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{{About|the chemical element}}
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{{Infobox hydrogen}}
<!-- Form of the first part of the intro matches agreed Elements form -->
'''Hydrogen''' is a [[chemical element]]; it has the [[chemical symbol|symbol]]{{nbsp}}'''H''' and [[atomic number]]{{nbsp}}1. It is the [[List of atomic weights|lightest]] and [[abundance of the chemical elements|most abundant]] chemical element in the universe, constituting about 75% of all [[baryon|normal matter]]. Under [[standard conditions]], hydrogen is a [[gas]] of [[diatomic molecule]]s with the [[chemical formula|formula]]{{nbsp}}{{chem2|H2}}, called '''dihydrogen'''<!--[[dihydrogen]] is a redirect to hydrogen; also called '''diprotium'''-->, or sometimes '''hydrogen gas''', '''molecular hydrogen''', or simply hydrogen. Dihydrogen is colorless, odorless, non-toxic, and highly [[combustible]]. [[Star]]s, including the [[Sun]], mainly consist of hydrogen in a [[plasma state]], while on Earth, hydrogen is found as the gas{{nbsp}}{{chem2|H2}} (dihydrogen) and in [[molecule]]s, such as in [[water]] and [[organic compound]]s. The most common [[isotope of hydrogen]], {{sup|1}}H, consists of one [[proton]], one [[electron]], and no [[neutron]]s.

Hydrogen gas was first produced artificially in the 17th{{nbsp}}century by the [[chemical reaction|reaction]] of [[acid]]s with metals. [[Henry Cavendish]], in{{nbsp}}1766–1781, identified hydrogen gas as a distinct substance and discovered its property of producing water when burned: this is the origin of hydrogen's name, which means {{gloss|water-former}} (from {{langx|grc|[[wikt:ὕδωρ#Ancient Greek|ὕδωρ]]|húdōr|water}}, and {{langx|grc|[[wikt:γεννάω#Ancient Greek|γεννάω]]|gennáō|I bring forth|label=none}}). Understanding the [[Hydrogen spectral series|colors of light absorbed and emitted by hydrogen]] was a crucial part of the development of [[quantum mechanics]].

Hydrogen, typically [[nonmetallic]] except under [[extreme pressure]], readily forms [[covalent bond]]s with most nonmetals, contributing to the formation of compounds like water and various organic substances. Its role is crucial in [[acid–base reaction]]s, which mainly involve proton exchange among [[solubility|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]], {{chem2|H+}}, called a proton. Although tightly bonded to water molecules, protons strongly affect the behavior of [[aqueous solution]]s, as reflected in the importance of [[pH]]. Hydride, on the other hand, is rarely observed because it tends to deprotonate solvents, yielding{{nbsp}}{{chem2|H2}}.

In the [[early universe]], neutral hydrogen atoms [[Big Bang nucleosynthesis|formed about 370,000 years after the Big Bang]] as the universe expanded and plasma had cooled enough for electrons to remain bound to protons. After stars [[star formation|began to form]], most of the hydrogen in the [[intergalactic medium]] was re-ionized.

Nearly all [[hydrogen production]] is done by transforming [[fossil fuel]]s, particularly [[steam reforming]] of [[natural gas]]. It can also be produced from water or saline by [[electrolysis]], but this process is more expensive. Its main industrial uses include fossil fuel processing and [[ammonia production]] for fertilizer. Emerging uses for hydrogen include the use of [[fuel cell]]s to generate electricity.
 
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== Properties ==
===Atomic hydrogen===
==== Electron energy levels ====
{{Main|Hydrogen atom}}
The [[ground state]] [[energy level]] of the electron in a hydrogen atom is −13.6{{nbsp}}[[electronvolts]]{{nbsp}}(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}}[[nanometer]]s 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 are referred to by consecutive [[quantum number]]s, with <math>n=1</math> being the ground state. The [[hydrogen spectral series]] corresponds to emission of light due to transitions from higher to lower energy levels.<ref>{{Cite book |last=Levine |first=Ira N. |title=Quantum chemistry |date=1970 |publisher=Pearson |isbn=978-0-321-89060-3 |edition=2 |series=Pearson advanced chemistry series |location=Boston}}</ref>{{rp|105}} Each energy level is further split by [[spin (physics)|spin]] interactions between the electron and proton into four [[Hyperfine structure|hyperfine]] levels.<ref>{{Cite book |last1=Feynman |first1=Richard P.  |title=The Feynman lectures on physics |last2=Leighton |first2=Robert B. |last3=Sands |first3=Matthew L. |date=2011 |publisher=Basic Books |isbn=978-0-465-02414-8 |edition=New millennium |location=New York |chapter=The Hyperfine Splitting in Hydrogen |oclc=671704374 |chapter-url=https://www.feynmanlectures.caltech.edu/III_12.html}}</ref>

High-precision values for the hydrogen atom energy levels are required for definitions of physical constants. Quantum calculations have identified nine contributions to the energy levels. The [[eigenvalue]] from the [[Dirac equation]] is the largest contribution. Other terms include [[Relativistic quantum mechanics|relativistic]] recoil, the [[self-energy]], and the [[vacuum polarization]] terms.<ref>{{Cite journal |last1=Tiesinga |first1=Eite |last2=Mohr |first2=Peter J. |last3=Newell |first3=David B. |last4=Taylor |first4=Barry N. |date=2021-09-23 |title=CODATA Recommended Values of the Fundamental Physical Constants: 2018* |journal=Journal of Physical and Chemical Reference Data |volume=50 |issue=3 |page=033105 |doi=10.1063/5.0064853 |issn=0047-2689 |pmc=9888147 |pmid=36726646|bibcode=2021JPCRD..50c3105T }}</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)]]
Hydrogen has three naturally-occurring isotopes, denoted {{chem|1|H}}, {{chem|2|H}} and{{nbsp}}{{chem|3|H}}. Other, highly-unstable [[nuclide]]s{{nbsp}}({{chem|4|H}} to {{chem|7|H}}) have been [[nucleosynthesis|synthesized]] in laboratories but not observed in nature.<ref name="Gurov">{{cite journal
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'''{{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 ({{xref|see [[diproton]] for a discussion of why others do not exist}}).<ref>{{NUBASE2020}}</ref>

'''{{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 nuclei in the universe are thought to have been produced in [[Big Bang nucleosynthesis]], and have endured since then.<ref>{{Cite journal |last1=Particle Data Group |last2=Workman |first2=R L |last3=Burkert |first3=V D |last4=Crede |first4=V |last5=Klempt |first5=E |last6=Thoma |first6=U |last7=Tiator |first7=L |last8=Agashe |first8=K |last9=Aielli |first9=G |last10=Allanach |first10=B C |last11=Amsler |first11=C |last12=Antonelli |first12=M |last13=Aschenauer |first13=E C |last14=Asner |first14=D M |last15=Baer |first15=H |date=2022-08-08 |title=Review of Particle Physics |url=https://academic.oup.com/ptep/article/doi/10.1093/ptep/ptac097/6651666 |journal=Progress of Theoretical and Experimental Physics |language=en |volume=2022 |issue=8 |article-number=083C01 |doi=10.1093/ptep/ptac097 |issn=2050-3911|hdl=1854/LU-01HQG4F6CV7P2F3WWNH4RRN8HD |hdl-access=free }}</ref>{{rp|loc=24.2}} 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
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}}</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
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|date=11 November 1991
|title=Breakthrough in Nuclear Fusion Offers Hope for Power of Future
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|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{{nbsp}}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 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}}</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}}</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 hydrogen's 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{{nbsp}}T (instead of {{chem|2|H}} and {{chem|3|H}}) are sometimes used for deuterium and tritium, but the symbol{{nbsp}}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]]{{nbsp}}(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>

[[Antihydrogen]] ({{physics particle|anti=yes|H}}) is the [[antimatter]] counterpart to hydrogen. It consists of an [[antiproton|{{shy|anti|proton}}]] with a [[positron]].<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> The [[exotic atom]] [[muonium]] (symbol Mu), composed of an [[antimuon]] and an [[electron]], is the {{shy|anti|matter}} analogue of hydrogen; {{abbr|IUPAC|international Union of Pure and Applied Chemistry}}{{nbsp}}nomenclature incorporates such hypothetical compounds as muonium chloride{{nbsp}}(MuCl) and sodium muonide{{nbsp}}(NaMu), analogous to [[hydrogen chloride]] and [[sodium hydride]] respectively.<ref name="iupac">{{cite journal
 |doi=10.1351/pac200173020377
 |author=W. H. Koppenol
 |author2=IUPAC
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 |archive-date=14 May 2011
|url-status=live
 }}</ref>

===Dihydrogen===

Under [[standard conditions]], hydrogen is a [[gas]] of [[diatomic molecule]]s with the [[chemical formula|formula]]{{nbsp}}{{chem2|H2}}, officially called "dihydrogen",<ref>[http://old.iupac.org/publications/books/rbook/Red_Book_2005.pdf Nomenclature of Inorganic Chemistry IUPAC Recommendations 2005] - Full text (PDF)<br />2004 version with separate chapters as pdf: [http://www.iupac.org/reports/provisional/abstract04/connelly_310804.html IUPAC Provisional Recommendations for the Nomenclature of Inorganic Chemistry (2004)] {{webarchive|url=https://web.archive.org/web/20080219122415/http://www.iupac.org/reports/provisional/abstract04/connelly_310804.html |date=2008-02-19 }}</ref>{{rp|308}} but also called "molecular hydrogen",<ref name="britannica">{{Cite encyclopedia|title=Hydrogen|url=https://www.britannica.com/science/hydrogen|url-status=live|access-date=25 December 2021|encyclopedia=[[Encyclopædia Britannica]]|archive-date=24 December 2021|archive-url=https://web.archive.org/web/20211224165150/https://www.britannica.com/science/hydrogen}}</ref> or simply hydrogen. Dihydrogen is a colorless, odorless, flammable gas.<ref name="britannica"/>

==== 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, hydrogen in the container rises and burns as it mixes with the air.]]

Hydrogen gas is highly flammable, reacting with [[oxygen]] in air to produce liquid water:
{{bi|{{chem2|2 H2(g) + O2(g) → 2 H2O(l)}}}}
The [[Enthalpy of combustion|amount of heat released]] per [[mole (unit)|mole]] of hydrogen is {{val|−286|u=[[kilojoules]]}}{{nbsp}}(kJ), or {{val|141.865|u=[[megajoules]]}}{{nbsp}}(MJ) for a {{convert|1|kilogram|adj=on|spell=in|abbr=out}} mass.<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 {{val|4|–|74|u=%}}<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 {{val|5|–|95|u=%}}. 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> In a high-pressure [[hydrogen leak]], the shock wave from the leak itself can heat air to the autoignition temperature, leading to flaming and possibly explosion.<ref>{{Cite journal |last1=Yamada |first1=Eisuke |last2=Kitabayashi |first2=Naoki |last3=Hayashi |first3=A. Koichi |last4=Tsuboi |first4=Nobuyuki |date=2011-02-01 |title=Mechanism of high-pressure hydrogen auto-ignition when spouting into air |url=https://linkinghub.elsevier.com/retrieve/pii/S0360319910009468 |journal=International Journal of Hydrogen Energy |series=The Third Annual International Conference on Hydrogen Safety |volume=36 |issue=3 |pages=2560–2566 |doi=10.1016/j.ijhydene.2010.05.011 |bibcode=2011IJHE...36.2560Y |issn=0360-3199|url-access=subscription }}</ref>

Hydrogen flames emit faint blue and [[ultraviolet]] light.<ref>{{cite journal |last1=Schefer |first1=E. W. |last2=Kulatilaka |first2=W. D. |last3=Patterson |first3=B. D. |last4=Settersten |first4=T. B. |date=June 2009 |title=Visible emission of hydrogen flames |url=https://zenodo.org/record/1258847 |url-status=live |journal=Combustion and Flame |volume=156 |issue=6 |pages=1234–1241 |bibcode=2009CoFl..156.1234S |doi=10.1016/j.combustflame.2009.01.011 |archive-url=https://web.archive.org/web/20210129015717/https://zenodo.org/record/1258847 |archive-date=29 January 2021 |access-date=30 June 2019}}</ref> [[Flame detector]]s are used to detect hydrogen fires as they are nearly invisible to the naked eye in daylight.<ref>{{Cite web |title=Making Visible the Invisible {{!}} NASA Spinoff |url=https://spinoff.nasa.gov/spinoff1999/er5.htm |access-date=2025-02-09 |website=spinoff.nasa.gov}}</ref><ref name="spinoff-2016" />

==== Spin isomers ====
{{Main|Spin isomers of hydrogen}}
Molecular {{chem2|H2}} exists as two [[nuclear isomer]]s that differ 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 '''{{shy|ortho|hydrogen}}''' form, the spins of the two nuclei are parallel, forming a spin [[triplet state]] having a [[total molecular spin]] <math>S = 1</math>; in the '''{{shy|para|hydrogen}}''' form the spins are {{shy|anti|parallel}} 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 |url-access=subscription }}</ref> The ortho form is an [[excited state]], having higher energy than the para form by {{val|1.455|ul=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 these isomers differ because each has distinct [[Rotational–vibrational spectroscopy|rotational quantum states]].<!-- This link is less direct than [[Rotational spectroscopy]] but presently the subject better (June 2021).--><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 sufficient heat to evaporate most of the liquid if the conversion to {{shy|para|hydrogen}} does not occur 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 {{shy|inter|conversion}}, such as [[ferric oxide]] and [[activated carbon]] compounds, are therefore 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|bibcode=1957JAChS..79.5385S }}</ref>

==== Phases ====
[[File:Phase diagram of hydrogen.png|thumb|[[Phase diagram]] of hydrogen with a [[logarithmic scale]]. The left edge corresponds to about one [[atmosphere (unit)|atmosphere]].<ref>{{Cite journal |last=Stevenson |first=D J |date=May 1982 |title=Interiors of the Giant Planets |url=https://www.annualreviews.org/doi/10.1146/annurev.ea.10.050182.001353 |journal=Annual Review of Earth and Planetary Sciences |language=en |volume=10 |issue=1 |pages=257–295 |doi=10.1146/annurev.ea.10.050182.001353 |bibcode=1982AREPS..10..257S |issn=0084-6597|url-access=subscription }}</ref>|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.]]
[[Liquid hydrogen]] can exist at temperatures below hydrogen's [[critical point (thermodynamics)|critical point]] of {{convert|33|K|C F|abbr=out|lk=in}}.<ref>{{cite web|url=https://webbook.nist.gov/cgi/cbook.cgi?ID=C1333740&Mask=4 |title=Hydrogen |website=NIST Chemistry WebBook, SRD 69 |publisher=[[National Institute of Standards and Technology]] |access-date=2025-01-14 |year=2023}}</ref> However, for it to be in a fully liquid state at [[atmospheric pressure]], H<sub>2</sub> needs to be cooled to {{cvt|20.28|K|C F}}. Hydrogen was liquefied by [[James Dewar]] in{{nbsp}}1898 by using [[regenerative cooling]] and his invention, the [[vacuum flask]].<ref>{{cite journal |author1=James Dewar |author1-link=James Dewar |title=Liquid Hydrogen |journal=Science |date=1900 |volume=11 |issue=278 |pages=641–651 |doi=10.1126/science.11.278.641 |pmid=17813562 |bibcode=1900Sci....11..641D |language=en}}</ref>

Liquid hydrogen becomes [[solid hydrogen]] at [[standard pressure]] below hydrogen's [[melting point]] of {{cvt|14.01|K|C F}}. Distinct solid phases exist, known as Phase{{nbsp}}I through Phase{{nbsp}}V, each exhibiting a characteristic molecular arrangement.<ref name="Helled2020">{{cite journal|first1=Ravit |last1=Helled |first2=Guglielmo |last2=Mazzola |first3=Ronald |last3=Redmer |title=Understanding dense hydrogen at planetary conditions |date=2020-09-01 |journal=Nature Reviews Physics |volume=2 |issue=10 |pages=562–574 |doi=10.1038/s42254-020-0223-3 |arxiv=2006.12219|bibcode=2020NatRP...2..562H }}</ref> Liquid and solid phases can exist in combination at the [[triple point]]; this mixture is known as [[slush hydrogen]].<ref>{{cite book |last=Ohira |first=K. |chapter=Slush hydrogen production, storage, and transportation |date=2016 |title=Compendium of Hydrogen Energy |pages=53–90 |publisher=Elsevier |doi=10.1016/b978-1-78242-362-1.00003-1 |isbn=978-1-78242-362-1}}</ref>

[[Metallic hydrogen]], a phase obtained at extremely high pressures (in excess of {{convert|400|e6Pa|abbr=unit|lk=on}}), is an electrical conductor. It is believed to exist deep within [[giant planet]]s like [[Jupiter]].<ref name="Helled2020"/><ref>{{cite book|last1=Frankoi |first1=A. |display-authors=etal |title=Astronomy 2e |year=2022 |publisher=OpenStax |chapter-url=https://openstax.org/books/astronomy-2e/pages/11-2-the-giant-planets |chapter=11.2 The Giant Planets |page=370 |isbn=978-1-951693-50-3}}</ref>

When [[ionized]], hydrogen becomes a [[plasma (physics)|plasma]]. This is the form in which hydrogen exists within [[star]]s.<ref>{{Cite book|last=Phillips |first=K. J. H. |date=1995 |title=Guide to the Sun |page=|publisher=[[Cambridge University Press]] |url=https://books.google.com/books?id=idwBChjVP0gC&q=Guide+to+the+Sun+phillips |isbn=978-0-521-39788-9 |url-status=live |archive-url= https://web.archive.org/web/20180115215631/https://books.google.com/books?id=idwBChjVP0gC&printsec=frontcover&dq=Guide+to+the+Sun+phillips&hl=en&sa=X&ved=0ahUKEwiBj4Gbj5bXAhXrrVQKHfnAAKUQ6AEIKDAA |archive-date=15 January 2018 }}</ref>

{{clear}}

==== Thermal and physical properties ====
{|class="wikitable mw-collapsible"
|- 
|+Thermal and physical properties of hydrogen (H{{sub|2}}) at atmospheric pressure<ref>{{Cite book |last=Holman |first=Jack P. |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. |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>
|-
!Temperature (K)
!Density (kg/m<sup>3</sup>)
![[Specific heat]] (kJ/kg K)
![[Dynamic viscosity]] (kg/m s)
![[Kinematic viscosity]] (m<sup>2</sup>/s)
![[Thermal conductivity]] (W/m K)
![[Thermal diffusivity]] (m<sup>2</sup>/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 ==
{{Main|Timeline of hydrogen technologies}}

=== 18th century ===
[[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>
Boyle did not note that the gas was flammable, but hydrogen would play a key role in overturning the [[phlogiston theory]] of combustion.<ref name=Ramsay-1896>{{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 |page=19}}</ref>

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{{nbsp}}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>

[[File:Antoine-Laurent Lavoisier by Louis Jean Desire Delaistre (cropped).jpg|thumb|{{langr|fr|[[Antoine Lavoisier]]}}, who identified the element that came to be known as hydrogen]]
In 1783, {{langr|fr|[[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|{{langr|fr|Laplace|cat=no}}]] reproduced Cavendish's finding that water is produced when hydrogen is burned.<ref name="nbb" /> {{langr|fr|Lavoisier}} produced hydrogen for his experiments on [[mass conservation]] by treating metallic [[iron]] with a stream of water 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:
*{{chem2|Fe + H2O -> FeO + H2}}
*{{chem2|2 Fe + 3 H2O -> Fe2O3 + 3 H2}}
*{{chem2|3 Fe + 4 H2O -> Fe3O4 + 4 H2}}

Many metals react similarly with water, leading to the production of hydrogen.<ref>{{Cite journal |last1=Northwood |first1=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|url-access=subscription }}</ref> In some situations, this H<sub>2</sub>-producing process is problematic, for instance in the case of [[zirconium cladding]] on [[nuclear fuel rod]]s.<ref>{{cite journal |doi=10.1016/j.jnucmat.2019.02.042 |title=Hydrogen in zirconium alloys: A review |date=2019 |last1=Motta |first1=Arthur T. |last2=Capolungo |first2=Laurent |last3=Chen |first3=Long-Qing |last4=Cinbiz |first4=Mahmut Nedim |last5=Daymond |first5=Mark R. |last6=Koss |first6=Donald A. |last7=Lacroix |first7=Evrard |last8=Pastore |first8=Giovanni |last9=Simon |first9=Pierre-Clément A. |last10=Tonks |first10=Michael R. |last11=Wirth |first11=Brian D. |author11-link=Brian Wirth|last12=Zikry |first12=Mohammed A. |journal=Journal of Nuclear Materials |volume=518 |pages=440–460 |bibcode=2019JNuM..518..440M }}</ref>

===19th century===
By 1806 hydrogen was used to fill [[balloon]]s.<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>
{{langr|fr|[[François Isaac de Rivaz]]}} built the first [[de Rivaz engine|{{langr|fr|de{{nbsp}}Rivaz|cat=no}} engine]], an internal combustion engine powered by a mixture of hydrogen and oxygen, in{{nbsp}}1806. [[Edward Daniel Clarke]] invented the hydrogen gas blowpipe in{{nbsp}}1819. The [[Döbereiner's lamp|{{langr|de|Döbereiner|cat=no}}'s lamp]] and [[limelight]] were invented in{{nbsp}}1823. Hydrogen was [[Liquid hydrogen|liquefied]] for the first time by [[James Dewar]] in{{nbsp}}1898 by using [[regenerative cooling]] and his invention, the [[vacuum flask]]. He produced [[solid hydrogen]] the next year.<ref name="nbb" />

One of the first [[quantum effects]] to be explicitly noticed, although not understood at the time, was [[James Clerk Maxwell]]'s observation 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 ([[Quantization (physics)|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>

===20th century===
The existence of the [[hydride anion]] was suggested by [[Gilbert N. Lewis|Gilbert{{nbsp}}N. Lewis]] in{{nbsp}}1916 for [[Group 1 elements|group{{nbsp}}1]] and [[Group 2 elements|group{{nbsp}}2]] salt-like compounds. In{{nbsp}}1920, {{langr|de|Moers}} electrolyzed molten [[lithium hydride]]{{nbsp}}(LiH), producing a [[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>

[[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 [[Hydrogen spectral series|spectrum of light]] produced from it or absorbed by it, has been central to the [[History of atomic theory|development of the theory of atomic 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> The energy levels of hydrogen can be calculated fairly accurately using the [[Bohr model|{{langr|da|Bohr|cat=no}} 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 [[wikt:discretization|discretization]] of [[angular momentum]] postulated in early [[quantum mechanics]] by {{langr|da|Bohr}}, the electron in the {{langr|da|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>

Hydrogen's unique position as the only neutral atom for which the [[Schrödinger equation|{{langr|de|Schrödinger|cat=no}} equation]] can be directly solved, has significantly contributed to the understanding of quantum mechanics through the exploration of its energetics.<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> 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.<ref>{{Cite journal |last=Wilson |first=E. Bright |date=1977 |title=Impact of the Heitler-London hydrogen molecule paper on chemistry |url=https://onlinelibrary.wiley.com/doi/10.1002/qua.560120807 |journal=International Journal of Quantum Chemistry |language=en |volume=12 |issue=S11 |pages=17–28 |doi=10.1002/qua.560120807 |issn=1097-461X|url-access=subscription }}</ref>

==== Hydrogen-lifted airship ====
[[File:Hindenburg over New York 1937 (cropped).jpg|alt=Airship Hindenburg over New York|thumb|The [[Hindenburg-class airship|{{lang|de|Hindenburg|cat=no}}]] over [[New York City]] in{{nbsp}}1937]]
Because {{chem2|H2}} has 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 |url={{Google books|OI0fTJhydh4C|page=|keywords=|text=|plainurl=yes}} |title=History of industrial gases |date=2003 |publisher=Kluwer Academic/Plenum Publishers |isbn=978-0-306-47277-0 |location=New York, N.Y. |pages=47–56 |access-date=20 May 2015}}</ref> The first hydrogen-filled balloon was invented by {{langr|fr|[[Jacques Charles]]}} in{{nbsp}}1783. Hydrogen provided the lift for the first reliable form of air-travel following the{{nbsp}}1852 invention of the first hydrogen-lifted airship by {{langr|fr|[[Henri Giffard]]}}. German count [[Ferdinand von Zeppelin|{{langr|de|Ferdinand von{{nbsp}}Zeppelin|cat=no}}]] promoted the idea of rigid airships lifted by hydrogen that later were called {{langr|de|[[Zeppelin]]s}}, the first of which had its maiden flight in{{nbsp}}1900.<ref name="nbb" /> Regularly-scheduled flights started in{{nbsp}}1910 and by the outbreak of World War{{nbsp}}I in August{{nbsp}}1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships in the form of [[blimp]]s were used as observation platforms and bombers during World War{{nbsp}}II, especially on the [[US Eastern seaboard|US{{nbsp}}Eastern seaboard]].<ref>{{Cite web |last=Kratz |first=Jessie |date=2017-10-27 |title=Beyond the Hindenburg: Airships Throughout History |url=https://prologue.blogs.archives.gov/2017/10/27/beyond-the-hindenburg-airships-throughout-history/ |access-date=2025-04-09 |website=Pieces of History |language=en-US}}</ref>

The first non-stop transatlantic crossing was made by the British airship{{nbsp}}''[[R34 (airship)|R34]]'' in{{nbsp}}1919 and  regular passenger service resumed in the{{nbsp}}1920s. Hydrogen was used in the [[LZ 129 Hindenburg|{{lang|de|Hindenburg|cat=no}}]], which caught fire over [[New Jersey]] on 6{{nbsp}}May 1937.<ref name="nbb" /> The hydrogen that filled the airship was ignited, possibly by static electricity, and burst into flames.<ref>{{Cite web |last=Follows |first=Mike |date=July 2, 2015 |title=What ignited the Hindenburg? |url=https://edu.rsc.org/feature/what-ignited-the-hindenburg/2000137.article |access-date=2025-02-19 |website=RSC Education |language=en}}</ref> Following this [[Hindenburg disaster|disaster]], 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]].<ref>{{Cite web |last=Rappe |first=Mollie |date=May 9, 2023 |title=Researchers switch from helium to hydrogen weather balloons |url=https://phys.org/news/2023-05-helium-hydrogen-weather-balloons.html |access-date=2025-02-19 |website=phys.org |language=en}}</ref>

==== Deuterium and tritium ====
[[Deuterium]] was discovered in December{{nbsp}}1931 by [[Harold Urey]], and [[tritium]] was prepared in{{nbsp}}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{{nbsp}}1932.<ref name="nbb" />

==Chemistry==
===Reactions of H<sub>2</sub>===
[[File:HFe H2 dppe 2.svg|thumb|right|A [[dihydrogen complex]] of iron, [HFe(H<sub>2</sub>)(dppe)<sub>2</sub>]<sup>+</sup>]]

{{chem2|H2}} is relatively unreactive. The thermodynamic basis of this low reactivity is the very strong {{nowr|H–H bond}}, with a [[bond dissociation energy]] of {{val|435.7|ul=kJ/mol}}.<ref>{{RubberBible87th}}</ref>  It does form coordination complexes called [[dihydrogen complex]]es. These species provide insights into the early steps in the interactions of hydrogen with metal catalysts.  According to [[neutron diffraction]], the metal and two H{{nbsp}}atoms form a triangle in these complexes. The {{nowr|H-H bond}} remains intact but is elongated. They are acidic.<ref>{{Cite book
| edition = 1
| publisher = Springer
| isbn = 0-306-46465-9
| last = Kubas
| first = Gregory J.
| title = Metal Dihydrogen and σ-Bond Complexes: Structure, Theory, and Reactivity
| date = 2001-08-31
}}</ref>

Although exotic on Earth, the {{chem2|H3+}}{{nbsp}}ion is common in the universe. It is a triangular species, like the aforementioned dihydrogen complexes. It is 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>

Hydrogen reacts with [[chlorine]] to produce{{nbsp}}[[HCl]], and with [[bromine]] to produce{{nbsp}}[[Hydrogen bromide|HBr]], via a [[chain reaction]]. The reaction requires initiation. For example, in the case of Br<sub>2</sub>, the dibromine molecule is split apart: {{chem2|Br2 + (UV light)-> 2Br•}}. Propagating reactions consume hydrogen molecules and produce{{nbsp}}HBr, as well as Br{{nbsp}}and H{{nbsp}}atoms: 
{{bi|{{chem2|Br• + H2 -> HBr + H}}}}
{{bi|{{chem2|H + Br2 -> HBr +Br}}}}
Finally the terminating reaction:
{{bi|{{chem2|H + HBr -> H2 + Br•}}}}
{{bi|{{chem2|2 Br• -> Br2}}}}
consumes the remaining atoms.<ref>{{Cite book |last=Laidler |first=Keith J. |url=https://archive.org/details/chemicalkinetics0000laid_s2p0/page/290/mode/2up |title=Chemical kinetics |date=1998 |publisher=HarperCollins |isbn=978-0-06-043862-3 |edition=3. ed., [Nachdr.] |location=New York, NY}}</ref>{{rp|289}}

The addition of H<sub>2</sub> to [[Saturated and unsaturated compounds|unsaturated]] organic compounds, such as [[alkene]]s and [[alkyne]]s, is called [[hydrogenation]]. Even if the reaction is [[exothermic|energetically favorable]], it does not occur spontaneously even at higher temperatures. In the presence of a [[catalyst]] like finely divided [[platinum]] or [[nickel]], the reaction proceeds at room temperature.<ref>{{Cite book |last1=Vollhardt |first1=Kurt Peter C. |title=Organic chemistry: structure and function |last2=Schore |first2=Neil Eric |date=2003 |publisher=W.H. Freeman and Co |isbn=978-0-7167-4374-3 |edition=4. |location=New York}}</ref>{{rp|477}}

===Hydrogen-containing compounds===
{{Main|Hydrogen compounds}}
Hydrogen can exist in both +1 and −1{{nbsp}}[[oxidation state]]s, forming compounds through [[ionic bonding|ionic]] and [[covalent bonding]]. The element is part of a wide range of substances, including water, [[hydrocarbon]]s, and numerous other [[organic compound]]s.<ref name="hydrocarbon">{{cite web| title=Structure and Nomenclature of Hydrocarbons| publisher=Purdue University| url=https://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 H<sup>+</sup>{{nbsp}}ion—commonly referred to as a proton due to its single proton and absence of electrons—is central to [[acid–base reaction|acid–base chemistry]], although the proton does not move freely. In the [[Brønsted–Lowry acids|{{langr|no|Brønsted|cat=no}}–Lowry]] framework, acids are defined by their ability to donate H<sup>+</sup>{{nbsp}}ions to bases.<ref>{{Cite book |last=Laurence |first=Christian |title=Lewis basicity and affinity scales: data and measurement |date=2010 |publisher=Wiley |isbn=978-0-470-68189-3 |location=Chichester}}</ref>

Hydrogen forms a vast variety of compounds with [[carbon]], known as hydrocarbons, and an even greater diversity with other elements ([[heteroatom]]s), giving rise to the broad class of organic compounds often associated with living organisms.<ref name="hydrocarbon"/>

[[File:NaH.jpg|thumb|A sample of [[sodium hydride]] ]]
Hydrogen compounds with hydrogen in the oxidation state{{nbsp}}−1 are known as [[hydride]]s, which are usually formed between hydrogen and metals. The hydrides can be ionic (aka saline), covalent, or metallic. With heating, H<sub>2</sub> reacts efficiently with the [[alkali earth metals|alkali]] and [[alkaline earth metal]]s to give the [[ionic hydrides]] of the formulas{{nbsp}}MH and MH<sub>2</sub>, respectively. These salt-like crystalline compounds have high melting points and all react with water to liberate hydrogen. Covalent hydrides include [[borane]]s and polymeric [[aluminium hydride]]. [[Transition metal]]s form [[metal hydride]]s via continuous dissolution of hydrogen into the metal.<ref name=UllmannH2/> A well-known hydride is [[lithium aluminium hydride]]: the {{chem2|[AlH4]-}}{{nbsp}}anion carries hydridic centers firmly attached to the Al(III).<ref>{{Greenwood&Earnshaw2nd|page=228}}</ref> Perhaps the most extensive series of hydrides are the [[borane]]s, compounds consisting only of boron and hydrogen.<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>

Hydrides can bond to these [[electropositive]] elements not only as a terminal [[ligand]] but also as [[bridging ligand]]s. In diborane{{nbsp}}({{chem2|B2H6}}), four hydrogen atoms are terminal, while two bridge between the two boron atoms.<ref name="Miessler" />

=== Hydrogen bonding ===
{{Main|Hydrogen bond}}
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]] like oxygen or nitrogen. This phenomenon, called [[hydrogen bonding]], is critical to the stability of many biological molecules.<ref>{{Cite journal |last1=Pimentel |first1=G C |last2=McClellan |first2=A L |date=October 1971 |title=Hydrogen Bonding |url=https://www.annualreviews.org/doi/10.1146/annurev.pc.22.100171.002023 |journal=Annual Review of Physical Chemistry |language=en |volume=22 |issue=1 |pages=347–385 |doi=10.1146/annurev.pc.22.100171.002023 |bibcode=1971ARPC...22..347P |issn=0066-426X|url-access=subscription }}</ref>{{rp|375}}<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 bonding alters molecule structures, [[viscosity]], [[solubility]], melting and boiling points, and even [[protein folding]] dynamics.<ref>{{Cite journal |last1=Xie |first1=Zhenkai |last2=Luo |first2=Rui |last3=Ying |first3=Tianping |last4=Gao |first4=Yurui |last5=Song |first5=Boqin |last6=Yu |first6=Tongxu |last7=Chen |first7=Xu |last8=Hao |first8=Munan |last9=Chai |first9=Congcong |last10=Yan |first10=Jiashu |last11=Huang |first11=Zhiheng |last12=Chen |first12=Zhiguo |last13=Du |first13=Luojun |last14=Zhu |first14=Chongqin |last15=Guo |first15=Jiangang |date=November 2024 |title=Dynamic-to-static switch of hydrogen bonds induces a metal–insulator transition in an organic–inorganic superlattice |url=https://www.nature.com/articles/s41557-024-01566-1 |journal=Nature Chemistry |language=en |volume=16 |issue=11 |pages=1803–1810 |doi=10.1038/s41557-024-01566-1 |pmid=39143300 |bibcode=2024NatCh..16.1803X |issn=1755-4330|url-access=subscription }}</ref>

===Protons and acids ===
{{Further|Acid–base reaction}}
[[File:Base pair AT.svg|thumb|right|An "A-T [[base pair]]" in DNA illustrating how hydrogen bonds are critical to the [[genetic code]]. The drawing illustrates that in many chemical depictions, {{nowr|C-H bonds}} are not always shown explicitly, an indication of their pervasiveness.]]
In water, hydrogen bonding plays an important role in reaction thermodynamics. A hydrogen bond can shift over to proton transfer.
Under the [[Brønsted–Lowry acid–base theory|{{langr|da|Brønsted|cat=no}}–Lowry acid–base theory]], acids are proton donors, while bases are proton acceptors.<ref>{{Cite book |last=Punekar |first=Narayan S. |url=https://link.springer.com/10.1007/978-981-97-8179-9_28 |title=ENZYMES: Catalysis, Kinetics and Mechanisms |date=2025 |publisher=Springer Nature Singapore |isbn=978-981-97-8178-2 |location=Singapore |pages=333–345 |language=en |doi=10.1007/978-981-97-8179-9_28}}</ref>{{rp|loc=28}}
A bare proton{{nbsp}}({{chem2|H+}}) essentially cannot exist in anything other than a vacuum. Otherwise it attaches to other atoms, ions, or molecules. Even chemical species as inert as [[methane]] can be protonated. The term "proton" is used loosely and metaphorically to refer to solvated hydrogen [[cation]]s attached to other solvated chemical species; it is denoted{{nbsp}}"{{chem2|H+}}" without any implication that any single protons exist freely in solution as a species. To avoid the implication of the naked proton in solution, acidic aqueous solutions are sometimes considered to contain the "[[hydronium]] ion"{{nbsp}}({{chem2|[H3O]+}}), or still more accurately, {{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
|bibcode=1977JAChS..99.6983P }}</ref>

The concentration of these solvated protons determines the [[pH]] of a solution, a [[logarithmic scale]] that reflects its acidity or basicity. Lower pH{{nbsp}}values indicate higher concentrations of hydronium ions, corresponding to more acidic conditions.<ref name="housecroft" />

==Occurrence==
===Cosmic===
[[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]].<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> and >90% by number of atoms.<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> In the [[early universe]], protons [[Big Bang baryogenesis|formed in the first second after the Big Bang]]; neutral hydrogen atoms formed about 370,000{{nbsp}}years later during the [[Recombination (cosmology)|recombination epoch]] as the universe expanded 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 |bibcode=2018PhRvD..98c0001T |hdl=11384/78286 }} (Revised September 2017) by [[Keith Olive|Keith A. Olive]] and [[John A. Peacock]].</ref>

In astrophysics, neutral hydrogen in the [[interstellar medium]] is called ''H{{nbsp}}I'' and ionized hydrogen is called ''H{{nbsp}}II''.<ref>{{Cite book |last1=Kaplan |first1=S. A. |url=https://www.degruyter.com/document/doi/10.4159/harvard.9780674493988/html |title=The Interstellar Medium |last2=Pikelner |first2=S. B. |date=1970-12-31 |publisher=Harvard University Press |isbn=978-0-674-49397-1 |pages=1–77 |chapter=1. Interstellar Hydrogen |doi=10.4159/harvard.9780674493988}}</ref> Radiation from stars ionizes H{{nbsp}}I to H{{nbsp}}II, creating [[Strömgren sphere|spheres of ionized H{{nbsp}}II]] around stars. In the [[chronology of the universe]] neutral hydrogen dominated until the birth of stars during the era of [[reionization]], which then produced bubbles of ionized hydrogen that grew and merged over hundreds of millions of years.<ref>{{Cite journal |last=Dijkstra |first=Mark |date=January 2014 |title=Lyα Emitting Galaxies as a Probe of Reionisation |url=https://www.cambridge.org/core/journals/publications-of-the-astronomical-society-of-australia/article/ly-emitting-galaxies-as-a-probe-of-reionisation/51F95FB047C1F0418D1DA56D39470C22 |journal=Publications of the Astronomical Society of Australia |language=en |volume=31 |article-number=e040 |doi=10.1017/pasa.2014.33 |arxiv=1406.7292 |bibcode=2014PASA...31...40D |issn=1323-3580}}</ref>
These are the source of the 21-centimeter [[hydrogen line]], at {{val|1420|ul=MHz}}, that is detected in order to probe primordial hydrogen. 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 {{nowr|''z'' {{=}} 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>

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 lower-mass stars, and through the [[CNO cycle|CNO{{nbsp}}cycle]] of [[nuclear fusion]] in 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>

[[Protonated molecular hydrogen]]{{nbsp}}({{chem2|H3+}}) is found in the [[interstellar medium]], where it is generated by ionization of molecular hydrogen by [[cosmic ray]]s. This ion has also been observed in the [[Primary atmosphere|upper atmosphere of Jupiter]]. The ion is long-lived 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>

===Terrestrial===
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 existing within [[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> Elemental hydrogen is normally in the form of a gas, {{chem2|H2}}, at [[standard conditions]]. It is present in a very low concentration in Earth's atmosphere (around {{val|0.53|u=[[parts per million]]}} on a [[mole (unit)|molar basis]]<ref name="Grinter">{{cite journal |last1=Rhys Grinter |last2=Kropp |first2=A. |last3=Venugopal |display-authors=etal |date=2023 |title=Structural basis for bacterial energy extraction from atmospheric hydrogen |journal=Nature |volume=615 |issue=7952 |pages=541–547 |bibcode=2023Natur.615..541G |doi=10.1038/s41586-023-05781-7 |pmc=10017518 |pmid=36890228}}</ref>) because of its light weight, which enables it to [[atmospheric escape|escape the atmosphere]] more rapidly than heavier gases. Despite its low concentration in the atmosphere, terrestrial hydrogen is sufficiently abundant to support the metabolism of several varieties of bacteria.<ref>{{cite journal |doi=10.1042/BST20230120 |title=Developing high-affinity, oxygen-insensitive &#91;NiFe&#93;-hydrogenases as biocatalysts for energy conversion |date=2023 |last1=Greening |first1=Chris |last2=Kropp |first2=Ashleigh |last3=Vincent |first3=Kylie |last4=Grinter |first4=Rhys |journal=Biochemical Society Transactions |volume=51 |issue=5 |pages=1921–1933 |pmid=37743798 |pmc=10657181 }}</ref>

Large underground deposits of hydrogen gas have been discovered in several countries including [[Mali]], [[France]] and [[Australia]].<ref name="Pearce-2024">{{Cite web |last=Pearce |first=Fred |date=January 25, 2024 |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 Environment 360|Yale E360]] |language=en-US}}</ref> As of 2024, it is uncertain how much underground hydrogen can be extracted economically.<ref name="Pearce-2024" />

== Production and storage==
{{Main|Hydrogen production}}

===Industrial routes===
Nearly all of the world's current supply of hydrogen gas{{nbsp}}({{chem2|H2}}) is produced from fossil fuels.<ref>{{cite news |last1=Reed |first1=Stanley |last2=Ewing |first2=Jack |date=13 July 2021 |title=Hydrogen Is One Answer to Climate Change. Getting It Is the Hard Part |url=https://www.nytimes.com/2021/07/13/business/hydrogen-climate-change.html |work=The New York Times}}</ref><ref name="rosenow-2022">{{cite journal |last1=Rosenow |first1=Jan |date=27 September 2022 |title=Is heating homes with hydrogen all but a pipe dream? An evidence review |journal=Joule |volume=6 |issue=10 |pages=2225–2228 |bibcode=2022Joule...6.2225R |doi=10.1016/j.joule.2022.08.015 |s2cid=252584593 |doi-access=free}}  Article in press.</ref>{{rp|1}} 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 methane reforming]]{{nbsp}}(SMR), the reaction of water and methane.<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 |page=249 |url-access=registration}}</ref><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 ({{cvt|1000|–|1400|K|C F|disp=sqbr}}), [[steam]] (water vapor) reacts with [[methane]] to yield [[carbon monoxide]] and{{nbsp}}{{chem2|H2}}.
{{bi|{{chem2|CH4 + H2O → CO + 3 H2}}}}
Producing one [[tonne]] of hydrogen through this process emits {{val|6.6|–|9.3}}{{nbsp}}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> The production of natural gas feedstock also produces emissions such as [[Gas venting|vented]] and [[Fugitive gas emissions|fugitive methane]], which further contributes to the overall carbon footprint of hydrogen.<ref name="Griffiths-20212">{{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 |article-number=102208 |bibcode=2021ERSS...8002208G |doi=10.1016/j.erss.2021.102208 |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>

This reaction is favored at low pressures but is nonetheless conducted at high pressures{{nbsp}}({{cvt|2.0|MPa|atm inHg|lk=on|disp=sqbr}}) because high-pressure{{nbsp}}{{chem2|H2}} is the most marketable product, and [[pressure swing adsorption]]{{nbsp}}(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 (fuel)|coke]] or carbon:
{{bi|{{chem2|CH4 → C + 2 H2}}}}

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

Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the [[Haber process|{{langr|de|Haber|cat=no}} 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}}</ref>

====Partial oxidation of hydrocarbons====
Other methods for CO and {{chem2|H2}} production include partial oxidation of hydrocarbons:<ref name="uigi"/>

{{bi|{{chem2|2 CH4 + O2 → 2 CO + 4 H2}}}}

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

{{bi|{{chem2|C + H2O → CO + H2}}}}

[[Olefin]] production units may produce substantial quantities of byproduct hydrogen, particularly from [[Petroleum cracking|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 |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.  
{{bi|{{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 |article-number=4695 |pmid=25146255 |bibcode=2014NatCo...5.4695G |s2cid=205329127 |doi-access=free }}</ref> The hydrogen created through electrolysis using renewable energy is commonly referred to as "[[green hydrogen]]".<ref name="RoyalSociety-2021">{{Cite web |date=June 2021 |title=The role of hydrogen and ammonia in meeting the net zero challenge |url=https://royalsociety.org/-/media/policy/projects/climate-change-science-solutions/climate-science-solutions-hydrogen-ammonia.pdf |website=The Royal Society}}</ref>

[[Electrolysis]] of [[brine]] to yield [[chlorine]]<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> also produces high-purity hydrogen as a co-product, which is used for a variety of transformations such as [[hydrogenation]]s.<ref>{{Cite book |last1=Schmittinger |first1=Peter |chapter=Chlorine |date=2006-01-15 |title=Ullmann's Encyclopedia of Industrial Chemistry |place=Weinheim, Germany |publisher=Wiley-VCH Verlag GmbH & Co. KGaA |language=en |doi=10.1002/14356007.a06_399.pub2 |isbn=978-3-527-30673-2 |last2=Florkiewicz |first2=Thomas |last3=Curlin |first3=L. Calvert |last4=Lüke |first4=Benno |last5=Scannell |first5=Robert |last6=Navin |first6=Thomas |last7=Zelfel |first7=Erich |last8=Bartsch |first8=Rüdiger}}</ref>

The electrolysis process is more expensive than producing hydrogen from methane without [[carbon capture and storage]].<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>

Innovation in [[hydrogen electrolyzer]]s 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>

====Methane pyrolysis====
Hydrogen can be produced by [[pyrolysis]] of [[natural gas]] (methane), producing hydrogen gas and solid carbon with the aid of a catalyst and {{val|74|ul=kJ/mol}} input heat:
{{bi|{{chem2|CH4(g) → C(s) + 2 H2(g)}} (Δ''H''° {{=}} 74 kJ/mol)}}
The carbon may be sold as a manufacturing feedstock or fuel, or landfilled. This route could have a lower carbon footprint than existing hydrogen production processes, but mechanisms for removing the carbon and preventing it from reacting with the catalyst remain obstacles for industrial-scale use.<ref>{{Cite journal |last1=Rasul |first1=M. G. |last2=Hazrat |first2=M. A |last3=Sattar |first3=M. A. |last4=Jahirul |first4=M. I. |last5=Shearer |first5=M. J. |date=2022-11-15 |title=The future of hydrogen: Challenges on production, storage and applications |url=https://linkinghub.elsevier.com/retrieve/pii/S0196890422011049 |journal=Energy Conversion and Management |volume=272 |article-number=116326 |doi=10.1016/j.enconman.2022.116326 |bibcode=2022ECM...27216326R |issn=0196-8904|url-access=subscription }}</ref>{{rp|17}}<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>

====Thermochemical====
[[Water splitting]] is the process by which water is decomposed into its components. Relevant to the biological scenario is this equation:
{{bi|{{chem2|2 H2O  ->  4 H+ + O2 + 4 e-}}}}
The reaction occurs in the [[light-dependent reactions]] in all [[photosynthetic]] organisms. A few organisms, including the alga {{lang|la|[[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}}{{nbsp}}gas by specialized [[hydrogenase]]s 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 more efficiently generate {{chem2|H2}}{{nbsp}}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 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>

Relevant to the thermal water-splitting scenario is this simple equation:
{{bi|{{chem2|2 H2O  ->  2 H2 + O2}}}}
Over 200 thermochemical cycles can be used for [[water splitting]]. Many of these cycles such as the [[iron oxide cycle]], [[ceria based thermochemical cycles|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>

===Natural routes===
====Biohydrogen====
{{Further|Biohydrogen}}
{{chem2|H2}} is produced in organisms by enzymes called [[hydrogenase]]s. This process allows the host organism to use [[fermentation]] as a source of energy.<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> These same enzymes also can [[oxidize]]{{nbsp}}H<sub>2</sub>, such that the host organisms can subsist by reducing oxidized substrates using electrons extracted from{{nbsp}}H<sub>2</sub>.<ref>{{cite journal |author1=Chris Greening |author2=Ambarish Biswas |author3=Carlo R Carere |author4=Colin J Jackson |author5=Matthew C Taylor |author6=Matthew B Stott |author7=Gregory M Cook |author8=Sergio E Morales |title=Genomic and metagenomic surveys of hydrogenase distribution indicate H2 is a widely utilised energy source for microbial growth and survival |journal=The ISME Journal |date=2016 |volume=10 |issue=3 |pages=761–777 |doi=10.1038/ismej.2015.153 |pmid=26405831 |pmc=4817680 |bibcode=2016ISMEJ..10..761G |language=en}}</ref>

Hydrogenase enzymes feature [[iron]] or [[iron–nickel alloy|iron–nickel]] centers at their [[active site]]s.<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>

Some bacteria such as {{lang|la|[[Mycobacterium smegmatis]]}} can use the small amount of hydrogen in the atmosphere as a source of energy when other sources are lacking. Their hydrogenases feature small channels that exclude oxygen from the active site, permitting 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>

Confirming the existence of hydrogenase‐employing microbes [[Human gut flora|in the human gut]], {{chem2|H2}} occurs in human breath. The concentration in the breath of fasting people at rest is typically under {{val|5|u=[[parts per million]]}}{{nbsp}}(ppm), but can reach {{val|50|u=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|article-number=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>

====Serpentinization====
[[Serpentinization]] is a geological mechanism which produces highly-[[Reduction (chemistry)|reducing]] conditions.<ref name=FrostBeard2007>{{cite journal |last1=Frost |first1=B. R. |last2=Beard |first2=J. S. |title=On Silica Activity and Serpentinization |journal=Journal of Petrology |date=3 April 2007 |volume=48 |issue=7 |pages=1351–1368 |doi=10.1093/petrology/egm021|url=http://petrology.oxfordjournals.org/content/48/7/1351.full.pdf }}</ref> Under these conditions, water is capable of oxidizing [[ferrous ion|ferrous]]{{nbsp}}({{chem|Fe|2+}}) ions in [[fayalite]], generating hydrogen gas:<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 |url-status=live |journal=International Journal of Hydrogen Energy |language=en |volume=40 |issue=34 |pages=11094–11111 |bibcode=2015IJHE...4011094D |doi=10.1016/j.ijhydene.2014.12.035 |issn=0360-3199 |archive-url=https://web.archive.org/web/20220215183915/https://www.sciencedirect.com/science/article/abs/pii/S0360319914034119 |archive-date=15 February 2022 |access-date=4 February 2022|url-access=subscription }}</ref><ref>{{Cite journal| author1 = Sleep, N. H.| author2 = Meibom, A. | author3 = Fridriksson, Th. | author4 = Coleman, R. G. | author5 = Bird, D. K.| year = 2004| title = H<sub>2</sub>-rich fluids from serpentinization: Geochemical and biotic implications| journal = Proceedings of the National Academy of Sciences of the United States of America| volume = 101| issue = 35| pages = 12818–12823| doi = 10.1073/pnas.0405289101|bibcode = 2004PNAS..10112818S| pmid=15326313| pmc=516479| doi-access = free}}</ref>
{{bi|{{chem2|Fe2SiO4 + H2O → 2 Fe3O4 + SiO2 + H2}}}}

Closely related to this geological process is the [[Schikorr reaction|{{langr|de|Schikorr|cat=no}} reaction]]:
{{bi|{{chem2|3 Fe(OH)2 → Fe3O4 + 2 H2O + H2}}}}
This process also is relevant to the corrosion of [[iron]] and [[steel]] in [[Anoxic waters|oxygen-free]] [[groundwater]] and in [[reducing soil]]s below the [[water table]].<ref>{{cite journal |author1=Stephan Kaufhold |author2=Stephen Klimke |author3=Stefan Schloemer |author4=Theodor Alpermann |author5=Franz Renz |author6=Reiner Dohrmann |title=About the Corrosion Mechanism of Metal Iron in Contact with Bentonite |journal=ACS Earth and Space Chemistry |date=2020 |volume=4 |issue=5 |pages=711–721 |doi=10.1021/acsearthspacechem.0c00005 |bibcode=2020ESC.....4..711K |language=en}}</ref>

===Laboratory syntheses===
{{chem2|H2}} is produced in laboratory settings, such as in the small-scale [[electrolysis of water]] using metal [[electrodes]] and water containing an [[electrolyte]], which liberates hydrogen gas at the [[cathode]]:<ref name="housecroft" />
{{bi|{{chem2|2H+(aq) + 2 e− → H2(g)}}}}
Hydrogen is also often a by-product of other reactions. Many metals react with water to produce{{nbsp}}{{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 earth metals|alkali]] and [[alkaline earth metals]] as well as [[aluminium]], [[zinc]], [[manganese]], and [[iron]], react readily with aqueous acids.<ref name="housecroft">{{ cite book | title = Inorganic Chemistry | last1 = Housecroft | first1 = C. E. | last2 = Sharpe | first2 = A. G. | year = 2018 | publisher = Prentice Hall | edition = 5th | isbn = 978-1-292-13414-7 |pages = 219, 318–319}}</ref>
{{bi|{{chem2|Zn + 2 H+ → Zn(2+) + H2}}}}

Many metals, such as aluminium, are slow to react with water because they form [[passivation (chemistry)|passivated]] oxide coatings. An alloy of aluminium and [[gallium]], however, does react with water. In high-pH solutions, aluminium can react with {{nbsp}}{{chem2|H2}}:<ref name="housecroft" />

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

===Storage===
If H<sub>2</sub> is to be used as an energy source, its storage is important. It dissolves only poorly in solvents. For example, at [[room temperature]] and {{convert|0.1|mPa|atm|abbr=out|lk=on}}, {{approx}}{{nbsp}}{{val|0.05|u=[[mole (unit)|moles]]}} of hydrogen dissolve into {{convert|1|kg|abbr=out|spell=in}} of [[diethyl ether]].<ref name="UllmannH2">{{cite book |doi=10.1002/14356007.a13_297.pub3 |chapter=Hydrogen, 1. Properties and Occurrence |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2013 |last1=Lauermann |first1=Gerhard |last2=Häussinger |first2=Peter |last3=Lohmüller |first3=Reiner |last4=Watson |first4=Allan M. |pages=1–15 |isbn=978-3-527-30673-2 }}</ref> H<sub>2</sub> can be stored in compressed form, although compressing costs energy. Liquefaction is impractical given hydrogen's low [[critical temperature]]. In contrast, ammonia and many hydrocarbons can be liquified at room temperature under pressure. For these reasons, hydrogen ''carriers''—materials that reversibly bind{{nbsp}}H<sub>2</sub>—have attracted much attention. The key question is then the weight percent of H<sub>2</sub>-equivalents within the carrier material. For example, hydrogen can be reversibly absorbed into many [[rare earths]] 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 also a metallurgical problem, contributing to the [[hydrogen embrittlement|embrittlement]] of many metals,<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> 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>

The most problematic aspect of metal hydrides for storage is their modest H<sub>2</sub>{{nbsp}}content, often on the order of{{nbsp}}1%. For this reason, there is interest in storage of H<sub>2</sub> in compounds of low [[molecular weight]]. For example, [[ammonia borane]] ({{chem2|H3N\sBH3}}) contains 19.8{{nbsp}}[[weight percent]] of{{nbsp}}H<sub>2</sub>. The problem with this material is that after release of H<sub>2</sub>, the resulting boron nitride does not re-add H<sub>2</sub>: i.e., ammonia borane is an irreversible hydrogen carrier.<ref>{{cite journal |doi=10.1016/j.gee.2022.03.011 |title=Ammonia borane-enabled hydrogen transfer processes: Insights into catalytic strategies and mechanisms |date=2023 |last1=Zhao |first1=Wenfeng |last2=Li |first2=Hu |last3=Zhang |first3=Heng |last4=Yang |first4=Song |last5=Riisager |first5=Anders |journal=Green Energy & Environment |volume=8 |issue=4 |pages=948–971 |bibcode=2023GrEE....8..948Z |doi-access=free }}</ref> More attractive are [[hydrocarbon]]s such as [[tetrahydroquinoline]], which reversibly release some{{nbsp}}H<sub>2</sub> when heated in the presence of a catalyst:<ref>{{cite journal |doi=10.1021/acscatal.7b03547 |title=NHC-Based Iridium Catalysts for Hydrogenation and Dehydrogenation of N-Heteroarenes in Water under Mild Conditions |date=2018 |last1=Vivancos |first1=Ángela |last2=Beller |first2=Matthias |last3=Albrecht |first3=Martin |journal=ACS Catalysis |volume=8 |pages=17–21 }}</ref>
{{bi|{{chem2|C9H10NH <-> C9H7N + 2 H2}}}}

== Applications ==
{{See also|Hydrogen economy}}
[[File:2023-10-19 Clean Hydrogen Ladder 5.0.png|thumb|upright=2|Hydrogen Ladder: Ranking of hydrogen applications and 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 [[Sweetening (fossil fuels)|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=978-0-471-48494-3 }}</ref><ref name="UllmannHuse">{{cite book |author1=Peter Häussinger |author2=Reiner Lohmüller |author3=Allan M. Watson |title=Ullmann's Encyclopedia of Industrial Chemistry |date=2011 |publisher=Wiley |isbn=978-3-527-30673-2 |language=en |chapter= Hydrogen, 6. Uses |doi=10.1002/14356007.o13_o07}}</ref>
{{bi|{{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}} produces ammonia by the [[Haber process|{{langr|de|Haber|cat=no}} process]]:<ref name="UllmannHuse" />
{{bi|{{chem2|N2 + 3 H2 → 2 NH3}}}}
This process consumes a few percent of the energy budget in the entire industry and is the biggest consumer of hydrogen. The resulting ammonia is used extensively in [[fertilizer]] production; these fertilizers have become essential feedstocks in modern agriculture.<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 also used to convert [[unsaturated fat]]s and [[vegetable oil|oils]] to saturated fats and oils. The major application is the production of [[margarine]]. [[Methanol]] is produced by hydrogenation of carbon dioxide; the mixture of hydrogen and carbon dioxide used for this process is known as [[syngas]]. 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><ref name="housecroft" />

=== Fuel ===
The potential for using hydrogen{{nbsp}}(H<sub>2</sub>) as a fuel has been widely discussed. Hydrogen can be used in [[fuel cells]] to produce electricity,<ref>{{cite journal |doi=10.1155/2024/7271748 |title=A Recent Comprehensive Review of Fuel Cells: History, Types, and Applications |date=2024 |last1=Qasem |first1=Naef A. A. |last2=Abdulrahman |first2=Gubran A. Q. |journal=International Journal of Energy Research |issue=1 |article-number=7271748 |doi-access=free |bibcode=2024IJER.202471748Q }}</ref> or burned to generate heat.<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 |bibcode=2021ESAt....1..201L |doi=10.1039/D1EA00037C |doi-access=free}}{{Creative Commons text attribution notice|cc=by3|from this source=yes|url=|authors=|vrt=}}</ref> When hydrogen is consumed in fuel cells, the only emission at the point of use is water vapor.<ref name="Lewis-2021" /> When burned, hydrogen produces relatively little pollution at the point of combustion, but can lead to thermal formation of harmful [[NOx|nitrogen oxides]].<ref name="Lewis-2021" />

If hydrogen is produced with low or zero greenhouse gas emissions ([[green hydrogen]]), it can 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" /><ref name="Evans-2020" />

Hydrogen fuel can produce the intense heat required for industrial production of steel, cement, glass, and chemicals, thus contributing to the decarbonization 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">{{Cite book |author=IPCC |author-link=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 |isbn=978-1-009-15792-6 |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 |pages=91–92 |doi=10.1017/9781009157926 |ref={{harvid|IPCC AR6 WG3|2022}} |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}}</ref> For example, in [[steelmaking]], hydrogen could function as a clean fuel and also as a low-carbon catalyst, replacing coal-derived [[Coke (fuel)|coke]] (carbon):<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 |publisher=[[Rocky Mountain Institute]] |pages=2, 7, 8}}</ref>
{{bi|{{chem2|2FeO  +  C -> 2Fe  +  CO2}}}}
{{bi|vs}}
{{bi|{{chem2|FeO  +  H2 -> Fe  +  H2O}}}}
Hydrogen used to decarbonize 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|url-access=subscription }}</ref>

[[File:Shuttle Main Engine Test Firing cropped edited and reduced.jpg|thumb|A [[Space Shuttle Main Engine]] burns hydrogen with oxygen, producing a nearly invisible flame at full thrust.|alt=A black inverted funnel with blue glow emerging from its opening.]]
[[Liquid hydrogen]] and [[liquid oxygen]] together serve as [[cryogenic propellant]]s in [[liquid-propellant rocket]]s, as in the [[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>

Hydrogen produced when there is a surplus of [[variable renewable electricity]] could in principle be stored and later 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 |article-number=106785 |doi=10.1016/j.compchemeng.2020.106785 |issn=0098-1354 |doi-access=free}}</ref>  It can be further transformed into [[synthetic fuel]]s such as [[ammonia]] and [[methanol]].<ref>{{cite book |author=[[IRENA]] |url=https://www.irena.org/-/media/Files/IRENA/Agency/Publication/2021/March/IRENA_World_Energy_Transitions_Outlook_2021.pdf |title=World Energy Transitions Outlook: 1.5°C Pathway |year=2021 |isbn=978-92-9260-334-2 |pages=12, 22 |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 |archive-date=11 June 2021 |url-status=live}}</ref> Disadvantages of hydrogen fuel include high costs of storage and distribution due to hydrogen's explosivity, its large volume compared to other fuels, and its tendency to [[hydrogen embrittlement|embrittle materials]].<ref name="Griffiths-20212"/>

=== Nickel–hydrogen battery ===
The very long-lived, rechargeable [[nickel–hydrogen battery]] developed for satellite power systems uses pressurized gaseous{{nbsp}}H<sub>2</sub>.<ref>{{Cite book |last=Zimmerman |first=Albert H. |title=Nickel-hydrogen batteries: principles and practice |date=2009 |publisher=Aerospace press |isbn=978-1-884989-20-9 |location=El Segundo, Calif}}</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{{nbsp}}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{{nbsp}}years after launch and 13{{nbsp}}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>

=== Semiconductor industry ===
Hydrogen is employed in [[semiconductor manufacturing]] to saturate broken ("dangling") bonds of [[amorphous silicon]] and [[amorphous carbon]], which helps in stabilizing the materials' 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> Hydrogen, introduced as an unintended side-effect of production, acts as a shallow [[electron donor]] leading to [[N-type semiconductor|{{nowr|n-type}}]] conductivity in [[ZnO]], with important uses in [[transducers]] and [[phosphors]].<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=Spencer |first1=Joseph A. |last2=Mock |first2=Alyssa L. |last3=Jacobs |first3=Alan G. |last4=Schubert |first4=Mathias |last5=Zhang |first5=Yuhao |last6=Tadjer |first6=Marko J. |date=2022-03-04 |title=A review of band structure and material properties of transparent conducting and semiconducting oxides: Ga2O3, Al2O3, In2O3, ZnO, SnO2, CdO, NiO, CuO, and Sc2O3 |url=https://pubs.aip.org/aip/apr/article-abstract/9/1/011315/2835450/A-review-of-band-structure-and-material-properties |journal=Applied Physics Reviews |volume=9 |issue=1 |page=011315 |doi=10.1063/5.0078037 |issn=1931-9401|url-access=subscription }}</ref> Detailed analysis of ZnO and of [[MgO]] shows evidence of four and six-fold hydrogen multicentre bonds.<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>
The [[Dopant (semiconductor)|doping behavior]] of hydrogen varies with material.<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><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 ===
Beyond than the uses mentioned above, hydrogen is used in smaller scales in the following applications:
* 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 book |last1=Ujah |first1=Chika Oliver |url=https://onlinelibrary.wiley.com/doi/10.1002/9781394331925.ch6 |title=Advanced Welding Technologies |last2=N'Dedji Sodokin |first2=Rodolphe |last3=von Kallon |first3=Daramy Vandi |date=2025-05-05 |publisher=Wiley |isbn=978-1-394-33189-5 |editor-last=Kunar |editor-first=Sandip |edition=1 |pages=107–126 |language=en |chapter=Chapter 6 Atomic Hydrogen Welding |doi=10.1002/9781394331925.ch6 |editor-last2=Mandal |editor-first2=Gurudas}}</ref>

* Coolant: Hydrogen is used as a [[coolant]] in large electrical generators due to its high [[thermal conductivity]] and low density.<ref>{{Cite conference |last1=Kumar |first1=Rajendar |last2=Kumar |first2=Ashwani |date=June 2015 |title=Assessment of impact of hydrogen cooled generator on power system loadability enhancement |conference=2015 International Conference on Energy, Power and Environment: Towards Sustainable Growth (ICEPE) |publisher=IEEE |pages=1–6 |doi=10.1109/EPETSG.2015.7510166 |isbn=978-1-4673-6503-1}}</ref> The first [[hydrogen-cooled turbogenerator]] went into service using gaseous hydrogen as a coolant in the rotor and the stator in{{nbsp}}1937 in [[Dayton, Ohio]].<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>
* 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>
* Food industry: Hydrogen is an authorized [[food additive]] ([[E number#E900–E999 (glazing agents, gases and sweeteners)|E949]])<ref>{{cite web |title=Report from the Commission on Dietary Food Additive Intake |url=http://ec.europa.eu/food/fs/sfp/addit_flavor/flav15_en.pdf |url-status=live |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 |access-date=5 February 2008 |publisher=[[European Union]]}}</ref> that is used as a packaging gas,<ref>{{Cite journal |last1=EFSA Panel on Food Additives and Flavourings (FAF) |last2=Castle |first2=Laurence |last3=Andreassen |first3=Monica |last4=Aquilina |first4=Gabriele |last5=Bastos |first5=Maria Lourdes |last6=Boon |first6=Polly |last7=Fallico |first7=Biagio |last8=FitzGerald |first8=Reginald |last9=Frutos Fernandez |first9=Maria Jose |last10=Grasl-Kraupp |first10=Bettina |last11=Gundert-Remy |first11=Ursula |last12=Gürtler |first12=Rainer |last13=Houdeau |first13=Eric |last14=Kurek |first14=Marcin |last15=Louro |first15=Henriqueta |date=2025 |title=Re-evaluation of oxygen (E 948) and hydrogen (E 949) as food additives |journal=EFSA Journal |language=en |volume=23 |issue=8 |article-number=e9595 |doi=10.2903/j.efsa.2025.9595 |issn=1831-4732 |pmc=12329423 |pmid=40777209}}</ref> and also has [[antioxidant]] properties.<ref>{{Cite journal |last1=Yıldız |first1=Fatmanur |last2=LeBaron |first2=Tyler W. |last3=Alwazeer |first3=Duried |date=2025-03-01 |title=A comprehensive review of molecular hydrogen as a novel nutrition therapy in relieving oxidative stress and diseases: Mechanisms and perspectives |journal=Biochemistry and Biophysics Reports |volume=41 |article-number=101933 |doi=10.1016/j.bbrep.2025.101933 |issn=2405-5808 |pmc=11795818 |pmid=39911528}}</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> it also allows for leak testing in food packaging.

* Neutron moderation: [[Deuterium]] (hydrogen-2) is used in [[CANDU reactor|nuclear fission applications]] as a [[neutron moderator|moderator to slow neutrons]].
* 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>

* 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 radiation]] in [[Tritium radioluminescence|radioluminescent paint]] for instrument dials and emergency signage.<ref name="Traub95" />

== 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 =
 }}
}}
In hydrogen pipelines and steel storage vessels, hydrogen molecules are prone to reacting with metals, causing [[hydrogen embrittlement]] and leaks in the pipeline or storage vessel.<ref name="Li-2022">{{Cite journal |last1=Li |first1=Hao |last2=Cao |first2=Xuewen |last3=Liu |first3=Yang |last4=Shao |first4=Yanbo |last5=Nan |first5=Zilong |last6=Teng |first6=Lin |last7=Peng |first7=Wenshan |last8=Bian |first8=Jiang |date=2022-11-01 |title=Safety of hydrogen storage and transportation: An overview on mechanisms, techniques, and challenges |journal=Energy Reports |volume=8 |pages=6258–6269 |doi=10.1016/j.egyr.2022.04.067 |issn=2352-4847|doi-access=free |bibcode=2022EnRep...8.6258L }}Text was copied from this source, which is available under a [[creativecommons:by/4.0/|Creative Commons Attribution 4.0 International License]]</ref> Since it is lighter than air, hydrogen does not easily accumulate to form a combustible gas mixture.<ref name="Li-2022" /> However, even without ignition sources, high-pressure hydrogen leakage may cause spontaneous combustion and [[detonation]].<ref name="Li-2022" />

Hydrogen is flammable when mixed even in small amounts with air. Ignition can occur at a [[volumetric ratio]] of hydrogen to air as low as 4%.<ref>{{Cite journal |last1=Yang |first1=Fuyuan |last2=Wang |first2=Tianze |last3=Deng |first3=Xintao |last4=Dang |first4=Jian |last5=Huang |first5=Zhaoyuan |last6=Hu |first6=Song |last7=Li |first7=Yangyang |last8=Ouyang |first8=Minggao |date=2021-09-03 |title=Review on hydrogen safety issues: Incident statistics, hydrogen diffusion, and detonation process |url=https://linkinghub.elsevier.com/retrieve/pii/S0360319921025520 |journal=International Journal of Hydrogen Energy |volume=46 |issue=61 |pages=31467–31488 |doi=10.1016/j.ijhydene.2021.07.005 |bibcode=2021IJHE...4631467Y |issn=0360-3199|url-access=subscription }}</ref> In approximately 70% of hydrogen ignition accidents, the ignition source cannot be found, and it is widely believed by scholars that spontaneous ignition of hydrogen occurs.<ref name="Li-2022" />

Hydrogen fire, while being extremely hot, is almost invisible to the human eye, and thus can lead to accidental burns.<ref name="spinoff-2016">{{Cite web |date=2016 |title=Hydrogen Detection Tape Saves Time and Lives {{!}} NASA Spinoff |url=https://spinoff.nasa.gov/Spinoff2016/ps_5.html |access-date=2025-02-23 |website=spinoff.nasa.gov}}</ref> Hydrogen is non-toxic,<ref>{{Cite journal |last1=Abohamzeh |first1=Elham |last2=Salehi |first2=Fatemeh |last3=Sheikholeslami |first3=Mohsen |last4=Abbassi |first4=Rouzbeh |last5=Khan |first5=Faisal |date=2021-09-01 |title=Review of hydrogen safety during storage, transmission, and applications processes |journal=Journal of Loss Prevention in the Process Industries |volume=72 |article-number=104569 |doi=10.1016/j.jlp.2021.104569 |bibcode=2021JLPPI..7204569A |issn=0950-4230|doi-access=free }}</ref> but like most gases [[Inert gas asphyxiation|it can cause asphyxiation]] in the absence of adequate ventilation.<ref>{{Cite web |last=U.S. Department of Energy |title=Current Safe Operating Practices |url=https://www.energy.gov/eere/fuelcells/current-safe-operating-practices |access-date=2025-02-24 |website=Energy.gov |language=en}}</ref>

== 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|Methane pyrolysis}} (for hydrogen)
* {{annotated link|Natural hydrogen}}
* {{annotated link|Pyrolysis}}
{{div col end}}

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

== Further reading ==
{{Library resources box
 |onlinebooks=yes
 |by=no
 |lcheading= Hydrogen
 |label=Hydrogen
 }}
* {{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 ==
* {{webarchive|url=https://web.archive.org/web/20250308000802/http://www.physics.drexel.edu/~tim/open/hydrofin/|title=Basic quantum mechanical hydrogen atom model}} (Timothy Jones, Drexel University)
* [http://www.periodicvideos.com/videos/001.htm Hydrogen] at ''[[The Periodic Table of Videos]]'' (University of Nottingham)
* [http://militzer.berkeley.edu/diss/node5.html High temperature hydrogen phase diagram] from Burkhard Militzer (University of California, Berkeley)
* [http://hyperphysics.phy-astr.gsu.edu/Hbase/quantum/hydwf.html#c3 Wavefunction of hydrogen] at ''[[HyperPhysics]]'' (Georgia State University)

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