |title=Die Entdeckung des para-Wasserstoffs (The discovery of para-hydrogen) |access-date=9 November 2020 |archive-date=16 November 2020 |archive-url=https://web.archive.org/web/20201116064055/https://www.mpibpc.mpg.de/146336/para-Wasserstoff |url-status=live }}</ref> and it converts to the para form over the course of several minutes when cooled to low temperature.<ref>{{cite journal|last1=Milenko|first1=Yu. Ya.|last2=Sibileva|first2=R. M.|last3=Strzhemechny|first3=M. A.|title=Natural ortho-para conversion rate in liquid and gaseous hydrogen|journal=Journal of Low Temperature Physics|date=1997|volume=107|issue=1–2|pages=77–92
|doi=10.1007/BF02396837|bibcode = 1997JLTP..107...77M |s2cid=120832814}}</ref> The thermal properties of the forms differ because they differ in their allowed [[Rotational–vibrational spectroscopy|rotational quantum states]]<!-- This link is less direct than [[Rotational spectroscopy]] but presently the subject better (June 2021).-->, resulting in different thermal properties such as the heat capacity.<ref name="NASA">{{cite web|last=Hritz|first=J.|date=March 2006|url=http://smad-ext.grc.nasa.gov/gso/manual/chapter_06.pdf|title=CH. 6&nbsp;– Hydrogen|work=NASA Glenn Research Center Glenn Safety Manual, Document GRC-MQSA.001|publisher=NASA|access-date=5 February 2008|archive-url=https://web.archive.org/web/20080216050326/http://smad-ext.grc.nasa.gov/gso/manual/chapter_06.pdf|archive-date=16 February 2008}}</ref>

The ortho-to-para ratio in {{chem2|H2}} is an important consideration in the [[liquefaction]] and storage of [[liquid hydrogen]]: the conversion from ortho to para is [[exothermic]] and produces enough heat to evaporate most of the liquid if not converted first to parahydrogen during the cooling process.<ref name="Amos98">{{cite web|url=http://www.nrel.gov/docs/fy99osti/25106.pdf|title=Costs of Storing and Transporting Hydrogen|publisher=National Renewable Energy Laboratory|date=1 November 1998|first1=Wade A.|last1=Amos|pages=6–9|access-date=19 May 2015|archive-url=https://web.archive.org/web/20141226131234/http://www.nrel.gov/docs/fy99osti/25106.pdf|archive-date=26 December 2014|url-status=live}}</ref> [[Catalyst]]s for the ortho-para interconversion, such as [[ferric oxide]] and [[activated carbon]] compounds, are used during hydrogen cooling to avoid this loss of liquid.<ref name="Svadlenak">{{cite journal|last1=Svadlenak|first1=R. E.|last2=Scott|first2=A. B.|title=The Conversion of Ortho- to Parahydrogen on Iron Oxide-Zinc Oxide Catalysts|journal=Journal of the American Chemical Society|date=1957|volume=79|issue=20|pages=5385–5388|doi=10.1021/ja01577a013}}</ref>

=== Phases ===
[[File:Hydrogen ampoule.jpg|thumb|Hydrogen gas is colorless and transparent, here contained in a glass [[ampoule]].]]
[[File:Phase diagram of hydrogen.png|thumb|[[Phase diagram]] of hydrogen. The temperature and pressure scales are [[logarithmic scale|logarithmic]], so one unit corresponds to a 10× change. The left edge corresponds to 10{{sup|5}} Pa, or about one atmosphere.{{image reference needed|date=December 2022}}|alt=Phase diagram of hydrogen on logarithmic scales. Lines show boundaries between phases, with the end of the liquid-gas line indicating the critical point. The triple point of hydrogen is just off-scale to the left.]]
* [[Gas]]eous hydrogen
* [[Liquid hydrogen]]
* [[Slush hydrogen]]
* [[Solid hydrogen]]
* [[Metallic hydrogen]]
* [[Plasma (physics)|Plasma]] hydrogen

=== Compounds ===
{{Main|Hydrogen compounds}}

==== Covalent and organic compounds ====
While {{chem2|H2}} is not very reactive under standard conditions, it does form compounds with most elements. Hydrogen can form compounds with elements that are more [[electronegative]], such as [[halogen]]s (F, Cl, Br, I), or [[oxygen]]; in these compounds hydrogen takes on a partial positive charge.<ref>{{cite web|last=Clark|first=J.|title=The Acidity of the Hydrogen Halides|work=Chemguide|date=2002|url=http://www.chemguide.co.uk/inorganic/group7/acidityhx.html#top|access-date=9 March 2008|archive-url=https://web.archive.org/web/20080220174205/http://www.chemguide.co.uk/inorganic/group7/acidityhx.html#top|archive-date=20 February 2008}}</ref> When bonded to a more electronegative element, particularly [[fluorine]], [[oxygen]], or [[nitrogen]], hydrogen can participate in a form of medium-strength noncovalent bonding with another electronegative element with a lone pair, a phenomenon called [[hydrogen bond]]ing that is critical to the stability of many biological molecules.<ref>{{cite web|last=Kimball|first=J. W.|title=Hydrogen|work=Kimball's Biology Pages|date=7 August 2003|url=http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/HydrogenBonds.html|access-date=4 March 2008|archive-url=https://web.archive.org/web/20080304040611/http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/HydrogenBonds.html|archive-date=4 March 2008|url-status=live}}</ref><ref>IUPAC Compendium of Chemical Terminology, Electronic version, [http://goldbook.iupac.org/H02899.html Hydrogen Bond] {{Webarchive|url=https://web.archive.org/web/20080319045705/http://goldbook.iupac.org/H02899.html |date=19 March 2008 }}</ref> Hydrogen also forms compounds with less electronegative elements, such as [[metal]]s and [[metalloid]]s, where it takes on a partial negative charge. These compounds are often known as [[hydride]]s.<ref>{{cite web|last=Sandrock|first=G.|title=Metal-Hydrogen Systems|publisher=Sandia National Laboratories|date=2 May 2002|url=http://hydpark.ca.sandia.gov/DBFrame.html|access-date=23 March 2008|archive-url=https://web.archive.org/web/20080224162206/http://hydpark.ca.sandia.gov/DBFrame.html|archive-date=24 February 2008}}</ref>

Hydrogen forms many compounds with [[carbon]] called the [[hydrocarbon]]s, and even more with [[heteroatom]]s that, due to their association with living things, are called [[organic compound]]s.<ref name="hydrocarbon">{{cite web| title=Structure and Nomenclature of Hydrocarbons| publisher=Purdue University| url=http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/organic.html| access-date=23 March 2008| archive-url=https://web.archive.org/web/20120611084045/http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/organic.html| archive-date=11 June 2012}}</ref> The study of their properties is known as [[organic chemistry]]<ref>{{cite web| title=Organic Chemistry| work=Dictionary.com| publisher=Lexico Publishing Group| date=2008| url=http://dictionary.reference.com/browse/organic%20chemistry| access-date=23 March 2008| archive-url=https://web.archive.org/web/20080418093054/http://dictionary.reference.com/browse/organic%20chemistry| archive-date=18 April 2008| url-status=live}}</ref> and their study in the context of living [[organism]]s is called [[biochemistry]].<ref>{{cite web
|title=Biochemistry
|work=Dictionary.com
|publisher=Lexico Publishing Group
|date=2008
|url=http://dictionary.reference.com/browse/biochemistry
|access-date=23 March 2008
|archive-url=https://web.archive.org/web/20080329151719/http://dictionary.reference.com/browse/biochemistry
|archive-date=29 March 2008
|url-status=live
}}</ref> By some definitions, "organic" compounds are only required to contain carbon. However, most of them also contain hydrogen, and because it is the carbon-hydrogen bond that gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry.<ref name="hydrocarbon" /> Millions of [[hydrocarbon]]s are known, and they are usually formed by complicated pathways that seldom involve elemental hydrogen.

Hydrogen is highly soluble in many [[Rare earth element|rare earth]] and [[transition metal]]s<ref name="Takeshita">{{cite journal
|last1=Takeshita|first1=T.
|last2=Wallace|first2=W. E.
|last3=Craig|first3=R. S.
|title=Hydrogen solubility in 1:5 compounds between yttrium or thorium and nickel or cobalt
|journal=[[Inorganic Chemistry (journal)|Inorganic Chemistry]]
|volume=13|issue=9|pages=2282–2283
|date=1974
|doi=10.1021/ic50139a050
}}</ref> and is soluble in both nanocrystalline and [[amorphous metal]]s.<ref name="Kirchheim1">{{cite journal
|last1=Kirchheim|first1=R.
|last2=Mutschele|first2=T.
|last3=Kieninger|first3=W.
|title=Hydrogen in amorphous and nanocrystalline metals
|journal=Materials Science and Engineering
|date=1988|volume=99|issue=1–2
|pages=457–462
|doi=10.1016/0025-5416(88)90377-1
|last4=Gleiter
|first4=H.
|last5=Birringer
|first5=R.
|last6=Koble
|first6=T.
}}</ref> Hydrogen [[solubility]] in metals is influenced by local distortions or impurities in the [[crystal lattice]].<ref name="Kirchheim2">{{cite journal
|last=Kirchheim|first=R.
|title=Hydrogen solubility and diffusivity in defective and amorphous metals
|journal=[[Progress in Materials Science]]
|volume=32|issue=4|pages=262–325
|date=1988
|doi=10.1016/0079-6425(88)90010-2
}}</ref> These properties may be useful when hydrogen is purified by passage through hot [[palladium]] disks, but the gas's high solubility is a metallurgical problem, contributing to the [[hydrogen embrittlement|embrittlement]] of many metals,<ref name="Rogers 1999 1057–1064" /> complicating the design of pipelines and storage tanks.<ref name="Christensen">{{cite news
|last1=Christensen
|first1=C. H.
|last2=Nørskov
|first2=J. K.
|last3=Johannessen
|first3=T.
|date=9 July 2005
|title=Making society independent of fossil fuels&nbsp;– Danish researchers reveal new technology
|publisher=[[Technical University of Denmark]]
|url=http://news.mongabay.com/2005/0921-hydrogen_tablet.html
|access-date=19 May 2015
|archive-url=https://web.archive.org/web/20150521085421/http://news.mongabay.com/2005/0921-hydrogen_tablet.html
|archive-date=21 May 2015
|url-status=live
}}</ref>

==== Hydrides ====
{{Main|Hydride}}
[[File:NaH.jpg|thumb|A sample of [[sodium hydride]]]]
Hydrogen compounds are often called [[hydride]]s, a term that is used fairly loosely. The term "hydride" suggests that the H atom has acquired a negative or anionic character, denoted {{chem2|H−}}; and is used when hydrogen forms a compound with a more [[electropositive]] element. The existence of the [[hydride|hydride anion]], suggested by [[Gilbert N. Lewis]] in 1916 for group 1 and 2 salt-like hydrides, was demonstrated by Moers in 1920 by the electrolysis of molten [[lithium hydride]] (LiH), producing a [[Stoichiometry|stoichiometric]] quantity of hydrogen at the anode.<ref name="Moers">{{cite journal|last=Moers|first=K.|title=Investigations on the Salt Character of Lithium Hydride|journal=Zeitschrift für Anorganische und Allgemeine Chemie|date=1920|volume=113|issue=191|pages=179–228|doi=10.1002/zaac.19201130116|url=https://zenodo.org/record/1428170|access-date=24 August 2019|archive-url=https://web.archive.org/web/20190824162148/https://zenodo.org/record/1428170/files/article.pdf|archive-date=24 August 2019|url-status=live}}</ref> For hydrides other than group 1 and 2 metals, the term is quite misleading, considering the low electronegativity of hydrogen. An exception in group 2 hydrides is {{chem2|BeH2}}, which is polymeric. In [[lithium aluminium hydride]], the {{chem2|[AlH4]-}} anion carries hydridic centers firmly attached to the Al(III).

Although hydrides can be formed with almost all main-group elements, the number and combination of possible compounds varies widely; for example, more than 100 binary borane hydrides are known, but only one binary aluminium hydride.<ref name="Downs">{{cite journal
|last1=Downs|first1=A. J.
|last2=Pulham|first2=C. R.
|title=The hydrides of aluminium, gallium, indium, and thallium: a re-evaluation
|journal=Chemical Society Reviews
|date=1994|volume=23|pages=175–184
|doi=10.1039/CS9942300175
|issue=3
}}</ref> Binary [[indium]] hydride has not yet been identified, although larger complexes exist.<ref name="Hibbs">{{cite journal
|last1=Hibbs|first1=D. E.
|last2=Jones|first2=C.|last3=Smithies|first3=N. A.
|title=A remarkably stable indium trihydride complex: synthesis and characterisation of [InH<sub>3</sub>P(C<sub>6</sub>H<sub>11</sub>)<sub>3</sub>]
|journal=Chemical Communications
|date=1999|pages=185–186|doi=10.1039/a809279f
|issue=2}}</ref>

In [[inorganic chemistry]], hydrides can also serve as [[bridging ligand]]s that link two metal centers in a [[coordination complex]]. This function is particularly common in [[group 13 element]]s, especially in [[borane]]s ([[boron]] hydrides) and [[aluminium]] complexes, as well as in clustered [[carborane]]s.<ref name="Miessler" />

==== Protons and acids ====
{{Further|Acid–base reaction}}
Oxidation of hydrogen removes its electron and gives [[Hydrogen ion|{{chem2|H+}}]], which contains no electrons and a [[atomic nucleus|nucleus]] which is usually composed of one proton. That is why {{chem2|H+}} is often called a proton. This species is central to discussion of [[acid]]s. Under the [[Brønsted–Lowry acid–base theory]], acids are proton donors, while bases are proton acceptors.

A bare proton, {{chem2|H+}}, cannot exist in solution or in ionic crystals because of its strong attraction to other atoms or molecules with electrons. Except at the high temperatures associated with plasmas, such protons cannot be removed from the [[electron cloud]]s of atoms and molecules, and will remain attached to them. However, the term 'proton' is sometimes used loosely and metaphorically to refer to positively charged or [[cation]]ic hydrogen attached to other species in this fashion, and as such is denoted "{{chem2|H+}}" without any implication that any single protons exist freely as a species.

To avoid the implication of the naked "solvated proton" in solution, acidic aqueous solutions are sometimes considered to contain a less unlikely fictitious species, termed the "[[hydronium]] ion" ({{chem2|[H3O]+}}). However, even in this case, such solvated hydrogen cations are more realistically conceived as being organized into clusters that form species closer to {{chem2|[H9O4]+}}.<ref name="Okumura">{{cite journal
|last1=Okumura|first1=A. M.
|last2=Yeh|first2=L. I.|last3=Myers|first3=J. D.|last4=Lee|first4=Y. T.
|title=Infrared spectra of the solvated hydronium ion: vibrational predissociation spectroscopy of mass-selected H<sub>3</sub>O+•(H<sub>2</sub>O<sub>)n</sub>•(H<sub>2</sub>)<sub>m</sub>
|journal=Journal of Physical Chemistry
|date=1990|volume=94|issue=9|pages=3416–3427|doi=10.1021/j100372a014
}}</ref> Other [[oxonium ion]]s are found when water is in acidic solution with other solvents.<ref name="Perdoncin">{{cite journal
|last1=Perdoncin|first1=G.|last2=Scorrano|first2=G.
|title=Protonation Equilibria in Water at Several Temperatures of Alcohols, Ethers, Acetone, Dimethyl Sulfide, and Dimethyl Sulfoxide
|journal=Journal of the American Chemical Society
|date=1977|volume=99|issue=21|pages=6983–6986
|doi=10.1021/ja00463a035
}}</ref>

Although exotic on Earth, one of the most common ions in the universe is the {{chem2|H3+}} ion, known as [[Trihydrogen cation|protonated molecular hydrogen]] or the trihydrogen cation.<ref name="Carrington">{{cite journal
|last1=Carrington|first1=A.|last2=McNab|first2=I. R.
|title=The infrared predissociation spectrum of triatomic hydrogen cation (H<sub>3</sub><sup>+</sup>)
|journal=Accounts of Chemical Research
|date=1989|volume=22|issue=6|pages=218–222
|doi=10.1021/ar00162a004}}</ref>

=== Isotopes ===
{{Main|Isotopes of hydrogen}}
[[File:Blausen 0530 HydrogenIsotopes.png|thumb|The three naturally-occurring isotopes of hydrogen: hydrogen-1 (protium), hydrogen-2 (deuterium), and hydrogen-3 (tritium)|alt=Diagram showing the structure of each of Hydrogen-1 (mass number 1, 1 electron, 1 proton), Hydrogen-2 or deuterium (mass number 2, 1 electron, 1 proton, 1 neutron), and Hydrogen-3 or tritium (mass number 3, 1 electron, 1 proton, 2 neutrons)]]
[[File:Hydrogen discharge tube.jpg|thumb|Hydrogen discharge (spectrum) tube]]
[[File:Deuterium discharge tube.jpg|thumb|Deuterium discharge (spectrum) tube]]
Hydrogen has three naturally occurring isotopes, denoted {{chem|1|H}}, {{chem|2|H}} and {{chem|3|H}}. Other, highly unstable nuclei ({{chem|4|H}} to {{chem|7|H}}) have been synthesized in the laboratory but not observed in nature.<ref name="Gurov">{{cite journal
|author=Gurov, Y. B.
|author2=Aleshkin, D. V.
|author3=Behr, M. N.
|author4=Lapushkin, S. V.
|author5=Morokhov, P. V.
|author6=Pechkurov, V. A.
|author7=Poroshin, N. O.
|author8=Sandukovsky, V. G.
|author9=Tel'kushev, M. V.
|author10=Chernyshev, B. A.
|author11=Tschurenkova, T. D.
|title=Spectroscopy of superheavy hydrogen isotopes in stopped-pion absorption by nuclei
|journal=Physics of Atomic Nuclei
|date=2004|volume=68|issue=3|pages=491–97
|doi=10.1134/1.1891200
|bibcode = 2005PAN....68..491G |s2cid=122902571
}}</ref><ref name="Korsheninnikov">{{cite journal
|title=Experimental Evidence for the Existence of <sup>7</sup>H and for a Specific Structure of <sup>8</sup>He
|journal=Physical Review Letters
|date=2003|volume=90|issue=8|page=082501
|doi=10.1103/PhysRevLett.90.082501|pmid=12633420
|bibcode=2003PhRvL..90h2501K
|display-authors=8
|last1=Korsheninnikov
|first1=A.
|last2=Nikolskii
|first2=E.
|last3=Kuzmin
|first3=E.
|last4=Ozawa
|first4=A.
|last5=Morimoto
|first5=K.
|last6=Tokanai
|first6=F.
|last7=Kanungo
|first7=R.
|last8=Tanihata
|first8=I.
|last9=Timofeyuk
|first9=N.}}</ref>
* '''{{chem|1|H}}''' is the most common hydrogen isotope, with an abundance of >99.98%. Because the [[atomic nucleus|nucleus]] of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name ''protium''.<ref>{{cite journal
|last1=Urey|first1=H. C.
|last2=Brickwedde|first2=F. G.|last3=Murphy|first3=G. M.
|title=Names for the Hydrogen Isotopes
|journal=Science|date=1933|volume=78
|issue=2035|pages=602–603
|doi=10.1126/science.78.2035.602
|pmid=17797765|bibcode = 1933Sci....78..602U }}</ref> It is the only stable isotope with no neutrons; see [[Isotopes of helium#Helium-2 (diproton)|diproton]] for a discussion of why others do not exist.
* '''{{chem|2|H}}''', the other stable hydrogen isotope, is known as [[deuterium]] and contains one proton and one [[neutron]] in the nucleus. Nearly all deuterium in the universe is thought to have been produced at the time of the [[Big Bang]], and has endured since then. Deuterium is not radioactive, and is not a significant toxicity hazard. Water enriched in molecules that include deuterium instead of normal hydrogen is called [[heavy water]]. Deuterium and its compounds are used as a non-radioactive label in chemical experiments and in solvents for {{chem|1|H}}-[[NMR spectroscopy]].<ref>{{cite journal
|author=Oda, Y.
|author2=Nakamura, H.
|author3=Yamazaki, T.
|author4=Nagayama, K.
|author5=Yoshida, M.
|author6=Kanaya, S.
|author7=Ikehara, M.
|title=1H NMR studies of deuterated ribonuclease HI selectively labeled with protonated amino acids
|journal=[[Journal of Biomolecular NMR]]
|date=1992|volume=2|issue=2|pages=137–47
|doi=10.1007/BF01875525
|pmid=1330130|s2cid=28027551
}}</ref> Heavy water is used as a [[neutron moderator]] and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial [[nuclear fusion]].<ref>{{cite news
|last=Broad
|first=W. J.
|date=11 November 1991
|title=Breakthrough in Nuclear Fusion Offers Hope for Power of Future
|work=The New York Times
|url=https://query.nytimes.com/gst/fullpage.html?res=9D0CE4D81030F932A25752C1A967958260
|access-date=12 February 2008
|archive-date=29 January 2021
|archive-url=https://web.archive.org/web/20210129015717/https://www.nytimes.com/1991/11/11/us/breakthrough-in-nuclear-fusion-offers-hope-for-power-of-future.html
|url-status=live
}}</ref>
* '''{{chem|3|H}}''' is known as [[tritium]] and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into [[helium-3]] through [[beta decay]] with a [[half-life]] of 12.32 years.<ref name="Miessler" /> It is radioactive enough to be used in [[Radioluminescent paint|luminous paint]] to enhance the visibility of data displays, such as for painting the hands and dial-markers of watches. The watch glass prevents the small amount of radiation from escaping the case.<ref name="Traub95">{{cite web|last1=Traub|first1=R. J.|last2=Jensen|first2=J. A.|title=Tritium radioluminescent devices, Health and Safety Manual|url=http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/27/001/27001618.pdf|publisher=International Atomic Energy Agency|access-date=20 May 2015|page=2.4|date=June 1995|archive-url=https://web.archive.org/web/20150906043743/http://www.iaea.org/inis/collection/NCLCollectionStore/_Public/27/001/27001618.pdf|archive-date=6 September 2015|url-status=live}}</ref> Small amounts of tritium are produced naturally by cosmic rays striking atmospheric gases; tritium has also been released in [[nuclear testing|nuclear weapons tests]].<ref>{{cite web| author=Staff| date=15 November 2007| url=http://www.epa.gov/rpdweb00/radionuclides/tritium.html| publisher=U.S. Environmental Protection Agency| title=Tritium| access-date=12 February 2008| archive-url=https://web.archive.org/web/20080102171148/http://www.epa.gov/rpdweb00/radionuclides/tritium.html| archive-date=2 January 2008| url-status=live}}</ref> It is used in nuclear fusion,<ref>{{cite web| last=Nave| first=C. R.| title=Deuterium-Tritium Fusion| work=HyperPhysics| publisher=Georgia State University| date=2006| url=http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/fusion.html| access-date=8 March 2008| archive-url=https://web.archive.org/web/20080316055852/http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/fusion.html| archive-date=16 March 2008| url-status=live}}</ref> as a tracer in [[isotope geochemistry]],<ref>{{cite journal| first1=C.| last1=Kendall| first2=E.| last2=Caldwell| journal=Isotope Tracers in Catchment Hydrology| title=Chapter 2: Fundamentals of Isotope Geochemistry| editor1=C. Kendall| editor2=J. J. McDonnell| publisher=US Geological Survey| date=1998| doi=10.1016/B978-0-444-81546-0.50009-4| url=http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html#2.5.1| access-date=8 March 2008| archive-url=https://web.archive.org/web/20080314192517/http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html#2.5.1| archive-date=14 March 2008| pages=51–86}}</ref> and in specialized [[self-powered lighting]] devices.<ref>{{cite web| title=The Tritium Laboratory| publisher=University of Miami| date=2008| url=http://www.rsmas.miami.edu/groups/tritium/| access-date=8 March 2008| archive-url=https://web.archive.org/web/20080228061358/http://www.rsmas.miami.edu/groups/tritium/| archive-date=28 February 2008| df=dmy-all}}</ref> Tritium has also been used in chemical and biological labeling experiments as a [[radiolabel]].<ref name="holte">{{cite journal| last1=Holte| first1=A. E.| last2=Houck| first2=M. A.| last3=Collie| first3=N. L.| title=Potential Role of Parasitism in the Evolution of Mutualism in Astigmatid Mites| journal=Experimental and Applied Acarology| volume=25| issue=2| pages=97–107| date=2004|doi=10.1023/A:1010655610575| pmid=11513367| s2cid=13159020}}</ref>

Unique among the elements, distinct names are assigned to its isotopes in common use. During the early study of radioactivity, heavy radioisotopes were given their own names, but these are mostly no longer used. The symbols D and T (instead of {{chem|2|H}} and {{chem|3|H}}) are sometimes used for deuterium and tritium, but the symbol P was already used for [[phosphorus]] and thus was not available for protium.<ref>{{cite web|last=van der Krogt|first=P.|date=5 May 2005|url=http://elements.vanderkrogt.net/element.php?sym=H|publisher=Elementymology & Elements Multidict|title=Hydrogen|access-date=20 December 2010|archive-url=https://web.archive.org/web/20100123001440/http://elements.vanderkrogt.net/element.php?sym=H|archive-date=23 January 2010}}</ref> In its [[IUPAC nomenclature|nomenclatural]] guidelines, the [[International Union of Pure and Applied Chemistry]] (IUPAC) allows any of D, T, {{chem|2|H}}, and {{chem|3|H}} to be used, though {{chem|2|H}} and {{chem|3|H}} are preferred.<ref>§ IR-3.3.2, [http://old.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap3-3.04.pdf Provisional Recommendations] {{Webarchive|url=https://web.archive.org/web/20160209043933/http://old.iupac.org/reports/provisional/abstract04/RB-prs310804/Chap3-3.04.pdf |date=9 February 2016 }}, Nomenclature of Inorganic Chemistry, Chemical Nomenclature and Structure Representation Division, IUPAC. Accessed on line 3 October 2007.</ref>

The [[exotic atom]] [[muonium]] (symbol Mu), composed of an anti[[muon]] and an [[electron]], can also be considered a light radioisotope of hydrogen.<ref name="Gold">{{cite book|author=IUPAC|title=Compendium of Chemical Terminology|title-link=Compendium of Chemical Terminology|publisher=[[Blackwell Scientific Publications]]|year=1997|isbn=978-0-86542-684-9|editor=A. D. McNaught, A. Wilkinson|edition=2nd|chapter=Muonium|doi=10.1351/goldbook.M04069|author-link=International Union of Pure and Applied Chemistry|access-date=15 November 2016|chapter-url=http://goldbook.iupac.org/M04069.html|archive-url=https://web.archive.org/web/20080313121643/http://goldbook.iupac.org/M04069.html|archive-date=13 March 2008|url-status=live}}</ref> Because muons decay with lifetime {{val|2.2|u=[[Microsecond|µs]]}}, muonium is too unstable for observable chemistry.<ref name="Hughes">{{cite journal|author=V. W. Hughes|display-authors=etal|year=1960|title=Formation of Muonium and Observation of its Larmor Precession|journal=[[Physical Review Letters]]|volume=5|issue=2|pages=63–65|bibcode=1960PhRvL...5...63H|doi=10.1103/PhysRevLett.5.63}}</ref> Nevertheless, muonium compounds are important test cases for [[Quantum simulator|quantum simulation]], due to the mass difference between the antimuon and the proton,<ref>{{Cite journal|last1=Bondi|first1=D. K.|last2=Connor|first2=J. N. L.|last3=Manz|first3=J.|last4=Römelt|first4=J.|date=20 October 1983|title=Exact quantum and vibrationally adiabatic quantum, semiclassical and quasiclassical study of the collinear reactions Cl + MuCl, Cl + HCl, Cl + DCl|journal=Molecular Physics|volume=50|issue=3|pages=467–488|doi=10.1080/00268978300102491|bibcode=1983MolPh..50..467B |issn=0026-8976}}</ref> and IUPAC nomenclature incorporates such hypothetical compounds as muonium chloride (MuCl) and sodium muonide (NaMu), analogous to [[hydrogen chloride]] and [[sodium hydride]] respectively.<ref name="iupac">{{cite journal
 |doi=10.1351/pac200173020377
 |author=W. H. Koppenol
 |author2=IUPAC
 |author2-link=International Union of Pure and Applied Chemistry
 |year=2001
 |title=Names for muonium and hydrogen atoms and their ions
 |url=http://www.iupac.org/publications/pac/2001/pdf/7302x0377.pdf
 |journal=[[Pure and Applied Chemistry]]
 |volume=73
 |issue=2
 |pages=377–380
 |s2cid=97138983
 |access-date=15 November 2016
|archive-url=https://web.archive.org/web/20110514000319/http://www.iupac.org/publications/pac/2001/pdf/7302x0377.pdf
 |archive-date=14 May 2011
|url-status=live
 }}</ref>

=== Thermal and physical properties ===
Table of thermal and physical properties of hydrogen (H{{sub|2}}) at atmospheric pressure:<ref>{{Cite book |last=Holman |first=Jack P. |url=https://www.worldcat.org/oclc/46959719 |title=Heat transfer |date=2002 |publisher=McGraw-Hill |isbn=0-07-240655-0 |edition=9th |location=New York, NY |pages=600–606 |language=English |oclc=46959719}}</ref><ref>{{cite book |author-link1=Frank P. Incropera |last1=Incropera |last2=Dewitt |last3=Bergman |last4=Lavigne |first1=Frank P. |first2=David P. |first3=Theodore L. |first4=Adrienne S. |url=https://www.worldcat.org/oclc/62532755 |title=Fundamentals of heat and mass transfer |date=2007 |publisher=John Wiley and Sons, Inc |isbn=978-0-471-45728-2 |edition=6th |location=Hoboken, NJ |pages=941–950 |language=English |oclc=62532755 }}</ref>
{|class="wikitable mw-collapsible mw-collapsed"
|Temperature (K)
|Density (kg/m^3)
|Specific heat (kJ/kg K)
|Dynamic viscosity (kg/m s)
|Kinematic viscosity (m^2/s)
|Thermal conductivity (W/m K)
|Thermal diffusivity (m^2/s)
|Prandtl Number
|-
|100
|0.24255
|11.23
|4.21E-06
|1.74E-05
|6.70E-02
|2.46E-05
|0.707
|-
|150
|0.16371
|12.602
|5.60E-06
|3.42E-05
|0.0981
|4.75E-05
|0.718
|-
|200
|0.1227
|13.54
|6.81E-06
|5.55E-05
|0.1282
|7.72E-05
|0.719
|-
|250
|0.09819
|14.059
|7.92E-06
|8.06E-05
|0.1561
|1.13E-04
|0.713
|-
|300
|0.08185
|14.314
|8.96E-06
|1.10E-04
|0.182
|1.55E-04
|0.706
|-
|350
|0.07016
|14.436
|9.95E-06
|1.42E-04
|0.206
|2.03E-04
|0.697
|-
|400
|0.06135
|14.491
|1.09E-05
|1.77E-04
|0.228
|2.57E-04
|0.69
|-
|450
|0.05462
|14.499
|1.18E-05
|2.16E-04
|0.251
|3.16E-04
|0.682
|-
|500
|0.04918
|14.507
|1.26E-05
|2.57E-04
|0.272
|3.82E-04
|0.675
|-
|550
|0.04469
|14.532
|1.35E-05
|3.02E-04
|0.292
|4.52E-04
|0.668
|-
|600
|0.04085
|14.537
|1.43E-05
|3.50E-04
|0.315
|5.31E-04
|0.664
|-
|700
|0.03492
|14.574
|1.59E-05
|4.55E-04
|0.351
|6.90E-04
|0.659
|-
|800
|0.0306
|14.675
|1.74E-05
|5.69E-04
|0.384
|8.56E-04
|0.664
|-
|900
|0.02723
|14.821
|1.88E-05
|6.90E-04
|0.412
|1.02E-03
|0.676
|-
|1000
|0.02424
|14.99
|2.01E-05
|8.30E-04
|0.448
|1.23E-03
|0.673
|-
|1100
|0.02204
|15.17
|2.13E-05
|9.66E-04
|0.488
|1.46E-03
|0.662
|-
|1200
|0.0202
|15.37
|2.26E-05
|1.12E-03
|0.528
|1.70E-03
|0.659
|-
|1300
|0.01865
|15.59
|2.39E-05
|1.28E-03
|0.568
|1.96E-03
|0.655
|-
|1400
|0.01732
|15.81
|2.51E-05
|1.45E-03
|0.61
|2.23E-03
|0.65
|-
|1500
|0.01616
|16.02
|2.63E-05
|1.63E-03
|0.655
|2.53E-03
|0.643
|-
|1600
|0.0152
|16.28
|2.74E-05
|1.80E-03
|0.697
|2.82E-03
|0.639
|-
|1700
|0.0143
|16.58
|2.85E-05
|1.99E-03
|0.742
|3.13E-03
|0.637
|-
|1800
|0.0135
|16.96
|2.96E-05
|2.19E-03
|0.786
|3.44E-03
|0.639
|-
|1900
|0.0128
|17.49
|3.07E-05
|2.40E-03
|0.835
|3.73E-03
|0.643
|-
|2000
|0.0121
|18.25
|3.18E-05
|2.63E-03
|0.878
|3.98E-03
|0.661
|}

== History ==

=== Discovery and use ===
{{Main|Timeline of hydrogen technologies}}

==== Robert Boyle ====
[[File:Portret van Robert Boyle, RP-P-OB-4578 (cropped).jpg|thumb|[[Robert Boyle]], who discovered the reaction between [[iron filings]] and dilute acids]]
In 1671, Irish scientist [[Robert Boyle]] discovered and described the reaction between [[iron]] filings and dilute [[acid]]s, which results in the production of hydrogen gas.<ref>{{Cite book |last=Boyle |first=R. |url=https://quod.lib.umich.edu/e/eebo2/A29057.0001.001?rgn=main;view=fulltext |title=Tracts written by the Honourable Robert Boyle containing new experiments, touching the relation betwixt flame and air, and about explosions, an hydrostatical discourse occasion'd by some objections of Dr. Henry More against some explications of new experiments made by the author of these tracts: To which is annex't, an hydrostatical letter, dilucidating an experiment about a way of weighing water in water, new experiments, of the positive or relative levity of bodies under water, of the air's spring on bodies under water, about the differing pressure of heavy solids and fluids |publisher=Printed for Richard Davis |year=1672 |pages=64–65}}</ref><ref>{{cite web
|first=M.
|last=Winter
|date=2007
|url=http://education.jlab.org/itselemental/ele001.html
|title=Hydrogen: historical information
|publisher=WebElements Ltd
|access-date=5 February 2008
|archive-url=https://web.archive.org/web/20080410102154/http://education.jlab.org/itselemental/ele001.html
|archive-date=10 April 2008
}}</ref>

{{Blockquote|text=Having provided a saline spirit [hydrochloric acid], which by an uncommon way of preparation was made exceeding sharp and piercing, we put into a vial, capable of containing three or four ounces of water, a convenient quantity of filings of steel, which were not such as are commonly sold in shops to Chymists and Apothecaries, (those being usually not free enough from rust) but such as I had a while before caus'd to be purposely fil'd off from a piece of good steel. This metalline powder being moistn'd in the viol with a little of the menstruum, was afterwards drench'd with more; whereupon the mixture grew very hot, and belch'd up copious and stinking fumes; which whether they consisted altogether of the volatile sulfur of the Mars [iron], or of metalline steams participating of a sulfureous nature, and join'd with the saline exhalations of the menstruum, is not necessary to be here discuss'd. But whencesoever this stinking smoak proceeded, so inflammable it was, that upon the approach of a lighted candle to it, it would readily enough take fire, and burn with a blewish and somewhat greenish flame at the mouth of the viol for a good while together; and that, though with little light, yet with more strength than one would easily suspect.|author=Robert Boyle|title=Tracts written by the Honourable Robert Boyle containing new experiments, touching the relation betwixt flame and air...}}

The word "sulfureous" may be somewhat confusing, especially since Boyle did a similar experiment with iron and sulfuric acid.<ref>{{Cite journal |last=Szydło |first=Z. A. |date=2020 |title=Hydrogen - Some Historical Highlights |journal=Chemistry-Didactics-Ecology-Metrology |volume=25 |issue=1–2 |pages=5–34|doi=10.2478/cdem-2020-0001 |s2cid=231776282 |doi-access=free }}</ref> However, in all likelihood, "sulfureous" should here be understood to mean "combustible".<ref>{{Cite book |last=Ramsay |first=W. |url=https://www.gutenberg.org/files/52778/52778-h/52778-h.htm |title=The gases of the atmosphere: The history of their discovery |publisher=Macmillan |year=1896 |pages=19}}</ref>

==== Henry Cavendish ====
In 1766, [[Henry Cavendish]] was the first to recognize hydrogen gas as a discrete substance, by naming the gas from a [[metal-acid reaction]] "inflammable air". He speculated that "inflammable air" was in fact identical to the hypothetical substance "[[Phlogiston theory|phlogiston]]"<ref>{{cite book |last = Musgrave
 |first = A.
 |chapter = Why did oxygen supplant phlogiston? Research programmes in the Chemical Revolution
 |title = Method and appraisal in the physical sciences
 |series = The Critical Background to Modern Science, 1800–1905
 |editor = Howson, C.
 |year = 1976
 |publisher = Cambridge University Press
 |access-date = 22 October 2011
 |chapter-url = http://ebooks.cambridge.org/chapter.jsf?bid=CBO9780511760013&cid=CBO9780511760013A009
 |doi = 10.1017/CBO9780511760013
 |isbn = 978-0-521-21110-9
 |url-access = registration
 |url = https://archive.org/details/methodappraisali0000unse
}}</ref><ref name="cav766">{{cite journal|last1=Cavendish|first1=Henry|title=Three Papers, Containing Experiments on Factitious Air, by the Hon. Henry Cavendish, F. R. S.|journal=Philosophical Transactions|date=12 May 1766|volume=56|pages=141–184|jstor=105491|bibcode=1766RSPT...56..141C|doi=10.1098/rstl.1766.0019|doi-access=free}}</ref> and further finding in 1781 that the gas produces water when burned. He is usually given credit for the discovery of hydrogen as an element.<ref name="Nostrand">{{cite encyclopedia| title=Hydrogen| encyclopedia=Van Nostrand's Encyclopedia of Chemistry| pages=797–799| publisher=Wylie-Interscience| year=2005| isbn=978-0-471-61525-5}}</ref><ref name="nbb">{{cite book| last=Emsley| first=John| title=Nature's Building Blocks| publisher=Oxford University Press| year=2001| location=Oxford| pages=183–191| isbn=978-0-19-850341-5}}</ref>

==== Antoine Lavoisier ====
[[File:Antoine-Laurent Lavoisier by Louis Jean Desire Delaistre (cropped).jpg|thumb|[[Antoine Lavoisier]], who identified the element that came to be known as hydrogen]]
In 1783, [[Antoine Lavoisier]] identified the element that came to be known as hydrogen<ref>{{cite book| last=Stwertka| first=Albert| title=A Guide to the Elements| url=https://archive.org/details/guidetoelements00stwe| url-access=registration| publisher=Oxford University Press| year=1996| pages=[https://archive.org/details/guidetoelements00stwe/page/16 16–21]| isbn=978-0-19-508083-4}}</ref> when he and [[Pierre-Simon Laplace|Laplace]] reproduced Cavendish's finding that water is produced when hydrogen is burned.<ref name="nbb" /> Lavoisier produced hydrogen for his experiments on mass conservation by reacting a flux of steam with metallic [[iron]] through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions:
:1) {{chem2|Fe + H2O -> FeO + H2}}

:2) {{chem2|Fe + 3 H2O -> Fe2O3 + 3 H2}}

:3) {{chem2|Fe + 4 H2O -> Fe3O4 + 4 H2}}

Many metals such as [[zirconium]] undergo a similar reaction with water leading to the production of hydrogen.<ref>{{Cite journal |last=Northwood |first=D. O. |last2=Kosasih |first2=U. |date=1983 |title=Hydrides and delayed hydrogen cracking in zirconium and its alloys |url=https://journals.sagepub.com/doi/full/10.1179/imtr.1983.28.1.92 |journal=International Metals Reviews |language=en |volume=28 |issue=1 |pages=92–121 |doi=10.1179/imtr.1983.28.1.92 |issn=0308-4590}}</ref>

==== 19th century ====
[[François Isaac de Rivaz]] built the first [[de Rivaz engine]], an internal combustion engine powered by a mixture of hydrogen and oxygen in 1806. [[Edward Daniel Clarke]] invented the hydrogen gas blowpipe in 1819. The [[Döbereiner's lamp]] and [[limelight]] were invented in 1823.<ref name="nbb" />

Hydrogen was [[Liquid hydrogen|liquefied]] for the first time by [[James Dewar]] in 1898 by using [[regenerative cooling]] and his invention, the [[vacuum flask]].<ref name="nbb" /> He produced [[solid hydrogen]] the next year.<ref name="nbb" />

==== Hydrogen-lifted airship ====
[[File:Hindenburg over New York 1937 (cropped).jpg|alt=Airship Hindenburg over New York|thumb|The [[Hindenburg-class airship|Hindenburg]] over [[New York City]] in 1937]]
The first hydrogen-filled [[balloon]] was invented by [[Jacques Charles]] in 1783.<ref name="nbb" /> Hydrogen provided the lift for the first reliable form of air-travel following the 1852 invention of the first hydrogen-lifted [[airship]] by [[Henri Giffard]].<ref name="nbb" /> German count [[Ferdinand von Zeppelin]] promoted the idea of rigid airships lifted by hydrogen that later were called [[Zeppelin]]s; the first of which had its maiden flight in 1900.<ref name="nbb" /> Regularly scheduled flights started in 1910 and by the outbreak of World War I in August 1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships were used as observation platforms and bombers during the war.

The first non-stop transatlantic crossing was made by the British airship ''[[R34 (airship)|R34]]'' in 1919. Regular passenger service resumed in the 1920s and the discovery of [[helium]] reserves in the United States promised increased safety, but the U.S. government refused to sell the gas for this purpose. Therefore, {{chem2|H2}} was used in the [[LZ 129 Hindenburg|''Hindenburg'']] airship, which was destroyed in a midair fire over [[New Jersey]] on 6 May 1937.<ref name="nbb" /> The incident was broadcast live on radio and filmed. Ignition of leaking hydrogen is widely assumed to be the cause, but later investigations pointed to the ignition of the [[aluminium|aluminized]] fabric coating by [[static electricity]]. But the damage to hydrogen's reputation as a [[lifting gas]] was already done and commercial hydrogen airship travel [[Rigid airship#Demise|ceased]]. Hydrogen is still used, in preference to non-flammable but more expensive helium, as a lifting gas for [[Weather balloon#Materials and equipment|weather balloons]].

==== Deuterium and tritium ====
[[Deuterium]] was discovered in December 1931 by [[Harold Urey]], and [[tritium]] was prepared in 1934 by [[Ernest Rutherford]], [[Mark Oliphant]], and [[Paul Harteck]].<ref name="Nostrand" /> [[Heavy water]], which consists of deuterium in the place of regular hydrogen, was discovered by Urey's group in 1932.<ref name="nbb" />

==== Hydrogen-cooled turbogenerator ====
The first [[hydrogen-cooled turbogenerator]] went into service using gaseous hydrogen as a [[coolant]] in the rotor and the stator in 1937 at [[Dayton, Ohio|Dayton]], Ohio, owned by the Dayton Power & Light Co.<ref>{{cite book|url=https://archive.org/stream/chronologicalhis00natirich/chronologicalhis00natirich_djvu.txt|title=A chronological history of electrical development from 600 B.C|author=National Electrical Manufacturers Association|year=1946|page=102|publisher=New York, N.Y., National Electrical Manufacturers Association|access-date=9 February 2016|archive-url=https://web.archive.org/web/20160304141424/http://www.archive.org/stream/chronologicalhis00natirich/chronologicalhis00natirich_djvu.txt|archive-date=4 March 2016|url-status=live}}</ref> This was justified by the high thermal conductivity and very low viscosity of hydrogen gas, thus lower drag than air. This is the most common coolant used for generators 60 MW and larger; smaller generators are usually [[air cooling|air-cooled]].

==== Nickel–hydrogen battery ====
The [[nickel–hydrogen battery]] was used for the first time in 1977 aboard the U.S. Navy's Navigation technology satellite-2 (NTS-2).<ref>{{Cite journal|title=NTS-2 Nickel-Hydrogen Battery Performance 31|journal=Journal of Spacecraft and Rockets|volume=17|pages=31–34|doi=10.2514/3.57704|year=1980|last1=Stockel|first1=J.F|last2=j.d. Dunlop|last3=Betz|first3=F|bibcode=1980JSpRo..17...31S}}</ref> The [[International Space Station]],<ref>{{cite conference|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020070612_2002115777.pdf|work=IECEC '02. 2002 37th Intersociety Energy Conversion Engineering Conference, 2002|pages=45–50|date=July 2002|access-date=11 November 2011|doi=10.1109/IECEC.2002.1391972|title=Validation of international space station electrical performance model via on-orbit telemetry|last1=Jannette|first1=A. G.|last2=Hojnicki|first2=J. S.|last3=McKissock|first3=D. B.|last4=Fincannon|first4=J.|last5=Kerslake|first5=T. W.|last6=Rodriguez|first6=C. D.|isbn=0-7803-7296-4|archive-url=https://web.archive.org/web/20100514100504/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020070612_2002115777.pdf|archive-date=14 May 2010|url-status=live|hdl=2060/20020070612|hdl-access=free}}</ref> [[2001 Mars Odyssey|Mars Odyssey]]<ref>{{cite book|doi=10.1109/AERO.2002.1035418 |date=2002|last1=Anderson|first1=P. M.|last2=Coyne|first2=J. W.|title=Proceedings, IEEE Aerospace Conference |chapter=A lightweight, high reliability, single battery power system for interplanetary spacecraft |isbn=978-0-7803-7231-3|volume=5|pages=5–2433|s2cid=108678345}}</ref> and the [[Mars Global Surveyor]]<ref>{{cite web|url=http://www.astronautix.com/craft/marveyor.htm|title=Mars Global Surveyor|publisher=Astronautix.com|access-date=6 April 2009|archive-url=https://web.archive.org/web/20090810180658/http://www.astronautix.com/craft/marveyor.htm|archive-date=10 August 2009}}</ref> are equipped with nickel-hydrogen batteries. In the dark part of its orbit, the [[Hubble Space Telescope]] is also powered by nickel-hydrogen batteries, which were finally replaced in May 2009,<ref>{{cite web|url=http://www.nasa.gov/mission_pages/hubble/servicing/SM4/main/SM4_Essentials.html|title=Hubble servicing mission 4 essentials|date=7 May 2009|editor=Lori Tyahla|access-date=19 May 2015|publisher=NASA|archive-url=https://web.archive.org/web/20150313073737/http://www.nasa.gov/mission_pages/hubble/servicing/SM4/main/SM4_Essentials.html|archive-date=13 March 2015|url-status=live}}</ref> more than 19 years after launch and 13 years beyond their design life.<ref>{{cite web|url=http://www.nasa.gov/mission_pages/hubble/servicing/series/battery_story.html|title=Extending Hubble's mission life with new batteries|date=25 November 2008|first1=Susan|last1=Hendrix|editor=Lori Tyahla|access-date=19 May 2015|publisher=NASA|archive-url=https://web.archive.org/web/20160305002850/http://www.nasa.gov/mission_pages/hubble/servicing/series/battery_story.html|archive-date=5 March 2016|url-status=live}}</ref>

=== Role in quantum theory ===
[[File:Emission_spectrum-H_labeled.svg|thumb|Hydrogen emission spectrum lines in the four visible lines of the [[Balmer series]]|alt=A line spectrum showing black background with narrow lines superimposed on it: one violet, one blue, one cyan, and one red.]]
Because of its simple atomic structure, consisting only of a proton and an electron, the [[hydrogen atom]], together with the spectrum of light produced from it or absorbed by it, has been central to the development of the theory of [[atom]]ic structure.<ref>{{cite book |last=Crepeau |first=R.
|title=Niels Bohr: The Atomic Model |series=Great Scientific Minds
|date=1 January 2006 |isbn=978-1-4298-0723-4
}}</ref> Furthermore, study of the corresponding simplicity of the hydrogen molecule and the corresponding cation [[H2+|{{chem2|H2+}}]] brought understanding of the nature of the [[chemical bond]], which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.

One of the first quantum effects to be explicitly noticed (but not understood at the time) was a Maxwell observation involving hydrogen, half a century before full [[Quantum mechanics|quantum mechanical theory]] arrived. Maxwell observed that the [[specific heat capacity]] of {{chem2|H2}} unaccountably departs from that of a [[diatomic]] gas below room temperature and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theory, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in {{chem2|H2}} because of its low mass. These widely spaced levels inhibit equal partition of heat energy into rotational motion in hydrogen at low temperatures. Diatomic gases composed of heavier atoms do not have such widely spaced levels and do not exhibit the same effect.<ref name="Berman">{{cite journal
|last1=Berman|first1=R.|last2=Cooke|first2=A. H.|last3=Hill|first3=R. W.
|title=Cryogenics|journal=Annual Review of Physical Chemistry
|date=1956|volume=7|pages=1–20
|doi=10.1146/annurev.pc.07.100156.000245|bibcode = 1956ARPC....7....1B }}</ref>

[[Antihydrogen]] ({{physics particle|anti=yes|H}}) is the [[antimatter]] counterpart to hydrogen. It consists of an [[antiproton]] with a [[positron]]. Antihydrogen is the only type of antimatter atom to have been produced {{as of|2015|lc=y}}.<ref name="char15">{{cite journal|last1=Charlton|first1=Mike|last2=Van Der Werf|first2=Dirk Peter|title=Advances in antihydrogen physics|journal=Science Progress|date=1 March 2015|volume=98|issue=1|pages=34–62|doi=10.3184/003685015X14234978376369|pmid=25942774|pmc=10365473 |s2cid=23581065}}</ref><ref name="Keller15">{{cite journal|last1=Kellerbauer|first1=Alban|title=Why Antimatter Matters|journal=European Review|date=29 January 2015|volume=23|issue=1|pages=45–56|doi=10.1017/S1062798714000532|s2cid=58906869}}</ref>

== Cosmic prevalence and distribution ==
[[File:Nursery of New Stars - GPN-2000-000972.jpg|right|thumb|[[NGC 604]], a giant [[H II region|region of ionized hydrogen]] in the [[Triangulum Galaxy]]|alt=A white-green cotton-like clog on black background.]]
Hydrogen, as atomic H, is the most [[Natural abundance|abundant]] [[chemical element]] in the universe, making up 75% of [[Baryon|normal matter]] by [[mass]] and >90% by number of atoms. Most of the mass of the universe, however, is not in the form of chemical-element type matter, but rather is postulated to occur as yet-undetected forms of mass such as [[dark matter]] and [[dark energy]].<ref>{{cite web|first=S.|last=Gagnon|url=http://education.jlab.org/itselemental/ele001.html|title=Hydrogen|publisher=Jefferson Lab|access-date=5 February 2008|archive-url=https://web.archive.org/web/20080410102154/http://education.jlab.org/itselemental/ele001.html|archive-date=10 April 2008}}</ref>

Hydrogen is found in great abundance in stars and [[gas giant]] planets. [[Molecular cloud]]s of {{chem2|H2}} are associated with [[star formation]]. Hydrogen plays a vital role in powering [[star]]s through the [[proton-proton reaction]] in case of stars with very low to approximately 1 mass of the Sun and the [[CNO cycle]] of [[nuclear fusion]] in case of stars more massive than the [[Sun]].<ref>{{cite web
|last1=Haubold|first1=H.|last2=Mathai|first2=A. M.
|date=15 November 2007
|url=http://neutrino.aquaphoenix.com/un-esa/sun/sun-chapter4.html |archive-url=https://web.archive.org/web/20111211073137/http://neutrino.aquaphoenix.com/un-esa/sun/sun-chapter4.html
 |archive-date=11 December 2011 |title=Solar Thermonuclear Energy Generation
|publisher=[[Columbia University]]|access-date=12 February 2008
}}</ref>

=== States ===
Throughout the universe, hydrogen is mostly found in the [[atom]]ic and [[Plasma (physics)|plasma]] states, with properties quite distinct from those of molecular hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity and high emissivity (producing the light from the Sun and other stars). The charged particles are highly influenced by magnetic and electric fields. For example, in the [[solar wind]] they interact with the Earth's [[magnetosphere]] giving rise to [[Birkeland current]]s and the [[aurora]].

Hydrogen is found in the neutral atomic state in the [[interstellar medium]] because the atoms seldom collide and combine. They are the source of the 21-cm [[hydrogen line]] at 1420&nbsp;MHz that is detected in order to probe primordial hydrogen.<ref>{{Cite web|url=http://mysite.du.edu/~jcalvert/phys/hydrogen.htm|title=Hydrogen|website=mysite.du.edu|access-date=20 April 2008|archive-url=https://web.archive.org/web/20090418033147/http://mysite.du.edu/~jcalvert/phys/hydrogen.htm|archive-date=18 April 2009|url-status=live}}</ref> The large amount of neutral hydrogen found in the [[damped Lyman-alpha system]]s is thought to dominate the [[Physical cosmology|cosmological]] [[baryon]]ic density of the universe up to a [[redshift]] of ''z''&nbsp;=&nbsp;4.<ref>{{cite journal
|last1=Storrie-Lombardi|first1=L. J.
|last2=Wolfe|first2=A. M.
|title=Surveys for z > 3 Damped Lyman-alpha Absorption Systems: the Evolution of Neutral Gas
|journal=Astrophysical Journal
|date=2000|volume=543|pages=552–576
|arxiv=astro-ph/0006044
|doi=10.1086/317138
|bibcode=2000ApJ...543..552S
|issue=2|s2cid=120150880
}}</ref>

Under ordinary conditions on Earth, elemental hydrogen exists as the diatomic gas, {{chem2|H2}}. Hydrogen gas is very rare in Earth's atmosphere (around 0.53 [[part per million|ppm]] on a molar basis<ref name=Grinter>{{cite journal | last1 =Rhys Grinter |last2 = Kropp | first2 = A. | last3 = Venugopal | display-authors=etal | title = Structural basis for bacterial energy extraction from atmospheric hydrogen | journal = Nature | date = 2023 |volume = 615 |issue = 7952 |pages = 541–547 | doi = 10.1038/s41586-023-05781-7|pmid = 36890228 |pmc = 10017518 |bibcode = 2023Natur.615..541G }}</ref>) because of its light weight, which enables it to [[atmospheric escape|escape the atmosphere]] more rapidly than heavier gases. However, hydrogen is the third most abundant element on the Earth's surface,<ref name="ArgonneBasic">{{cite journal
|author=Dresselhaus, M.
|author-link=Mildred Dresselhaus
|display-authors=etal
|date=15 May 2003
|url=http://www.sc.doe.gov/bes/hydrogen.pdf
|title=Basic Research Needs for the Hydrogen Economy
|journal=APS March Meeting Abstracts
|volume=2004
|pages=m1.001
|publisher=Argonne National Laboratory, U.S. Department of Energy, Office of Science Laboratory
|access-date=5 February 2008
|archive-url=https://web.archive.org/web/20080213144956/http://www.sc.doe.gov/bes/hydrogen.pdf
|archive-date=13 February 2008
|bibcode=2004APS..MAR.m1001D
}}</ref> mostly in the form of [[chemical compound]]s such as [[hydrocarbon]]s and water.<ref name="Miessler">{{cite book|first1=G. L.|last1=Miessler|last2=Tarr|first2=D. A.|date=2003|title=Inorganic Chemistry|edition=3rd|publisher=Prentice Hall|isbn=978-0-13-035471-6|url-access=registration|url=https://archive.org/details/inorganicchemist03edmies}}</ref>

A molecular form called [[protonated molecular hydrogen]] ({{chem2|H3+}}) is found in the interstellar medium, where it is generated by ionization of molecular hydrogen from [[cosmic ray]]s. This ion has also been observed in the upper atmosphere of [[Jupiter]]. The ion is relatively stable in outer space due to the low temperature and density. {{chem2|H3+}} is one of the most abundant ions in the universe, and it plays a notable role in the chemistry of the interstellar medium.<ref>{{cite web|author=McCall Group|author2=Oka Group|date=22 April 2005|url=http://h3plus.uiuc.edu/|title=H3+ Resource Center|publisher=Universities of Illinois and Chicago|access-date=5 February 2008|archive-url=https://web.archive.org/web/20071011211244/http://h3plus.uiuc.edu/|archive-date=11 October 2007}}</ref> Neutral [[triatomic hydrogen]] {{chem2|H3}} can exist only in an excited form and is unstable.<ref name="couple">{{citation|year=2003|publisher=Department of Molecular and Optical Physics, University of Freiburg, Germany|author=Helm, H.|display-authors=etal|title=Dissociative Recombination of Molecular Ions with Electrons|pages=275–288|doi=10.1007/978-1-4615-0083-4_27|chapter=Coupling of Bound States to Continuum States in Neutral Triatomic Hydrogen|isbn=978-1-4613-4915-0}}</ref> By contrast, the positive [[hydrogen molecular ion]] ({{chem2|H2+}}) is a rare molecule in the universe.

== Production ==
{{Main|Hydrogen production}}
Many methods exist for producing H<sub>2</sub>, but three dominate commercially: steam reforming often coupled to water-gas shift, partial oxidation of hydrocarbons, and water electrolysis.<ref name=KO/>

===Steam reforming===
[[File:SMR+WGS-1.png|thumb|Inputs and outputs of steam reforming (SMR) and water gas shift (WGS) reaction of natural gas, a process used in hydrogen production]]
Hydrogen is mainly produced by [[steam reforming|steam methane reforming]] (SMR), the reaction of water and methane.<ref>{{cite web |last1=Freyermuth |first1=George H |title=1934 Patent: "The manufacture of hydrogen from methane hydrocarbons by the action of steam at elevated temperature" |url=http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&p=1&u=/netahtml/PTO/srchnum.html&r=1&f=G&l=50&d=PALL&s1=1970695.PN. |website=Patent Full-Text Databases |publisher=United States Patent and Trademark Office |access-date=30 October 2020 |archive-date=1 October 2021 |archive-url=https://web.archive.org/web/20211001062417/https://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.html&r=1&f=G&l=50&d=PALL&s1=1970695.PN. |url-status=live }}</ref><ref name="rotech">{{cite book |last1=Press |first1=Roman J. |url=https://archive.org/details/introductiontohy0000unse/page/249/mode/2up |title=Introduction to Hydrogen Technology |last2=Santhanam |first2=K. S. V. |last3=Miri |first3=Massoud J. |last4=Bailey |first4=Alla V. |last5=Takacs |first5=Gerald A. |publisher=John Wiley & Sons |year=2008 |isbn=978-0-471-77985-8 |pages=249 |url-access=registration}}</ref><!--update?--> <ref name="Oxtoby">{{cite book
| first=D. W.|last=Oxtoby|date=2002
| title=Principles of Modern Chemistry
| edition=5th|publisher=Thomson Brooks/Cole
| isbn=978-0-03-035373-4}}</ref> Thus, at high temperature (1000–1400&nbsp;K, 700–1100°C or 1300–2000°F), steam (water vapor) reacts with [[methane]] to yield [[carbon monoxide]] and {{chem2|H2}}.

:{{chem2|CH4 + H2O → CO + 3 H2}}
Steam reforming is also  used for the industrial preparation of ammonia.

This reaction is favored at low pressures, Nonetheless, conducted at high pressures (2.0&nbsp;MPa, 20&nbsp;atm or 600&nbsp;[[inHg]]) because high-pressure {{chem2|H2}} is the most marketable product, and [[pressure swing adsorption]] (PSA) purification systems work better at higher pressures. The product mixture is known as "[[synthesis gas]]" because it is often used directly for the production of [[methanol]] and many other compounds. [[Hydrocarbon]]s other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly optimized technology is the formation of coke or carbon:
:{{chem2|CH4 → C + 2 H2}}

Therefore, steam reforming typically employs an excess of {{chem2|H2O}}. Additional hydrogen can be recovered from the steam by using carbon monoxide through the [[water gas shift reaction]] (WGS).  This process requires an [[iron oxide]] catalyst:<ref name="Oxtoby" />
:{{chem2|CO + H2O → CO2 + H2}}

Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the [[Haber process]] for [[ammonia production]], hydrogen is generated from natural gas.<ref>{{cite web| last=Funderburg| first=E.| title=Why Are Nitrogen Prices So High?| publisher=The Samuel Roberts Noble Foundation| date=2008| url=http://www.noble.org/Ag/Soils/NitrogenPrices/Index.htm| access-date=11 March 2008| archive-url=https://web.archive.org/web/20010509065844/http://www.noble.org/ag/Soils/NitrogenPrices/Index.htm| archive-date=9 May 2001| df=dmy-all}}</ref>

===Partial oxidation of hydrocarbons===
Other methods for CO and {{chem2|H2}} production include partial oxidation of hydrocarbons:<ref>{{cite web| title=Hydrogen Properties, Uses, Applications| publisher=Universal Industrial Gases, Inc.| date=2007| url=http://www.uigi.com/hydrogen.html| access-date=11 March 2008| archive-url=https://web.archive.org/web/20080327014507/http://www.uigi.com/hydrogen.html| archive-date=27 March 2008| url-status=live}}</ref>

:{{chem2|2 CH4 + O2 → 2 CO + 4 H2}}

Although less important commercially, coal can serve as a prelude to the shift reaction above:<ref name="Oxtoby" />

:{{chem2|C + H2O → CO + H2}}

Olefin production units may produce substantial quantities of byproduct hydrogen particularly from cracking light feedstocks like [[ethane]] or [[propane]].<ref>{{Cite journal |last=Hannula |first=Ilkka |date=2015 |title=Co-production of synthetic fuels and district heat from biomass residues, carbon dioxide and electricity: Performance and cost analysis |url=http://dx.doi.org/10.1016/j.biombioe.2015.01.006 |journal=Biomass and Bioenergy |volume=74 |pages=26–46 |doi=10.1016/j.biombioe.2015.01.006 |bibcode=2015BmBe...74...26H |issn=0961-9534}}</ref>

=== Water electrolysis ===
[[File:Hydrogen production via Electrolysis.png|thumb|Inputs and outputs of the electrolysis of water production of hydrogen]]
[[Electrolysis of water]] is a conceptually simple method of producing hydrogen.  
:{{chem2|2 H2O(l) → 2 H2(g) + O2(g)}}
Commercial [[electrolyzer]]s use [[nickel]]-based catalysts in strongly alkaline solution. Platinum is a better catalyst but is expensive.<ref>{{cite journal |doi=10.1038/ncomms5695 |title=Nanoscale nickel oxide/Nickel heterostructures for active hydrogen evolution electrocatalysis |date=2014 |last1=Gong |first1=Ming |last2=Zhou |first2=Wu |last3=Tsai |first3=Mon-Che |last4=Zhou |first4=Jigang |last5=Guan |first5=Mingyun |last6=Lin |first6=Meng-Chang |last7=Zhang |first7=Bo |last8=Hu |first8=Yongfeng |last9=Wang |first9=Di-Yan |last10=Yang |first10=Jiang |last11=Pennycook |first11=Stephen J. |last12=Hwang |first12=Bing-Joe |last13=Dai |first13=Hongjie |journal=Nature Communications |volume=5 |page=4695 |pmid=25146255 |bibcode=2014NatCo...5.4695G |s2cid=205329127 |doi-access=free }}</ref>

[[Electrolysis]] of [[brine]] to yield [[chlorine]] also produces hydrogen as a co-product.<ref>{{cite web| last=Lees| first=A.| title=Chemicals from salt| publisher=BBC|date=2007|url=http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsrocks/chemicals_saltrev3.shtml|access-date=11 March 2008|archive-url = https://web.archive.org/web/20071026052022/http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsrocks/chemicals_saltrev3.shtml |archive-date = 26 October 2007}}</ref>

=== Methane pyrolysis ===

Hydrogen can be produced by [[pyrolysis]] of natural gas (methane).

This route has a lower carbon footprint than commercial hydrogen production processes.<ref>{{cite journal |last1=Von Wald |first1=Gregory A. |title=Optimization-based technoeconomic analysis of molten-media methane pyrolysis for reducing industrial sector CO2 emissions |url=https://pubs.rsc.org/en/content/articlelanding/2020/SE/D0SE00427H |journal=Sustainable Energy & Fuels |year=2020 |volume=4 |issue=9 |pages=4598–4613 |publisher=Royal Society of Chemistry |doi=10.1039/D0SE00427H |s2cid=225676190 |access-date=31 October 2020 |archive-date=8 November 2020 |archive-url=https://web.archive.org/web/20201108001230/https://pubs.rsc.org/en/content/articlelanding/2020/SE/D0SE00427H |url-status=live }}</ref><ref>{{cite journal |last1=Schneider |first1=Stefan |title=State of the Art of Hydrogen Production via Pyrolysis of Natural Gas |journal=ChemBioEng Reviews |year=2020 |volume=7 |issue=5 |pages=150–158 |publisher=Wiley Online Library |doi=10.1002/cben.202000014 |doi-access=free }}</ref><ref>{{cite web |last1=Cartwright |first1=Jon |title=The reaction that would give us clean fossil fuels forever |url=https://www.newscientist.com/article/mg23230940-200-crack-methane-for-fossil-fuels-without-tears/ |website=New Scientist |access-date=30 October 2020 |archive-date=26 October 2020 |archive-url=https://web.archive.org/web/20201026044037/https://www.newscientist.com/article/mg23230940-200-crack-methane-for-fossil-fuels-without-tears/ |url-status=live }}</ref><ref>{{cite web |last1=Karlsruhe Institute of Technology |title=Hydrogen from methane without CO2 emissions |url=https://phys.org/news/2013-04-hydrogen-methane-co2-emissions.html |website=Phys.Org |access-date=30 October 2020 |archive-date=21 October 2020 |archive-url=https://web.archive.org/web/20201021215453/https://phys.org/news/2013-04-hydrogen-methane-co2-emissions.html |url-status=live }}</ref> Developing a commercial methane pyrolysis process could expedite the expanded use of hydrogen in industrial and transportation applications. Methane pyrolysis is accomplished by passing methane through a molten metal catalyst containing dissolved [[nickel]]. Methane is converted to hydrogen gas and solid [[carbon]].<ref>{{cite journal
 |last1=Upham
 |first1=D. Chester
 |title=Catalytic molten metals for the direct conversion of methane to hydrogen and separable carbon
 |journal=Science
 |year=2017
 |volume=358
 |issue=6365
 |pages=917–921
 |publisher=American Association for Advancement of Science
 |doi=10.1126/science.aao5023
 |pmid=29146810
 |bibcode=2017Sci...358..917U
 |s2cid=206663568
 |doi-access=free
 }}</ref><ref>{{cite journal |last1=Clarke |first1=Palmer |title=Dry reforming of methane catalyzed by molten metal alloys |url=https://www.nature.com/articles/s41929-019-0416-2 |journal=Nature Catalysis |year=2020 |volume=3 |pages=83–89 |doi=10.1038/s41929-019-0416-2 |s2cid=210862772 |access-date=31 October 2020 |archive-date=29 January 2021 |archive-url=https://web.archive.org/web/20210129015717/https://www.nature.com/articles/s41929-019-0416-2 |url-status=live }}</ref>

:{{chem2|CH4(g) → C(s) + 2 H2(g)}} (ΔH° = 74 kJ/mol)
The carbon may be sold as a manufacturing feedstock or fuel, or landfilled.

Further research continues in several laboratories, including at Karlsruhe Liquid-metal Laboratory<ref>{{cite web |last1=Gusev |first1=Alexander |title=KITT/IASS – Producing CO2 Free Hydrogen From Natural Gas For Energy Usage |url=http://www.europeanenergyinnovation.eu/Latest-Research/Spring-2019/KITT-IASS-Producing-CO2-free-hydrogen-from-natural-gas-for-energy-usage |website=European Energy Innovation |publisher=Institute for Advanced Sustainability Studies |access-date=30 October 2020 |archive-date=29 January 2021 |archive-url=https://web.archive.org/web/20210129015717/http://www.europeanenergyinnovation.eu/Latest-Research/Spring-2019/KITT-IASS-Producing-CO2-free-hydrogen-from-natural-gas-for-energy-usage |url-status=live }}</ref> and at University of California – Santa Barbara.<ref>{{cite web |last1=Fernandez |first1=Sonia |title=Researchers develop potentially low-cost, low-emissions technology that can convert methane without forming CO2 |url=https://phys.org/news/2017-11-potentially-low-cost-low-emissions-technology-methane.html |website=Phys-Org |publisher=American Institute of Physics |access-date=19 October 2020 |archive-date=19 October 2020 |archive-url=https://web.archive.org/web/20201019193709/https://phys.org/news/2017-11-potentially-low-cost-low-emissions-technology-methane.html |url-status=live }}</ref> [[BASF]] built a methane pyrolysis pilot plant.<ref>{{cite web |last1=BASF |title=BASF researchers working on fundamentally new, low-carbon production processes, Methane Pyrolysis |url=https://www.basf.com/us/en/who-we-are/sustainability/we-produce-safely-and-efficiently/energy-and-climate-protection/carbon-management/interview-methane-pyrolysis.html |website=United States Sustainability |publisher=BASF |access-date=19 October 2020 |archive-date=19 October 2020 |archive-url=https://web.archive.org/web/20201019120013/https://www.basf.com/us/en/who-we-are/sustainability/we-produce-safely-and-efficiently/energy-and-climate-protection/carbon-management/interview-methane-pyrolysis.html |url-status=live }}</ref>

=== Thermochemical ===
More than 200 thermochemical cycles can be used for [[water splitting]]. Many of these cycles such as the [[iron oxide cycle]], [[cerium(IV) oxide–cerium(III) oxide cycle]], [[zinc zinc-oxide cycle]], [[sulfur-iodine cycle]], [[copper-chlorine cycle]] and [[hybrid sulfur cycle]] have been evaluated for their commercial potential to produce hydrogen and oxygen from water and heat without using electricity.<ref>{{cite web|url=http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf|title=Development of solar-powered thermochemical production of hydrogen from water|first1=Al|last1=Weimer|date=25 May 2005|publisher=Solar Thermochemical Hydrogen Generation Project|access-date=21 December 2008|archive-url=https://web.archive.org/web/20070417134156/http://www.hydrogen.energy.gov/pdfs/review05/pd28_weimer.pdf|archive-date=17 April 2007|url-status=live}}</ref> A number of labs (including in [[France]], [[Germany]], [[Greece]], [[Japan]], and the [[United States]]) are developing thermochemical methods to produce hydrogen from solar energy and water.<ref>{{cite web|url=http://www.hydrogen.energy.gov/pdfs/progress07/ii_f_1_perret.pdf|title=Development of Solar-Powered Thermochemical Production of Hydrogen from Water, DOE Hydrogen Program, 2007|author=Perret, R.|access-date=17 May 2008|archive-url=https://web.archive.org/web/20100527212241/http://www.hydrogen.energy.gov/pdfs/progress07/ii_f_1_perret.pdf|archive-date=27 May 2010}}</ref>

===Laboratory methods===
{{chem2|H2}} is produced in labs, often as a by-product of other reactions. Many metals react with water to produce {{chem2|H2}}, but the rate of hydrogen evolution depends on the metal, the pH, and the presence of alloying agents. Most often, hydrogen evolution is induced by acids. The alkali and alkaline earth metals, aluminium, zinc, manganese, and iron react readily with aqueous acids. This reaction is the basis of the [[Kipp's apparatus]], which once was used as a laboratory gas source:
:{{chem2|Zn + 2 H+ → Zn(2+) + H2}}

In the absence of acid, the evolution of {{chem2|H2}} is slower. Because iron is widely used structural material, its [[anaerobic corrosion]] is of technological significance:

:{{chem2|Fe + 2 H2O → Fe(OH)2 + H2}}

Many metals, such as [[aluminium]], are slow to react with water because they form passivated oxide coatings of oxides. An alloy of aluminium and [[gallium]], however, does react with water.<ref>{{cite journal|doi= 10.1016/j.ijhydene.2008.02.025|title= Activation of aluminium metal to evolve hydrogen from water|year= 2008|last1= Parmuzina|first1= A.V.|last2= Kravchenko|first2= O.V.|journal= International Journal of Hydrogen Energy|volume= 33|issue= 12|pages= 3073–3076|bibcode= 2008IJHE...33.3073P}}</ref> At high pH, aluminium can produce {{chem2|H2}}:

:{{chem2|2 Al + 6 H2O + 2 OH- → 2 [Al(OH)4]- + 3 H2}}

Some metal-containing compounds react with acids to evolve {{chem2|H2}}. Under anaerobic conditions, [[ferrous hydroxide]] ({{chem|Fe(OH)|2}}) can be oxidized by the protons of water to form [[magnetite]] and {{chem2|H2}}. This process is described by the [[Schikorr reaction]]:

:{{chem2|3 Fe(OH)2 → Fe3O4 + 2 H2O + H2}}

This process occurs during the anaerobic corrosion of [[iron]] and [[steel]] in [[Anoxic waters|oxygen-free]] [[groundwater]] and in reducing [[soil]]s below the [[water table]].

===Biohydrogen===
{{chem2|H2}} is produced by hydrogenase enzymes in some [[fermentation]].<ref>{{cite journal |doi=10.1021/cr050186q |title=&#91;NiFe&#93; and &#91;FeFe&#93; Hydrogenases Studied by Advanced Magnetic Resonance Techniques |date=2007 |last1=Lubitz |first1=Wolfgang |last2=Reijerse |first2=Eduard |last3=Van Gastel |first3=Maurice |journal=Chemical Reviews |volume=107 |issue=10 |pages=4331–4365 |pmid=17845059 }}</ref>

===Wells===
There is a well in Mali and deposits in several other countries, such as France.<ref>{{Cite web |title=Natural Hydrogen: A Potential Clean Energy Source Beneath Our Feet |url=https://e360.yale.edu/features/natural-geologic-hydrogen-climate-change |access-date=2024-01-27 |website=Yale E360 |language=en-US}}</ref>

== Applications ==

{{See also|Hydrogen economy}}
[[File:The_Hydrogen_Ladder,_Version_5.0.jpg|thumb|Some projected uses in the medium term, but analysts disagree<ref>{{Cite web |last=Barnard |first=Michael |date=2023-10-22 |title=What's New On The Rungs Of Liebreich's Hydrogen Ladder? |url=https://cleantechnica.com/2023/10/22/whats-new-on-the-rungs-of-liebreichs-hydrogen-ladder/ |access-date=2024-03-10 |website=CleanTechnica |language=en-US}}</ref>]]

=== Petrochemical industry ===
Large quantities of {{chem2|H2}} are used in the "upgrading" of [[fossil fuels]]. Key consumers of {{chem2|H2}} include [[hydrodesulfurization]], and [[hydrocracking]]. Many of these reactions can be classified as [[hydrogenolysis]], i.e., the cleavage of bonds by hydrogen. Illustrative is the separation of sulfur from liquid fossil fuels:<ref name=KO>{{cite book |doi=10.1002/0471238961.0825041803262116.a01.pub2 |chapter=Hydrogen |title=Kirk-Othmer Encyclopedia of Chemical Technology |date=2001 |last1=Baade |first1=William F. |last2=Parekh |first2=Uday N. |last3=Raman |first3=Venkat S. |isbn=9780471484943 }}</ref>
:{{chem2|R2S + 2 H2 → H2S + 2 RH}}

=== Hydrogenation ===
[[Hydrogenation]], the addition of {{chem2|H2}} to various substrates, is done on a large scale. Hydrogenation of {{chem2|N2}} to produce ammonia by the [[Haber process]], consumes a few percent of the energy budget in the entire industry. The resulting ammonia is used to supply most of the protein consumed by humans.<ref name="Smil_2004_Enriching">{{cite book |last1=Smil |first1=Vaclav |title=Enriching the Earth: Fritz Haber, Carl Bosch, and the Transformation of World Food Production |date=2004 |publisher=MIT |location=Cambridge, MA |isbn=978-0-262-69313-4 |edition=1st}}</ref> Hydrogenation is used to convert [[unsaturated fat]]s and [[vegetable oil|oils]] to saturated (trans) fats and oils. The major application is the production of [[margarine]]. [[Methanol]] is produced by hydrogenation of carbon dioxide. It is similarly the source of hydrogen in the manufacture of [[hydrochloric acid]]. {{chem2|H2}} is also used as a [[reducing agent]] for the conversion of some [[ore]]s to the metals.<ref>{{cite web|author=Chemistry Operations|date=15 December 2003|url=http://periodic.lanl.gov/1.shtml|title=Hydrogen|publisher=Los Alamos National Laboratory|access-date=5 February 2008|archive-url=https://web.archive.org/web/20110304203439/http://periodic.lanl.gov/1.shtml|archive-date=4 March 2011}}</ref>

=== Coolant ===
{{Main|Hydrogen-cooled turbo generator}}
Hydrogen is commonly used in power stations as a coolant in generators due to a number of favorable properties that are a direct result of its light diatomic molecules. These include low [[density]], low [[viscosity]], and the highest [[specific heat]] and [[thermal conductivity]] of all gases.

=== Energy carrier ===
Elemental hydrogen is widely discussed in the context of energy as an energy carrier with potential to help to decarbonize economies and mitigate greenhouse gas emissions.<ref name="IPCC-2022">{{Cite book |author=IPCC |url=https://ipcc.ch/report/ar6/wg3/downloads/report/IPCC_AR6_WGIII_FullReport.pdf |title=Climate Change 2022: Mitigation of Climate Change |publisher=Cambridge University Press (In Press) |year=2022 |editor1-last=Shukla |editor1-first=P.R. |series=Contribution of Working Group III to the [[IPCC Sixth Assessment Report|Sixth Assessment Report]] of the Intergovernmental Panel on Climate Change |place=Cambridge, UK and New York, NY, US |doi=10.1017/9781009157926 |ref={{harvid|IPCC AR6 WG3|2022}} |author-link=IPCC |editor2-last=Skea |editor2-first=J. |editor3-last=Slade |editor3-first=R. |editor4-last=Al Khourdajie |editor4-first=A. |editor5-last=van Diemen |editor5-first=R. |editor6-last=McCollum |editor6-first=D. |editor7-last=Pathak |editor7-first=M. |editor8-last=Some |editor8-first=S. |editor9-last=Vyas |editor9-first=P. |display-editors=4 |editor10-first=R. |editor10-last=Fradera |editor11-first=M. |editor11-last=Belkacemi |editor12-first=A. |editor12-last=Hasija |editor13-first=G. |editor13-last=Lisboa |editor14-first=S. |editor14-last=Luz |editor15-first=J. |editor15-last=Malley|pages=91–92|isbn=9781009157926 }}</ref><ref name="Evans-2020" /> This therefore requires hydrogen to be produced cleanly in quantities to be supplied in sectors and applications where cheaper and more energy-efficient [[Climate change mitigation|mitigation]] alternatives are limited. These include heavy industry and long-distance transport.<ref name="IPCC-2022" /> Hydrogen is a {{em|carrier}} of energy rather than an energy resource, because there is no naturally occurring source of hydrogen in useful quantities.<ref name="sustain">{{cite web
 |last = McCarthy
 |first = J.
 |title = Hydrogen
 |publisher = [[Stanford University]]
 |date = 31 December 1995
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 |access-date = 14 March 2008
 |archive-url = https://web.archive.org/web/20080314043136/http://www-formal.stanford.edu/jmc/progress/hydrogen.html
 |archive-date = 14 March 2008
 |df = dmy-all
}}</ref>

Hydrogen can be deployed as an energy source in [[fuel cell]]s to produce electricity or via combustion to generate heat.<ref name="Lewis-2021" /> When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapor.<ref name="Lewis-2021" /> Combustion of hydrogen can lead to the thermal formation of harmful [[NOx|nitrogen oxides]].<ref name="Lewis-2021" /> The overall lifecycle emissions of hydrogen depend on how it is produced. Nearly all the world's current supply of hydrogen is created from fossil fuels.<ref>{{Cite news |last1=Reed |first1=Stanley |last2=Ewing |first2=Jack |date=2021-07-13 |title=Hydrogen Is One Answer to Climate Change. Getting It Is the Hard Part. |work=[[The New York Times]] |url=https://www.nytimes.com/2021/07/13/business/hydrogen-climate-change.html |url-status=live |access-date=14 July 2021 |archive-url=https://web.archive.org/web/20210714190628/https://www.nytimes.com/2021/07/13/business/hydrogen-climate-change.html |archive-date=14 July 2021 |issn=0362-4331}}</ref><ref>{{cite book|author=[[IRENA]]|date=2019|title=Hydrogen: A renewable energy perspective|isbn=978-92-9260-151-5|url=https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2019/Sep/IRENA_Hydrogen_2019.pdf |access-date=17 October 2021|archive-date=29 September 2021|archive-url=https://web.archive.org/web/20210929023014/https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2019/Sep/IRENA_Hydrogen_2019.pdf|url-status=live|page=9}}.</ref> The main method is [[steam methane reforming]], in which hydrogen is produced from a chemical reaction between steam and [[methane]], the main component of natural gas. Producing one tonne of hydrogen through this process emits 6.6–9.3 tonnes of carbon dioxide.<ref name="Bonheure-2021">{{Cite web |last1=Bonheure |first1=Mike |last2=Vandewalle |first2=Laurien A. |last3=Marin |first3=Guy B. |last4=Van Geem |first4=Kevin M. |date=March 2021 |title=Dream or Reality? Electrification of the Chemical Process Industries |url=https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852 |url-status=live |archive-url=https://web.archive.org/web/20210717132733/https://www.aiche-cep.com/cepmagazine/march_2021/MobilePagedArticle.action?articleId=1663852 |archive-date=17 July 2021 |access-date=6 July 2021 |website=CEP Magazine |publisher=[[American Institute of Chemical Engineers]]}}</ref> While [[carbon capture and storage]] (CCS) could remove a large fraction of these emissions, the overall carbon footprint of hydrogen from natural gas is difficult to assess {{As of|2021|lc=y}}, in part because of emissions (including [[Gas venting|vented]] and [[Fugitive gas emissions|fugitive]] [[methane]]) created in the production of the natural gas itself.<ref name="Griffiths-2021">{{Cite journal |last1=Griffiths |first1=Steve |last2=Sovacool |first2=Benjamin K. |last3=Kim |first3=Jinsoo |last4=Bazilian |first4=Morgan |last5=Uratani |first5=Joao M. |display-authors=4 |date=2021 |title=Industrial decarbonization via hydrogen: A critical and systematic review of developments, socio-technical systems and policy options |url=https://www.sciencedirect.com/science/article/pii/S2214629621003017?dgcid=coauthor |url-status=live |journal=[[Energy Research & Social Science]] |volume=80 |page=39 |doi=10.1016/j.erss.2021.102208 |bibcode=2021ERSS...8002208G |issn=2214-6296 |archive-url=https://web.archive.org/web/20211016205152/https://www.sciencedirect.com/science/article/abs/pii/S2214629621003017?dgcid=coauthor |archive-date=16 October 2021 |access-date=11 September 2021}}</ref>

Electricity can be used to split water molecules, producing sustainable hydrogen, provided the electricity was generated sustainably. However, this [[electrolysis]] process is currently more expensive than creating hydrogen from methane without CCS and the efficiency of energy conversion is inherently low.<ref name="Evans-2020">{{Cite web |last1=Evans |first1=Simon |last2=Gabbatiss |first2=Josh |date=30 November 2020 |title=In-depth Q&A: Does the world need hydrogen to solve climate change? |url=https://www.carbonbrief.org/in-depth-qa-does-the-world-need-hydrogen-to-solve-climate-change |url-status=live |archive-url=https://web.archive.org/web/20201201155033/https://www.carbonbrief.org/in-depth-qa-does-the-world-need-hydrogen-to-solve-climate-change |archive-date=1 December 2020 |access-date=1 December 2020 |website=[[Carbon Brief]]}}</ref> Hydrogen can be produced when there is a surplus of [[Variable renewable energy|variable renewable electricity]], then stored and used to generate heat or to re-generate electricity.<ref>{{Cite journal |last1=Palys |first1=Matthew J. |last2=Daoutidis |first2=Prodromos |date=2020 |title=Using hydrogen and ammonia for renewable energy storage: A geographically comprehensive techno-economic study |journal=[[Computers & Chemical Engineering]] |volume=136 |pages=106785 |doi=10.1016/j.compchemeng.2020.106785 |issn=0098-1354 |doi-access=free}}</ref> Hydrogen created through electrolysis using renewable energy is commonly referred to as "[[green hydrogen]]".<ref>{{Cite web |title=Hydrogen industry must clean itself up before expanding into new… |url=https://www.canarymedia.com/articles/hydrogen/hydrogen-industry-must-clean-itself-up-before-expanding-into-new-uses-report-finds |access-date=2023-04-05 |website=Canary Media |date=31 August 2021 |language=en}}</ref> It can be further transformed into [[synthetic fuel]]s such as [[ammonia]] and [[methanol]].<ref>{{cite book|author=[[IRENA]]|year=2021|title=World Energy Transitions Outlook: 1.5°C Pathway |url=https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2021/March/IRENA_World_Energy_Transitions_Outlook_2021.pdf |isbn=978-92-9260-334-2|archive-date=11 June 2021|archive-url=https://web.archive.org/web/20210611230855/https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2021/March/IRENA_World_Energy_Transitions_Outlook_2021.pdf|url-status=live|pages=12, 22}}</ref>

Innovation in [[Electrolysis of water|hydrogen electrolyzers]] could make large-scale production of hydrogen from electricity more cost-competitive.<ref>{{Cite book|author1-link=International Energy Agency|last1=IEA|title=Net Zero by 2050: A Roadmap for the Global Energy Sector|year=2021|url=https://iea.blob.core.windows.net/assets/ad0d4830-bd7e-47b6-838c-40d115733c13/NetZeroby2050-ARoadmapfortheGlobalEnergySector.pdf|archive-date=23 May 2021|archive-url=https://web.archive.org/web/20210523155010/https://iea.blob.core.windows.net/assets/ad0d4830-bd7e-47b6-838c-40d115733c13/NetZeroby2050-ARoadmapfortheGlobalEnergySector.pdf|url-status=live
|pages=15, 75–76}}</ref> There is potential for hydrogen produced this way to play a significant role in decarbonizing energy systems where there are challenges and limitations to replacing fossil fuels with direct use of electricity.<ref name="IPCC-2022" />

Hydrogen fuel can produce the intense heat required for industrial production of steel, cement, glass, and chemicals, thus contributing to the decarbonisation of industry alongside other technologies, such as [[electric arc furnace]]s for steelmaking.<ref>{{Cite web |last=Kjellberg-Motton |first=Brendan |date=2022-02-07 |title=Steel decarbonisation gathers speed {{!}} Argus Media |url=https://www.argusmedia.com/en//news/2299399-steel-decarbonisation-gathers-speed |access-date=2023-09-07 |website=www.argusmedia.com |language=en}}</ref> However, it is likely to play a larger role in providing industrial feedstock for cleaner production of ammonia and organic chemicals.<ref name="IPCC-2022" /> For example, in [[steelmaking]], hydrogen could function as a clean energy carrier and also as a low-carbon catalyst, replacing coal-derived [[Coke (fuel)|coke]].<ref>{{Cite web |last1=Blank |first1=Thomas |last2=Molly |first2=Patrick |date=January 2020 |title=Hydrogen's Decarbonization Impact for Industry |url=https://rmi.org/wp-content/uploads/2020/01/hydrogen_insight_brief.pdf |url-status=live |archive-url=https://web.archive.org/web/20200922115313/https://rmi.org/wp-content/uploads/2020/01/hydrogen_insight_brief.pdf |archive-date=22 September 2020 |access-date= |publisher=[[Rocky Mountain Institute]] |pages=2, 7, 8}}</ref> Hydrogen used to decarbonise transportation is likely to find its largest applications in shipping, aviation and, to a lesser extent, heavy goods vehicles, through the use of hydrogen-derived synthetic fuels such as [[Green ammonia|ammonia]] and [[Green methanol|methanol]] and fuel cell technology.<ref name="IPCC-2022" /> For light-duty vehicles including cars, hydrogen is far behind other [[alternative fuel vehicle]]s, especially compared with the rate of adoption of [[battery electric vehicles]], and may not play a significant role in future.<ref>{{Cite journal |last=Plötz |first=Patrick |date=2022-01-31 |title=Hydrogen technology is unlikely to play a major role in sustainable road transport |url=https://www.nature.com/articles/s41928-021-00706-6 |journal=Nature Electronics |language=en |volume=5 |issue=1 |pages=8–10 |doi=10.1038/s41928-021-00706-6 |s2cid=246465284 |issn=2520-1131}}</ref>

Disadvantages of hydrogen as an energy carrier include high costs of storage and distribution due to hydrogen's explosivity, its large volume compared to other fuels, and its tendency to make pipes brittle.<ref name="Griffiths-2021" />

=== Semiconductor industry ===
Hydrogen is employed to saturate broken ("dangling") bonds of [[amorphous silicon]] and [[amorphous carbon]] that helps stabilizing material properties.<ref>{{cite journal
|last1=Le Comber| first1=P. G.
|title=Hall effect and impurity conduction in substitutionally doped amorphous silicon
|journal=Philosophical Magazine|doi=10.1080/14786437708232943
|volume=35
|issue=5
|pages=1173–1187
|date=1977
|last2=Jones
|first2=D. I.
|last3=Spear
|first3=W. E.|bibcode = 1977PMag...35.1173C }}</ref> It is also a potential [[electron donor]] in various oxide materials, including [[zinc oxide|ZnO]],<ref>{{cite journal|last=Van de Walle|first=C. G.|title=Hydrogen as a cause of doping in zinc oxide|journal=Physical Review Letters|volume=85|issue=5|doi=10.1103/PhysRevLett.85.1012|pages=1012–1015|date=2000|pmid=10991462|bibcode=2000PhRvL..85.1012V|hdl=11858/00-001M-0000-0026-D0E6-E|url=http://pubman.mpdl.mpg.de/pubman/item/escidoc:741885/component/escidoc:932688/PRL-85-1012-2000.pdf|access-date=1 August 2018|archive-url=https://web.archive.org/web/20170815000602/http://pubman.mpdl.mpg.de/pubman/item/escidoc:741885/component/escidoc:932688/PRL-85-1012-2000.pdf|archive-date=15 August 2017|url-status=live|hdl-access=free}}</ref><ref>{{cite journal
|last1=Janotti|first1= A.
|title=Hydrogen multicentre bonds|doi=10.1038/nmat1795
|journal=Nature Materials
|volume=6|pages=44–47
|date=2007
|pmid=17143265
|last2=Van De Walle
|first2=C. G.
|issue=1|bibcode = 2007NatMa...6...44J }}</ref> [[Tin dioxide|{{chem2|SnO2}}]], [[Cadmium oxide|CdO]], [[Magnesium oxide|MgO]],<ref>{{cite journal|last1=Kilic|first1=C.|title=n-type doping of oxides by hydrogen|doi=10.1063/1.1482783|journal=Applied Physics Letters|volume=81|issue=1|pages=73–75|date=2002|last2=Zunger|first2=Alex|bibcode=2002ApPhL..81...73K|s2cid=96415065}}</ref> [[Zirconium dioxide|{{chem2|ZrO2}}]], [[Hafnium(IV) oxide|{{chem2|HfO2}}]], [[Lanthanum(III) oxide|{{chem2|La2O3}}]], [[Yttrium(III) oxide|{{chem2|Y2O3}}]], [[Titanium dioxide|{{chem2|TiO2}}]], [[Strontium titanate|{{chem2|SrTiO3}}]], [[Lanthanum aluminate|{{chem2|LaAlO3}}]], [[Silicon dioxide|{{chem2|SiO2}}]], [[Aluminium oxide|{{chem2|Al2O3}}]], [[Zircon|{{chem2|ZrSiO4}}]], [[Hafnon|{{chem2|HfSiO4}}]], and [[Strontium zirconate|{{chem2|SrZrO3}}]].<ref>{{cite journal
|last1=Peacock| first1=P. W.|doi=10.1063/1.1609245
|title=Behavior of hydrogen in high dielectric constant oxide gate insulators
|journal=Applied Physics Letters
|volume=83
|issue=10
|pages=2025–2027
|date=2003
|last2=Robertson
|first2=J.
|bibcode = 2003ApPhL..83.2025P }}</ref>

=== Niche and evolving uses ===
*Shielding gas: Hydrogen is used as a [[shielding gas]] in [[welding]] methods such as [[atomic hydrogen welding]].<ref>{{cite journal
|last=Durgutlu| first=A.
|title=Experimental investigation of the effect of hydrogen in argon as a shielding gas on TIG welding of austenitic stainless steel
|journal=Materials & Design
|volume=25
|issue=1
|pages=19–23
|date=2003
|doi=10.1016/j.matdes.2003.07.004}}</ref><ref>{{cite web
|title=Atomic Hydrogen Welding| publisher=Specialty Welds
|date=2007
|url=http://www.specialwelds.com/underwater-welding/atomic-hydrogen-welding.htm|archive-url=https://web.archive.org/web/20110716115120/http://www.specialwelds.com/underwater-welding/atomic-hydrogen-welding.htm|archive-date=16 July 2011}}</ref>

*Cryogenic research: Liquid {{chem2|H2}} is used in [[cryogenic]] research, including [[superconductivity]] studies.<ref>{{cite journal
|last=Hardy
|first=W. N.
|title=From H2 to cryogenic H masers to HiTc superconductors: An unlikely but rewarding path
|journal=Physica C: Superconductivity
|volume=388–389
|pages=1–6
|date=2003
|doi=10.1016/S0921-4534(02)02591-1|bibcode = 2003PhyC..388....1H }}</ref>

*Buoyant lifting: Because {{chem2|H2}} is only 7% the density of air, it was once widely used as a [[lifting gas]] in balloons and [[airship]]s.<ref name="Almqvist03">{{cite book|last1=Almqvist|first1=Ebbe|title=History of industrial gases|date=2003|publisher=Kluwer Academic/Plenum Publishers|location=New York, N.Y.|isbn=978-0-306-47277-0|pages=47–56|url={{Google books|OI0fTJhydh4C|page=|keywords=|text=|plainurl=yes}}|access-date=20 May 2015}}</ref>
*Leak detection: Pure or mixed with nitrogen (sometimes called [[forming gas]]), hydrogen is a [[tracer gas]] for [[Leak detection|detection]] of minute leaks. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industries.<ref>{{cite conference
|first=M.
|last=Block
|title=Hydrogen as Tracer Gas for Leak Detection
|work=16th WCNDT 2004
|publisher=Sensistor Technologies
|date=3 September 2004
|location=Montreal, Canada
|url=http://www.ndt.net/abstract/wcndt2004/523.htm
|access-date=25 March 2008
|archive-url=https://web.archive.org/web/20090108102521/http://www.ndt.net/abstract/wcndt2004/523.htm
|archive-date=8 January 2009
}}</ref> Hydrogen is an authorized food additive (E 949) that allows food package leak testing, as well as having anti-oxidizing properties.<ref>{{cite web
|url=http://ec.europa.eu/food/fs/sfp/addit_flavor/flav15_en.pdf
|title=Report from the Commission on Dietary Food Additive Intake
|publisher=[[European Union]]
|access-date=5 February 2008
|archive-url=https://web.archive.org/web/20080216050325/http://ec.europa.eu/food/fs/sfp/addit_flavor/flav15_en.pdf
|archive-date=16 February 2008
|url-status=live
}}</ref>

*Neutron moderation: [[Deuterium]] (hydrogen-2) is used in [[CANDU reactor|nuclear fission applications]] as a [[neutron moderator|moderator]] to slow [[neutron]]s.
*Nuclear fusion fuel: Deuterium is used in [[nuclear fusion]] reactions.<ref name="nbb" />
*Isotopic labeling: Deuterium compounds have applications in chemistry and biology in studies of [[Kinetic isotope effect|isotope effects]] on reaction rates.<ref>{{cite journal|last1=Reinsch| first1=J.|first2=A. |last2=Katz|first3=J.|last3=Wean|first4=G.|last4=Aprahamian|first5=J. T.|last5=MacFarland
|title=The deuterium isotope effect upon the reaction of fatty acyl-CoA dehydrogenase and butyryl-CoA| journal=J. Biol. Chem.|volume=255
|issue=19|pages=9093–97|date=1980| doi=10.1016/S0021-9258(19)70531-6|pmid=7410413|doi-access=free}}</ref>

*Rocket fuel: [[Liquid hydrogen]] and [[liquid oxygen]] together serve as [[cryogenic propellant]]s in [[liquid-propellant rocket]]s, as in the [[RS-25|Space Shuttle main engines]]. [[NASA]] has investigated the use of [[rocket propellant]] made from atomic hydrogen, boron or carbon that is frozen into solid molecular hydrogen particles suspended in liquid helium. Upon warming, the mixture vaporizes to allow the atomic species to recombine, heating the mixture to high temperature.<ref>{{Cite web |url=https://ntrs.nasa.gov/api/citations/20030005922/downloads/20030005922.pdf |title=NASA/TM—2002-211915: Solid Hydrogen Experiments for Atomic Propellants |access-date=2 July 2021 |archive-date=9 July 2021 |archive-url=https://web.archive.org/web/20210709183557/https://ntrs.nasa.gov/api/citations/20030005922/downloads/20030005922.pdf |url-status=live }}</ref>
*Tritium uses: [[Tritium]] (hydrogen-3), produced in [[nuclear reactor]]s, is used in the production of [[hydrogen bomb]]s,<ref>{{cite journal| last=Bergeron| first=K. D.| title=The Death of no-dual-use| journal=Bulletin of the Atomic Scientists| volume=60| issue=1| pages=15–17| date=2004| url=http://find.galegroup.com/itx/start.do?prodId=SPJ.SP06| doi=10.2968/060001004| access-date=13 April 2008| archive-url=https://web.archive.org/web/20080419051641/http://find.galegroup.com/itx/start.do?prodId=SPJ.SP06| archive-date=19 April 2008| url-status=live| bibcode=2004BuAtS..60a..15B}}</ref> as an isotopic label in the biosciences,<ref name="holte" /> and as a source of [[beta particle|beta radiation]] in [[Tritium radioluminescence|radioluminescent paint]] for instrument dials and emergency signage.<ref name="Traub95" />

== Biological reactions ==
{{Further|Biohydrogen|Biological hydrogen production (Algae)}}
{{chem2|H2}} is a product of some types of [[Fermentation (biochemistry)|anaerobic metabolism]] and is produced by several [[microorganism]]s, usually via reactions [[catalysis|catalyzed]] by [[iron]]- or [[nickel]]-containing [[enzyme]]s called [[hydrogenase]]s. These enzymes catalyze the reversible [[redox]] reaction between {{chem2|H2}} and its component two protons and two electrons. Creation of hydrogen gas occurs in the transfer of reducing equivalents, produced during [[pyruvate]] [[fermentation (biochemistry)|fermentation]], to water.<ref>{{cite book|first1=R.|last1=Cammack|url=https://books.google.com/books?id=GTzajKoBoNwC&pg=PA202|last2=Robson|first2=R. L.|date=2001|pages=202–203|title=Hydrogen as a Fuel: Learning from Nature|publisher=Taylor & Francis Ltd|isbn=978-0-415-24242-4|access-date=3 September 2020|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015731/https://books.google.com/books?id=GTzajKoBoNwC&pg=PA202|url-status=live}}</ref> The natural cycle of hydrogen production and consumption by organisms is called the [[hydrogen cycle]].<ref name="Rhee6">{{cite journal|last1=Rhee|first1=T. S.|last2=Brenninkmeijer|first2=C. A. M.|last3=Röckmann|first3=T.|title=The overwhelming role of soils in the global atmospheric hydrogen cycle|journal=Atmospheric Chemistry and Physics|date=19 May 2006|volume=6|issue=6|pages=1611–1625|doi=10.5194/acp-6-1611-2006|bibcode=2006ACP.....6.1611R|url=https://hal.archives-ouvertes.fr/hal-00301903/file/acpd-5-11215-2005.pdf|access-date=24 August 2019|archive-url=https://web.archive.org/web/20190824162153/https://hal.archives-ouvertes.fr/hal-00301903/file/acpd-5-11215-2005.pdf|archive-date=24 August 2019|url-status=live|doi-access=free}}</ref>
Bacteria such as ''[[Mycobacterium smegmatis]]'' can use the small amount of hydrogen in the atmosphere as a source of energy when other sources are lacking, using a hydrogenase with small channels that exclude oxygen and so permits the reaction to occur even though the hydrogen concentration is very low and the oxygen concentration is as in normal air.<ref name=Grinter/><ref>{{cite journal |last1=Alex Wilkins |title=Soil bacteria enzyme generates electricity from hydrogen in the air |journal=New Scientist |date=Mar 8, 2023 |volume=257 |issue=3430 |page=13 |doi=10.1016/S0262-4079(23)00459-1 |bibcode=2023NewSc.257...13W |s2cid=257625443 |url=https://www.newscientist.com/article/2363552-soil-bacteria-enzyme-generates-electricity-from-hydrogen-in-the-air/}}</ref>

Hydrogen is the most abundant element in the human body by numbers of [[atom]]s but the third most abundant by mass. {{chem2|H2}} occurs in human breath due to the metabolic activity of hydrogenase-containing microorganisms in the [[large intestine]] and is a natural component of [[Flatulence|flatus]]. The concentration in the breath of fasting people at rest is typically less than 5 [[parts per million]] (ppm) but can be 50&nbsp;ppm when people with intestinal disorders consume molecules they cannot absorb during diagnostic [[hydrogen breath test]]s.<ref>{{cite journal|doi=10.1088/1752-7155/2/4/046002|title=Implementation and interpretation of hydrogen breath tests|year=2008|last1=Eisenmann|first1=Alexander|last2=Amann|first2=Anton|last3=Said|first3=Michael|last4=Datta|first4=Bettina|last5=Ledochowski|first5=Maximilian|journal=Journal of Breath Research|volume=2|issue=4|page=046002|pmid=21386189|bibcode=2008JBR.....2d6002E|s2cid=31706721|url=http://pdfs.semanticscholar.org/2f16/5a981d54c41da92c1ae81af44021a88f1b95.pdf|access-date=26 December 2020|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015732/http://pdfs.semanticscholar.org/2f16/5a981d54c41da92c1ae81af44021a88f1b95.pdf}}</ref>

[[Water splitting]], in which water is decomposed into its component protons, electrons, and oxygen, occurs in the [[Light-dependent reactions|light reactions]] in all [[photosynthetic]] organisms. Some such organisms, including the alga ''[[Chlamydomonas reinhardtii]]'' and [[cyanobacteria]], have evolved a second step in the [[dark reaction]]s in which protons and electrons are reduced to form {{chem2|H2}} gas by specialized hydrogenases in the [[chloroplast]].<ref>{{cite journal|last1=Kruse|first1=O.|last2=Rupprecht|first2=J.|last3=Bader|first3=K.|last4=Thomas-Hall|first4=S.|last5=Schenk|first5=P. M.|last6=Finazzi|first6=G.|last7=Hankamer|first7=B.|title=Improved photobiological H<sub>2</sub> production in engineered green algal cells|journal=The Journal of Biological Chemistry|date=2005|volume=280|issue=40|pages=34170–7|doi=10.1074/jbc.M503840200|pmid=16100118|s2cid=5373909|url=http://espace.library.uq.edu.au/view/UQ:75490/UQ75490_OA.pdf|access-date=24 August 2019|archive-date=29 January 2021|archive-url=https://web.archive.org/web/20210129015735/https://espace.library.uq.edu.au/data/UQ_75490/UQ75490_OA.pdf?Expires=1611885542&Key-Pair-Id=APKAJKNBJ4MJBJNC6NLQ&Signature=Qmpjq4YH0rwOJNqiSZ5M7-E5cYH~Dm2B-4kasb1eH66pVWPlvPNRj7TfcTKR1lDhF0--bkJdtE~yrSWwcZAA8FzxAA3MXY99mHTIOxyD3s73Dai1bwrLNuOkibXTVo6WbY5RKv7JAhXJ2sUV~TDIphC4Qikr0AWk5z-dwdY997n0NzcdTlqr0sn5n9WsOari3pJ0wRuL0w6Ged~HhrQ6ClrheilhtRo43U6HuaATFKEAuUM682rv4gvRCEVR1ljVOW0jwruB0SAJszTOZAbqNtb3V0SJh0x7wI8~ZZrp-XYqqzLDsWOB9w3ttyGSpLjcE2LvI7ty5vUljlfBGbnnLg__|url-status=live|doi-access=free}}</ref> Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to efficiently synthesize {{chem2|H2}} gas even in the presence of oxygen.<ref>{{cite web
|first1= Hamilton O.
|last1= Smith
|last2= Xu
|first2= Qing
|date= 2005
|url= http://www.hydrogen.energy.gov/pdfs/progress05/iv_e_6_smith.pdf
|title= IV.E.6 Hydrogen from Water in a Novel Recombinant Oxygen-Tolerant Cyanobacteria System
|work= FY2005 Progress Report
|publisher= United States Department of Energy
|access-date= 6 August 2016
|archive-url= https://web.archive.org/web/20161229231756/https://www.hydrogen.energy.gov/pdfs/progress05/iv_e_6_smith.pdf
|archive-date= 29 December 2016
|url-status= live
}}</ref> Efforts have also been undertaken with genetically modified [[Biological hydrogen production (Algae)|alga in a bioreactor]].<ref>{{cite news| last=Williams| first=C.| title=Pond life: the future of energy| work=Science| publisher=The Register| date=24 February 2006| url=https://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| access-date=24 March 2008| archive-url=https://web.archive.org/web/20110509143055/http://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| archive-date=9 May 2011| url-status=live}}</ref>

== Safety and precautions ==
{{Main|Hydrogen safety}}
{{Chembox
| container_only = yes
|Section7={{Chembox Hazards
| ExternalSDS =
| GHSPictograms = {{GHS02}}
| GHSSignalWord = Danger
| HPhrases = {{H-phrases|220}}
| PPhrases = {{P-phrases|202|210|271|403|377|381}}<ref>{{Cite web | url=http://isolab.ess.washington.edu/isolab/images/documents/msds_sds/hydrogen.pdf | title=MyChem: Chemical | access-date=1 October 2018 | archive-url=https://web.archive.org/web/20181001070437/http://isolab.ess.washington.edu/isolab/images/documents/msds_sds/hydrogen.pdf | archive-date=1 October 2018 }}</ref>
| NFPA-H = 0
| NFPA-F = 4
| NFPA-R = 0
| NFPA-S =
| NFPA_ref =
 }}
}}
Hydrogen poses a number of hazards to human safety, from potential [[detonation]]s and fires when mixed with air to being an [[Asphyxiant gas|asphyxiant]] in its pure, [[oxygen]]-free form.<ref name="NASAH2">{{cite web
|author=Brown, W. J.
|display-authors=etal
|url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970033338.pdf
|date=1997
|title=Safety Standard for Hydrogen and Hydrogen Systems |id=NSS 1740.16
|website=[[NASA]]
|access-date=12 July 2017
|archive-url=https://web.archive.org/web/20170501105215/https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19970033338.pdf
|archive-date=1 May 2017
|url-status=live
}}</ref> Also, [[liquid hydrogen]] is a [[cryogen]] and presents dangers (such as [[frostbite]]) associated with very cold liquids.<ref>{{cite web| title=Liquid Hydrogen MSDS| publisher=Praxair, Inc.| date=September 2004| url=http://www.hydrogenandfuelcellsafety.info/resources/mdss/Praxair-LH2.pdf| access-date=16 April 2008| archive-url=https://web.archive.org/web/20080527233910/http://www.hydrogenandfuelcellsafety.info/resources/mdss/Praxair-LH2.pdf| archive-date=27 May 2008| df=dmy-all}}</ref> Hydrogen dissolves in many metals and in addition to leaking out, may have adverse effects on them, such as [[hydrogen embrittlement]],<ref>{{cite journal| title='Bugs' and hydrogen embrittlement| journal=Science News| volume=128| issue=3| pages=41| date=20 July 1985|doi=10.2307/3970088| jstor=3970088}}</ref> leading to cracks and explosions.<ref>{{cite web|url=http://www.twi.co.uk/content/oilgas_casedown29.html|title=Union Oil Amine Absorber Tower|last=Hayes|first=B.|publisher=TWI|access-date=29 January 2010|archive-url=https://web.archive.org/web/20081120215355/http://www.twi.co.uk/content/oilgas_casedown29.html|archive-date=20 November 2008}}</ref> Hydrogen gas leaking into external air may spontaneously ignite. Moreover, hydrogen fire, while being extremely hot, is almost invisible, and thus can lead to accidental burns.<ref name="Cunn88">{{cite encyclopedia|last1=Walker|first1=James L.|last2=Waltrip|first2=John S.|last3=Zanker|first3=Adam|editor1=John J. McKetta|editor2=William Aaron Cunningham|title=Lactic acid to magnesium supply-demand relationships|date=1988|publisher=Dekker|location=New York|isbn=978-0-8247-2478-8|page=186|url={{Google books|8erDL_DnsgAC|page=PA186|keywords=|text=|plainurl=yes}}|access-date=20 May 2015|encyclopedia=Encyclopedia of Chemical Processing and Design|volume=28}}</ref>

Even interpreting the hydrogen data (including safety data) is confounded by a number of phenomena. Many physical and chemical properties of hydrogen depend on the [[Spin isomers of hydrogen|parahydrogen/orthohydrogen]] ratio (it often takes days or weeks at a given temperature to reach the equilibrium ratio, for which the data is usually given). Hydrogen detonation parameters, such as critical detonation pressure and temperature, strongly depend on the container geometry.<ref name="NASAH2" />

== See also ==
{{div col}}
* [[Combined cycle hydrogen power plant]]
* {{annotated link|Hydrogen economy}}
* {{annotated link|Hydrogen production}}
* {{annotated link|Hydrogen safety}}
* {{annotated link|Hydrogen technologies}}
* {{annotated link|Hydrogen transport}}
* {{annotated link|Liquid hydrogen}}
* {{annotated link|Methane pyrolysis}} (for hydrogen)
* {{annotated link|Natural hydrogen}}
* {{annotated link|Pyrolysis}}
{{div col end}}

== Notes ==
<references group="note" />

== References ==
{{Reflist|30em}}

== Further reading ==
{{Library resources box
 |onlinebooks=yes
 |by=no
 |lcheading= Hydrogen
 |label=Hydrogen
 }}
* [https://www.feynmanlectures.caltech.edu/III_12.html The Hyperfine Splitting in Hydrogen - The Feynman Lectures on Physics]
* {{cite book| title=Chart of the Nuclides| edition=17th| publisher= Knolls Atomic Power Laboratory|date=2010| url=http://www.nuclidechart.com/|isbn=978-0-9843653-0-2}}
* {{cite book|last=Newton|first=David E.|date=1994|title=The Chemical Elements|publisher=Franklin Watts|location=New York|isbn=978-0-531-12501-4|url=https://archive.org/details/chemicalelements00newt}}
* {{cite book|last=Rigden|first=John S.|date=2002|title=Hydrogen: The Essential Element|publisher=Harvard University Press|location=Cambridge, Massachusetts|isbn=978-0-531-12501-4|url=https://archive.org/details/chemicalelements00newt}}
* {{cite book|author=Romm, Joseph J.|title=The Hype about Hydrogen, Fact and Fiction in the Race to Save the Climate|publisher=Island Press|date=2004|isbn=978-1-55963-703-9|title-link=The Hype about Hydrogen}}
* {{cite book|last=Scerri|first=Eric|date=2007|title=The Periodic System, Its Story and Its Significance|publisher=Oxford University Press|location=New York|isbn=978-0-19-530573-9|url-access=registration|url=https://archive.org/details/periodictableits0000scer}}

== External links ==
{{Spoken Wikipedia|date=28 October 2006|En-Hydrogen (part 1).ogg|En-Hydrogen (part 2).ogg}}
* [https://web.archive.org/web/20060612225336/http://www.physics.drexel.edu/~tim/open/hydrofin/ Basic Hydrogen Calculations of Quantum Mechanics]
* [http://www.periodicvideos.com/videos/001.htm Hydrogen] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
<!-- access forbidden *[http://www.astro.washington.edu/users/larson/Astro150b/Lectures/JupSatUraNep/hydrogen_phase.gif Hydrogen phase diagram] -->
* [http://militzer.berkeley.edu/diss/node5.html High temperature hydrogen phase diagram]
* [http://hyperphysics.phy-astr.gsu.edu/Hbase/quantum/hydwf.html#c3 Wavefunction of hydrogen]

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