Editing Hydrogen (section)

==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]], the [[protons]] formed in the first second after the [[Big Bang]]; neutral hydrogen atoms formed about 370,000 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 }} (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 I'' and ionized hydrogen is called ''H 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 I to H II, creating [[Strömgren sphere|spheres]] of ionized H II around stars. In the [[chronology of the universe]] neutral hydrogen dominated until the birth of stars during the era of [[reionization]] led to bubbles of ionized hydrogen that grew and merged over 500  million 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 |pages=e040 |doi=10.1017/pasa.2014.33 |arxiv=1406.7292 |bibcode=2014PASA...31...40D |issn=1323-3580}}</ref>
They are the source of the 21-cm [[hydrogen line]] at 1420&nbsp;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 ''z''&nbsp;=&nbsp;4.<ref>{{cite journal
|last1=Storrie-Lombardi|first1=L. J.
|last2=Wolfe|first2=A. M.
|title=Surveys for z > 3 Damped Lyman-alpha Absorption Systems: the Evolution of Neutral Gas
|journal=Astrophysical Journal
|date=2000|volume=543|pages=552–576
|arxiv=astro-ph/0006044
|doi=10.1086/317138
|bibcode=2000ApJ...543..552S
|issue=2|s2cid=120150880
}}</ref>

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]] of [[nuclear fusion]] in case of stars more massive than the [[Sun]].<ref>{{cite web
|last1=Haubold|first1=H.|last2=Mathai|first2=A. M.
|date=15 November 2007|url=http://neutrino.aquaphoenix.com/un-esa/sun/sun-chapter4.html |archive-url =https://web.archive.org/web/20111211073137/http://neutrino.aquaphoenix.com/un-esa/sun/sun-chapter4.html
 |archive-date=11 December 2011 |title=Solar Thermonuclear Energy Generation
|publisher=[[Columbia University]]|access-date=12 February 2008
}}</ref>

A molecular form called [[protonated molecular hydrogen]] ({{chem2|H3+}}) is found in the interstellar medium, where it is generated by ionization of molecular hydrogen from [[cosmic ray]]s. This ion has also been observed in the [[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 in the form of [[chemical compound]]s such as [[hydrocarbon]]s and water.<ref name="Miessler">{{cite book|first1=G. L.|last1=Miessler|last2=Tarr|first2=D. A.|date=2003|title=Inorganic Chemistry|edition=3rd|publisher=Prentice Hall|isbn=978-0-13-035471-6|url-access=registration|url=https://archive.org/details/inorganicchemist03edmies}}</ref>  Elemental hydrogen is normally in the form of a gas, {{chem2|H2}}. It is present in a very low concentration in Earth's atmosphere (around 0.53 [[part per million|ppm]] on a molar basis<ref name="Grinter">{{cite journal |last1=Rhys Grinter |last2=Kropp |first2=A. |last3=Venugopal |display-authors=etal |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 our atmosphere, terrestrial hydrogen is sufficiently abundant to support the metabolism of several 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" />