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{{Two other uses|the chemistry of hydrogen|the physics of atomic hydrogen|Hydrogen atom|other meanings|Hydrogen (disambiguation)}}
{{Infobox hydrogen}}
 
ހައިޑްރަޖަން ({{en|Hydrogen}})އަކީ ކެމިކަލް ޢުންޞުރެކެވެ. ހައިޑްރަޖަންގެ ރަމްޒަކީ H އެވެ. އަޓޮމިކް ނަމްބަރަކީ 1 އެވެ.
 
'''Hydrogen''' is a [[chemical element]] with [[chemical symbol|symbol]]&nbsp;'''H''' and [[atomic number]]&nbsp;1. With an average [[atomic weight]] of {{val|1.00794|u=[[atomic mass unit|u]]}} ({{val|1.007825|u=[[atomic mass unit|u]]}} for [[hydrogen-1]]), hydrogen is the lightest element and its monatomic form (H<sub>1</sub>) is the [[abundance of the chemical elements|most abundant]] chemical substance, constituting roughly 75% of the Universe's [[baryon|baryonic]] mass.<ref>
{{cite web
|last=Palmer|first=D.
|title=Hydrogen in the Universe
|url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/971113i.html
|publisher=[[NASA]]
|date=13 September 1997
|accessdate=2008-02-05
}} Note that most of the universe's mass is not in the form of Baryon|chemical elements, however. See [[dark matter]] and [[dark energy]].</ref> Non-[[stellar remnant|remnant]] [[star]]s are mainly composed of hydrogen in its [[plasma (physics)|plasma]] state.
 
At [[standard temperature and pressure]], hydrogen is a [[Transparency (optics)|colorless]], [[odorless]], [[taste]]less, non-toxic, [[nonmetal]]lic, highly [[combustion|combustible]] [[Diatomic molecule|diatomic]] [[gas]] with the [[molecular formula]] H<sub>2</sub>. Naturally occurring atomic hydrogen is rare on Earth because hydrogen readily forms [[covalent bond|covalent]] compounds with most elements and is present in the water molecule and in most [[organic compound]]s. Hydrogen plays a particularly important role in [[acid-base reaction theories|acid-base chemistry]] with many reactions exchanging [[proton]]s between soluble molecules.
 
In [[ionic compound]]s, it can take a negative charge (an [[anion]] known as a [[hydride]] and written as H<sup>−</sup>), or as a positively charged [[chemical species|species]] H<sup>+</sup>. The latter [[cation]] is written as though composed of a bare proton, but in reality, hydrogen cations in [[ionic compound]]s always occur as more complex species.
 
The most common [[isotope]] of hydrogen is [[hydrogen-1|protium]] (name rarely used, symbol <sup>1</sup>H) with a single proton and no [[neutron]]s. As the simplest atom known, the [[hydrogen atom]] has been of theoretical use. For example, as the only neutral atom with an analytic solution to the [[Schrödinger equation]], the study of the energetics and bonding of the hydrogen atom played a key role in the development of [[quantum mechanics]].
 
Hydrogen gas was first artificially produced in the early 16th century, via the mixing of metals with strong acids. In 1766–81, [[Henry Cavendish]] was the first to recognize that hydrogen gas was a discrete substance,<ref>
{{Cite episode
|title = Discovering the Elements
|url = http://www.bbc.co.uk/programmes/b00q2mk5
|series = Chemistry: A Volatile History
|credits = Presenter: Professor Jim Al-Khalili
|network = [[BBC]]
|station = [[BBC Four]]
|airdate = 2010-01-21
|minutes = 25:40
}}</ref> and that it produces water when burned, a property which later gave it its name: in Greek, hydrogen means "water-former".
 
Industrial production is mainly from the steam reforming of natural gas, and less often from more energy-intensive [[hydrogen production]] methods like the [[electrolysis of water]].<ref>{{cite web
|title=Hydrogen Basics&nbsp;— Production
|url=http://www.fsec.ucf.edu/en/consumer/hydrogen/basics/production.htm
|publisher=[[Florida Solar Energy Center]]
|year=2007
|accessdate=2008-02-05
}}</ref> Most hydrogen is employed near its production site, with the two largest uses being [[fossil fuel]] processing (e.g., [[hydrocracking]]) and [[ammonia]] production, mostly for the fertilizer market.
 
Hydrogen is a concern in [[metallurgy]] as it can [[hydrogen embrittlement|embrittle]] 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
|year=1999
|doi=10.1126/science.159.3819.1057
|pmid=17775040
|bibcode = 1968Sci...159.1057R }}</ref> complicating the design of pipelines and storage tanks.<ref name="Christensen">{{cite news
|last=Christensen|first=C.H.
|coauthors=Nørskov, J.K.; Johannessen, 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://www.dtu.dk/English/About_DTU/News.aspx?guid=%7BE6FF7D39-1EDD-41A4-BC9A-20455C2CF1A7%7D
|accessdate=2008-03-28
}}</ref>
 
==Properties==
===Combustion===
[[Image:Shuttle Main Engine Test Firing cropped edited and reduced.jpg|thumb|upright|left|The [[Space Shuttle Main Engine]] burnt hydrogen with oxygen, producing a nearly invisible flame at full thrust.|alt=A black cup-like object hanging by its bottom with blue glow coming out of its opening.]]
 
Hydrogen gas (dihydrogen or molecular hydrogen)<!--[[dihydrogen]] is a redirect to [[hydrogen]]--><ref>
{{cite web
|url=http://www.usm.maine.edu/~newton/Chy251_253/Lectures/LewisStructures/Dihydrogen.html
|title=Dihydrogen
|work=O{{=}}CHem Directory
|publisher=[[University of Southern Maine]]
|accessdate=2009-04-06
}}</ref> is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume.<ref>
{{cite journal
|last=Carcassi|first=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
|year=2005
|doi=10.1016/j.energy.2004.02.012
}}</ref> The [[enthalpy of combustion]] for hydrogen is −286&nbsp;kJ/mol:<ref>
{{cite book
|author=Committee on Alternatives and Strategies for Future Hydrogen Production and Use, [[United States National Research Council|US National Research Council]], [[United States National Academy of Engineering|US National Academy of Engineering]]
|year=2004
|title=The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs
|page=240
|publisher=[[National Academies Press]]
|isbn=0-309-09163-2|url=http://books.google.com/?id=ugniowznToAC&pg=PA240
}}</ref>
: 2 H<sub>2</sub>(g) + O<sub>2</sub>(g) → 2 H<sub>2</sub>O(l) + 572&nbsp;kJ (286&nbsp;kJ/mol)<ref group="note">286&nbsp;kJ/mol: energy per mole of the combustible material (hydrogen)</ref>
 
Hydrogen gas forms explosive mixtures with air if it is 4–74% concentrated and with chlorine if it is 5–95% concentrated. The mixtures spontaneously explode by spark, heat or sunlight. The hydrogen [[autoignition temperature]], the temperature of spontaneous ignition in air, is {{convert|500|C|F}}.<ref>{{cite book
|url=http://books.google.com/?id=-CRRJBVv5d0C&pg=PA402|page=402
|title=A comprehensive guide to the hazardous properties of chemical substances
|publisher=Wiley-Interscience|isbn=0-471-71458-5
|year=2007
|author=Patnaik, P
}}</ref> Pure hydrogen-oxygen flames emit [[ultraviolet]] light and are nearly invisible to the naked eye, as
illustrated by the faint plume of the [[Space Shuttle Main Engine]] compared to the highly visible plume of a [[Space Shuttle Solid Rocket Booster]]. The detection of a burning hydrogen leak may require a [[flame detector]]; such leaks can be very dangerous. The [[Hindenburg disaster|destruction of the Hindenburg airship]] was an infamous example of hydrogen combustion; the cause is debated, but the visible flames were the result of combustible materials in the ship's skin.<ref>
{{cite web
|last=Dziadecki|first=J.
|title=Hindenburg Hydrogen Fire
|url=http://spot.colorado.edu/~dziadeck/zf/LZ129fire.htm
|year=2005
|accessdate=2007-01-16
}}</ref> Because hydrogen is buoyant in air, hydrogen flames tend to ascend rapidly and cause less damage than hydrocarbon fires. Two-thirds of the Hindenburg passengers survived the fire, and many deaths were instead the result of falls or burning diesel fuel.<ref>
{{cite web
|last=Kelly|first=M.
|title=The Hindenburg Disaster
|url=http://americanhistory.about.com/od/hindenburg/a/hindenburg.htm
|publisher=About.com:American history
|accessdate=2009-08-08
}}</ref>
 
H<sub>2</sub> reacts with every oxidizing element. Hydrogen can react spontaneously and violently at room temperature with [[chlorine]] and [[fluorine]] to form the corresponding hydrogen halides, [[hydrogen chloride]] and [[hydrogen fluoride]], which are also potentially dangerous [[acid]]s.<ref>
{{cite book
|last=Clayton|first=D.D.
|title=Handbook of Isotopes in the Cosmos: Hydrogen to Gallium
|year=2003
|publisher=[[Cambridge University Press]]
|isbn=0-521-82381-1
}}</ref>
 
===Electron energy levels===
{{Main|Hydrogen atom}}
[[Image:hydrogen atom.svg|thumb|left|Depiction of a hydrogen atom with size of central proton shown, and the atomic diameter shown as about twice the [[Bohr model]] radius (image not to scale).|alt=Drawing of a light-gray large sphere with a cut off quarter and a black small sphere and numbers 1.7x10<sup>−5</sup> illustrating their relative diameters.]]
 
The [[ground state]] [[energy level]] of the electron in a hydrogen atom is −13.6&nbsp;[[Electronvolt|eV]], which is equivalent to an ultraviolet [[photon]] of roughly 92&nbsp;[[metre|nm]] wavelength.<ref>{{cite web
|url=http://jupiter.phy.umist.ac.uk/~tjm/ISPhys/l7/ispl7.html
|title=Lecture 7, Emission Lines&nbsp;— Examples
|accessdate=2008-02-05|last=Millar|first=Tom
|date=December 10, 2003
|work=PH-3009 (P507/P706/M324) Interstellar Physics
|publisher=University of Manchester}}</ref>
 
The energy levels of hydrogen can be calculated fairly accurately using the [[Bohr model]] of the atom, which conceptualizes the electron as "orbiting" the proton in analogy to the Earth's orbit of the Sun. However, the [[electromagnetic force]] attracts electrons and protons to one another, while planets and celestial objects are attracted to each other by [[gravity]]. Because of the discretization of [[angular momentum]] postulated in early [[quantum mechanics]] by Bohr, the electron in the Bohr model can only occupy certain allowed distances from the proton, and therefore only certain allowed energies.<ref>{{cite web
|last=Stern|first=David P.|date=2005-05-16
|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)
|accessdate=2007-12-20}}</ref>
 
A more accurate description of the hydrogen atom comes from a purely quantum mechanical treatment that uses the [[Schrödinger equation]] or the [[Richard Feynman|Feynman]] [[path integral formulation]] to calculate the [[probability amplitude|probability density]] of the electron around the proton.<ref>{{cite web| last=Stern|first=David P.|date=2005-02-13| url=http://www-spof.gsfc.nasa.gov/stargaze/Q7.htm| title=Wave Mechanics| publisher=NASA Goddard Space Flight Center| accessdate=2008-04-16}}</ref> The most complicated treatments allow for the small effects of [[special relativity]] and [[vacuum polarization]]. In the quantum mechanical treatment, the electron in a ground state hydrogen atom has no angular momentum at all— an illustration of how different the "planetary orbit" conception of electron motion differs from reality.
 
===Elemental molecular forms===
{{See also|Spin isomers of hydrogen}}
[[File:Liquid hydrogen bubblechamber.jpg|thumb|upright|First tracks observed in [[liquid hydrogen]] [[bubble chamber]] at the [[Bevatron]]|alt=Two bright circles on dark background, both contain numerous thin black lines inside.]]
There exist two different [[spin isomers of hydrogen]] diatomic molecules that differ by the relative [[spin (physics)|spin]] of their nuclei.<ref name="uigi">{{cite web|author=Staff|year=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.|accessdate=2008-02-05}}</ref> In the [[orthohydrogen]] form, the spins of the two protons are parallel and form a triplet state with a molecular spin quantum number of 1 (½+½); in the [[parahydrogen]] form the spins are antiparallel and form a singlet with a molecular spin quantum number of 0 (½–½). At standard temperature and pressure, hydrogen gas contains about 25% of the para form and 75% of the ortho form, also known as the "normal form".<ref name="Tikhonov">{{cite journal|last=Tikhonov|first=Vladimir I.|coauthors=Volkov, Alexander A.|title=Separation of Water into Its Ortho and Para Isomers|journal=Science|year=2002|volume=296|issue=5577|doi=10.1126/science.1069513|pmid=12089435|page=2363}}</ref> The equilibrium ratio of orthohydrogen to parahydrogen depends on temperature, but because the ortho form is an [[excited state]] and has a higher energy than the para form, it is unstable and cannot be purified. At very low temperatures, the equilibrium state is composed almost exclusively of the para form. The liquid and gas phase thermal properties of pure parahydrogen differ significantly from those of the normal form because of differences in rotational heat capacities, as discussed more fully in [[Spin isomers of hydrogen]].<ref name="NASA">{{cite web|last=Hritz|first=James|month=March|year=2006|url=http://smad-ext.grc.nasa.gov/gso/manual/chapter_06.pdf|format=PDF|title=CH. 6&nbsp;– Hydrogen|work=NASA Glenn Research Center Glenn Safety Manual, Document GRC-MQSA.001|publisher=NASA|accessdate=2008-02-05}}</ref> The ortho/para distinction also occurs in other hydrogen-containing molecules or functional groups, such as water and [[Methylenes|methylene]], but is of little significance for their thermal properties.<ref>{{cite journal| last=Shinitzky| first=Meir| last2=Elitzur| first2=Avshalom C.| title=Ortho-para spin isomers of the protons in the methylene group| journal=Chirality| volume=18| issue=9| pages=754–756|year=2006|doi=10.1002/chir.20319| pmid=16856167| last1=Shinitzky| first1=M}}</ref>
 
The uncatalyzed interconversion between para and ortho H<sub>2</sub> increases with increasing temperature; thus rapidly condensed H<sub>2</sub> contains large quantities of the high-energy ortho form that converts to the para form very slowly.<ref>{{cite journal|last=Milenko|first=Yu. Ya.|coauthors=Sibileva, R. M.; Strzhemechny, M. A|title=Natural ortho-para conversion rate in liquid and gaseous hydrogen|journal=Journal of Low Temperature Physics|year=1997|volume=107|issue=1–2|pages=77–92
|doi=10.1007/BF02396837|bibcode = 1997JLTP..107...77M }}</ref> The ortho/para ratio in condensed H<sub>2</sub> is an important consideration in the preparation and storage of liquid hydrogen: the conversion from ortho to para is [[exothermic]] and produces enough heat to evaporate some of the hydrogen liquid, leading to loss of liquefied material. [[Catalyst]]s for the ortho-para interconversion, such as [[ferric oxide]], [[activated carbon]], platinized asbestos, rare earth metals, uranium compounds,
[[chromic oxide]], or some nickel<ref>{{cite web|url=http://www.mae.ufl.edu/NasaHydrogenResearch/h2webcourse/L11-liquefaction2.pdf|title=Ortho-Para conversion. Pag. 13|format=PDF}}</ref> compounds, are used during hydrogen cooling.<ref name="Svadlenak">{{cite journal|last=Svadlenak|first=R. Eldo|coauthors=Scott, Allen B|title=The Conversion of Ortho- to Parahydrogen on Iron Oxide-Zinc Oxide Catalysts|journal=Journal of the American Chemical Society|year=1957|volume=79|issue=20|pages=5385–5388|doi=10.1021/ja01577a013}}</ref>
 
===Phases===
*[[Compressed hydrogen]]
*[[Liquid hydrogen]]
*[[Slush hydrogen]]
*[[Solid hydrogen]]
*[[Metallic hydrogen]]
 
===Compounds===
{{further2|[[:Category:Hydrogen compounds|Hydrogen compounds]]}}
 
====Covalent and organic compounds====
While H<sub>2</sub> is not very reactive under standard conditions, it does form compounds with most elements. Hydrogen can form compounds with elements that are more [[electronegative]], such as [[halogen]]s (e.g., F, Cl, Br, I), or [[oxygen]]; in these compounds hydrogen takes on a partial positive charge.<ref>{{cite web
|last=Clark| first=Jim
|title=The Acidity of the Hydrogen Halides| work=Chemguide
|year=2002| url=http://www.chemguide.co.uk/inorganic/group7/acidityhx.html#top
|accessdate=2008-03-09}}</ref> When bonded to [[fluorine]], [[oxygen]], or [[nitrogen]], hydrogen can participate in a form of medium-strength noncovalent bonding called [[hydrogen bond]]ing, which is critical to the stability of many biological molecules.<ref>{{cite web
|last=Kimball| first=John W.
|title=Hydrogen| work=Kimball's Biology Pages| date=2003-08-07
|url=http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/H/HydrogenBonds.html
|accessdate=2008-03-04}}</ref><ref>IUPAC Compendium of Chemical Terminology, Electronic version, [http://goldbook.iupac.org/H02899.html Hydrogen Bond]</ref> Hydrogen also forms compounds with less electronegative elements, such as the [[metal]]s and [[metalloid]]s, in which it takes on a partial negative charge. These compounds are often known as [[hydride]]s.<ref>{{cite web
|last=Sandrock| first=Gary
|title=Metal-Hydrogen Systems| publisher=Sandia National Laboratories
|date=2002-05-02| url=http://hydpark.ca.sandia.gov/DBFrame.html
|accessdate=2008-03-23}}</ref>
 
Hydrogen forms a vast array of compounds with [[carbon]] called the [[hydrocarbon]]s, and an even vaster array with [[heteroatoms]] that, because of their general association with living things, are called [[organic compound]]s.<ref name="hydrocarbon">{{cite web| title=Structure and Nomenclature of Hydrocarbons|publisher=Purdue University| url=http://chemed.chem.purdue.edu/genchem/topicreview/bp/1organic/organic.html| accessdate=2008-03-23}}</ref> The study of their properties is known as [[organic chemistry]]<ref>{{cite web| title=Organic Chemistry| work=Dictionary.com| publisher=Lexico Publishing Group| year=2008| url=http://dictionary.reference.com/browse/organic%20chemistry| accessdate=2008-03-23}}</ref> and their study in the context of living [[organism]]s is known as [[biochemistry]].<ref>{{cite web
|title=Biochemistry| work=Dictionary.com
|publisher=Lexico Publishing Group
|year=2008
|url=http://dictionary.reference.com/browse/biochemistry
|accessdate=2008-03-23}}</ref> By some definitions, "organic" compounds are only required to contain carbon. However, most of them also contain hydrogen, and because it is the carbon-hydrogen bond which gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry.<ref name="hydrocarbon" /> Millions of [[hydrocarbon]]s are known, and they are usually formed by complicated synthetic pathways, which seldom involve elementary hydrogen.
 
====Hydrides====
Compounds of hydrogen are often called [[hydride]]s, a term that is used fairly loosely. The term "hydride" suggests that the H atom has acquired a negative or anionic character, denoted H<sup>−</sup>, and is used when hydrogen forms a compound with a more [[electropositive]] element. The existence of the hydride anion, suggested by [[Gilbert N. Lewis]] in 1916 for group I and II salt-like hydrides, was demonstrated by Moers in 1920 by the electrolysis of molten [[lithium hydride]] (LiH), producing a [[stoichiometric|stoichiometry]] quantity of hydrogen at the anode.<ref name="Moers">{{cite journal
|last=Moers|first=Kurt
|title=Investigations on the Salt Character of Lithium Hydride
|journal=Zeitschrift für Anorganische und Allgemeine Chemie
|year=1920|volume=113|issue=191|pages=179–228|doi=10.1002/zaac.19201130116
}}</ref> For hydrides other than group I and II metals, the term is quite misleading, considering the low electronegativity of hydrogen. An exception in group II hydrides is {{chem|BeH|2}}, which is polymeric. In [[lithium aluminium hydride]], the {{chem|AlH|4|-}} anion carries hydridic centers firmly attached to the Al(III).
 
Although hydrides can be formed with almost all main-group elements, the number and combination of possible compounds varies widely; for example, there are over 100 binary borane hydrides known, but only one binary aluminium hydride.<ref name="Downs">{{cite journal
|last=Downs|first=Anthony J.
|coauthors=Pulham, Colin R.
|title=The hydrides of aluminium, gallium, indium, and thallium: a re-evaluation
|journal=Chemical Society Reviews
|year=1994|volume=23|pages=175–184
|doi=10.1039/CS9942300175
|issue=3
}}</ref> Binary [[indium]] hydride has not yet been identified, although larger complexes exist.<ref name="Hibbs">{{cite journal
|last=Hibbs|first=David E.
|coauthors=Jones, Cameron; Smithies, Neil A.
|title=A remarkably stable indium trihydride complex: synthesis and characterisation of [InH<sub>3</sub>P(C<sub>6</sub>H<sub>11</sub>)<sub>3</sub>]
|journal=Chemical Communications
|year=1999|pages=185–186|doi=10.1039/a809279f
|issue=2}}</ref>
 
In [[inorganic chemistry]], hydrides can also serve as [[bridging ligand]]s that link two metal centers in a [[coordination complex]]. This function is particularly common in [[group 13 element]]s, especially in [[borane]]s ([[boron]] hydrides) and [[aluminium]] complexes, as well as in clustered [[carborane]]s.<ref name="Miessler" />
 
====Protons and acids====
{{See|Acid–base reaction}}
Oxidation of hydrogen removes its electron and gives H<sup>+</sup>, which contains no electrons and a [[atomic nucleus|nucleus]] which is usually composed of one proton. That is why {{chem|H|+}} is often called a proton. This species is central to discussion of [[acid]]s. Under the [[Bronsted-Lowry theory]], acids are proton donors, while bases are proton acceptors.
 
A bare proton, {{chem|H|+}}, cannot exist in solution or in ionic crystals, because of its unstoppable attraction to other atoms or molecules with electrons. Except at the high temperatures associated with plasmas, such protons cannot be removed from the [[electron cloud]]s of atoms and molecules, and will remain attached to them. However, the term 'proton' is sometimes used loosely and metaphorically to refer to positively charged or [[cation]]ic hydrogen attached to other species in this fashion, and as such is denoted "{{chem|H|+}}" without any implication that any single protons exist freely as a species.
 
To avoid the implication of the naked "solvated proton" in solution, acidic aqueous solutions are sometimes considered to contain a less unlikely fictitious species, termed the "[[hydronium]] ion" ({{chem|H|3|O|+}}). However, even in this case, such solvated hydrogen cations are thought more realistically physically to be organized into clusters that form species closer to {{chem|H|9|O|4|+}}.<ref name="Okumura">{{cite journal
|last=Okumura|first=Anthony M.
|coauthors=Yeh, L. I.; Myers, J. D.; Lee, 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
|year=1990|volume=94|issue=9|pages=3416–3427|doi=10.1021/j100372a014
}}</ref> Other [[oxonium ion]]s are found when water is in solution with other solvents.<ref name="Perdoncin">{{cite journal
|last=Perdoncin|first=Giulio|coauthors=Scorrano, Gianfranco
|title=Protonation Equilibria in Water at Several Temperatures of Alcohols, Ethers, Acetone, Dimethyl Sulfide, and Dimethyl Sulfoxide
|journal=Journal of the American Chemical Society
|year=1977|volume=99|issue=21|pages=6983–6986
|doi=10.1021/ja00463a035
}}</ref>
 
Although exotic on Earth, one of the most common ions in the universe is the {{chem|H|3|+}} ion, known as [[Trihydrogen cation|protonated molecular hydrogen]] or the trihydrogen cation.<ref name="Carrington">{{cite journal
|last=Carrington|first=Alan|coauthors=R. McNab, Iain
|title=The infrared predissociation spectrum of triatomic hydrogen cation (H<sub>3</sub><sup>+</sup>)
|journal=Accounts of Chemical Research
|year=1989|volume=22|issue=6|pages=218–222
|doi=10.1021/ar00162a004}}</ref>
 
===Isotopes===
{{Main|Isotopes of hydrogen}}
[[File:Hydrogen discharge tube.jpg|thumb|Hydrogen discharge (spectrum) tube]]
[[File:Deuterium discharge tube.jpg|thumb|Deuterium discharge (spectrum) tube]]
[[Image:Protium.svg|thumb|upright|Protium, the most common [[isotope]] of hydrogen, has one proton and one electron. Unique among all stable isotopes, it has no neutrons (see [[diproton]] for a discussion of why others do not exist).|alt=Schematic drawing of a positive atom in the center orbited by a negative particle.]]
 
Hydrogen has three naturally occurring isotopes, denoted {{chem|1|H}}, {{chem|2|H}} and {{chem|3|H}}. Other, highly unstable nuclei ({{chem|4|H}} to {{chem|7|H}}) have been synthesized in the laboratory but not observed in nature.<ref name="Gurov">{{cite journal
|author=Gurov, Yu. B.; Aleshkin, D. V.; Behr, M. N.; Lapushkin, S. V.; Morokhov, P. V.; Pechkurov, V. A.; Poroshin, N. O.; Sandukovsky, V. G.; Tel'kushev, M. V.; Chernyshev, B. A.; Tschurenkova, T. D
|title=Spectroscopy of superheavy hydrogen isotopes in stopped-pion absorption by nuclei
|journal=Physics of Atomic Nuclei
|year=2004|volume=68|issue=3|pages=491–97
|doi=10.1134/1.1891200
|bibcode = 2005PAN....68..491G }}</ref><ref name="Korsheninnikov">{{cite journal
|title=Experimental Evidence for the Existence of <sup>7</sup>H and for a Specific Structure of <sup>8</sup>He
|journal=Physical Review Letters
|year=2003|volume=90|issue=8|page=082501
|doi=10.1103/PhysRevLett.90.082501|bibcode=2003PhRvL..90h2501K
|last1=Korsheninnikov
|first1=A.
|last2=Nikolskii
|first2=E.
|last3=Kuzmin
|first3=E.
|last4=Ozawa
|first4=A.
|last5=Morimoto
|first5=K.
|last6=Tokanai
|first6=F.
|last7=Kanungo
|first7=R.
|last8=Tanihata
|first8=I.
|last9=Timofeyuk
|first9=N.}}</ref>
*'''{{chem|1|H}}''' is the most common hydrogen isotope with an abundance of more than 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
|last=Urey|first=Harold C.
|coauthors=Brickwedde, F. G.; Murphy, G. M.
|title=Names for the Hydrogen Isotopes
|journal=Science|year=1933|volume=78
|issue=2035|pages=602–603
|doi=10.1126/science.78.2035.602
|pmid=17797765|bibcode = 1933Sci....78..602U }}</ref>
*'''{{chem|2|H}}''', the other stable hydrogen isotope, is known as ''[[deuterium]]'' and contains one proton and one [[neutron]] in its nucleus. Essentially all deuterium in the universe is thought to have been produced at the time of the [[Big Bang]], and has endured since that time. Deuterium is not radioactive, and does not represent 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; Nakamura, H.; Yamazaki, T.; Nagayama, K.; Yoshida, M.; Kanaya, S.; Ikehara, M.
|title=1H NMR studies of deuterated ribonuclease HI selectively labeled with protonated amino acids
|journal=[[Journal of Biomolecular NMR]]
|year=1992|volume=2|issue=2|pages=137–47
|doi=10.1007/BF01875525
|pmid=1330130}}</ref> Heavy water is used as a [[neutron moderator]] and coolant for nuclear reactors. Deuterium is also a potential fuel for commercial [[nuclear fusion]].<ref>{{cite news
|last=Broad|first=William J.
|date=November 11, 1991
|title=Breakthrough in Nuclear Fusion Offers Hope for Power of Future
|work=The New York Times
|url=http://query.nytimes.com/gst/fullpage.html?res=9D0CE4D81030F932A25752C1A967958260&sec=&spon=&pagewanted=all
|accessdate=2008-02-12}}</ref>
*'''{{chem|3|H}}''' is known as ''[[tritium]]'' and contains one proton and two neutrons in its nucleus. It is radioactive, decaying into [[helium-3]] through [[beta decay]] with a [[half-life]] of 12.32 years.<ref name="Miessler" /> It is so radioactive that it can be used in [[Radioluminescent paint|luminous paint]], making it useful in such things as watches. The glass prevents the small amount of radiation from getting out.<ref>''The Elements'', Theodore Gray, Black Dog & Leventhal Publishers Inc., 2009</ref> Small amounts of tritium occur naturally because of the interaction of cosmic rays with atmospheric gases; tritium has also been released during [[nuclear testing|nuclear weapons tests]].<ref>{{cite web| author=Staff|date=November 15, 2007| url=http://www.epa.gov/rpdweb00/radionuclides/tritium.html| publisher=U.S. Environmental Protection Agency| title=Tritium|accessdate=2008-02-12}}</ref> It is used in nuclear fusion reactions,<ref>{{cite web| last=Nave| first=C. R.|title=Deuterium-Tritium Fusion| work=HyperPhysics| publisher=Georgia State University| year=2006| url=http://hyperphysics.phy-astr.gsu.edu/Hbase/nucene/fusion.html| accessdate=2008-03-08}}</ref> as a tracer in [[isotope geochemistry]],<ref>{{cite journal| first=Carol| last=Kendall| first2=Eric| last2=Caldwell| title=Fundamentals of Isotope Geochemistry| publisher=US Geological Survey| year=1998| url=http://wwwrcamnl.wr.usgs.gov/isoig/isopubs/itchch2.html#2.5.1| accessdate=2008-03-08}}</ref> and specialized in [[self-powered lighting]] devices.<ref>{{cite web| title=The Tritium Laboratory| publisher=University of Miami| year=2008| url=http://www.rsmas.miami.edu/groups/tritium/| accessdate=2008-03-08}}</ref> Tritium has also been used in chemical and biological labeling experiments as a [[radiolabel]].<ref name="holte">{{cite journal| last=Holte| first=Aurali E.| last2=Houck| first2=Marilyn A.| last3=Collie| first3=Nathan 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| publisher=Texas Tech University| location=Lubbock| year=2004|doi=10.1023/A:1010655610575}}</ref>
 
Hydrogen is the only element that has different names for its isotopes in common use today. During the early study of radioactivity, various heavy radioactive isotopes were given their own names, but such names are no longer used, except for deuterium and tritium. The symbols D and T (instead of {{chem|2|H}} and {{chem|3|H}}) are sometimes used for deuterium and tritium, but the corresponding symbol for protium, P, is already in use for [[phosphorus]] and thus is not available for protium.<ref>{{cite web
|last=van der Krogt|first=Peter|date=May 5, 2005
|url=http://elements.vanderkrogt.net/element.php?sym=H
|publisher=Elementymology & Elements Multidict
|title=Hydrogen|accessdate=2010-12-20}}</ref> In its [[IUPAC nomenclature|nomenclatural]] guidelines, the [[International Union of Pure and Applied Chemistry]] allows any of D, T, {{chem|2|H}}, and {{chem|3|H}} to be used, although {{chem|2|H}} and {{chem|3|H}} are preferred.<ref>§ IR-3.3.2, [http://www.iupac.org/reports/provisional/abstract04/connelly_310804.html Provisional Recommendations], Nomenclature of Inorganic Chemistry, Chemical Nomenclature and Structure Representation Division, IUPAC. Accessed on line October 3, 2007.</ref>
 
==History==
===Discovery and use===
{{Main|Timeline of hydrogen technologies}}
In 1671, [[Robert Boyle]] discovered and described the reaction between [[iron]] filings and dilute [[acid]]s, which results in the production of hydrogen gas.<ref>Boyle, Robert "Tracts written by the Honourable Robert Boyle containing new experiments, touching the relation betwixt flame and air..." (London, England: 1672).</ref><ref>{{cite web
|first=Mark|last=Winter|year=2007
|url=http://education.jlab.org/itselemental/ele001.html
|title=Hydrogen: historical information
|publisher=WebElements Ltd
|accessdate=2008-02-05}}</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]] "flammable air". He speculated that "flammable air" was in fact identical to the hypothetical substance called "[[Phlogiston theory|phlogiston]]"<ref>{{cite web
| title = Why did oxygen supplant phlogiston? Research programmes in the Chemical Revolution – Cambridge Books Online – Cambridge University Press
| accessdate = 2011-10-22
| url = http://ebooks.cambridge.org/chapter.jsf?bid=CBO9780511760013&cid=CBO9780511760013A009
}}</ref><ref>Just the Facts—Inventions & Discoveries, School Specialty Publishing, 2005</ref> and further finding in 1781 that the gas produces water when burned. He is usually given credit for its discovery 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=0-471-61525-0}}</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=0-19-850341-5}}</ref> In 1783, [[Antoine Lavoisier]] gave the element the name hydrogen (from the Greek ''ὕδρω'' ''hydro'' meaning water and ''γενῆς'' ''genes'' meaning creator)<ref name="Stwertka">{{cite book| last=Stwertka| first=Albert| title=A Guide to the Elements| publisher=Oxford University Press| year=1996| pages=16–21| isbn=0-19-508083-1}}</ref> when he and [[Laplace]] reproduced Cavendish's finding that water is produced when hydrogen is burned.<ref name="nbb" />
[[File:Antoine-Laurent Lavoisier (by Louis Jean Desire Delaistre)RENEW.jpg|thumb|180px|Antoine-Laurent de Lavoisier]]
Lavoisier produced hydrogen for his famous experiments on mass conservation by reacting a flux of steam with metallic [[iron]] through an incandescent iron tube heated in a fire. Anaerobic oxidation of iron by the protons of water at high temperature can be schematically represented by the set of following reactions:
 
:&nbsp;&nbsp; Fe + &nbsp;&nbsp; H<sub>2</sub>O → FeO + H<sub>2</sub>
 
:2 Fe + 3 H<sub>2</sub>O → Fe<sub>2</sub>O<sub>3</sub> + 3 H<sub>2</sub>
 
:3 Fe + 4 H<sub>2</sub>O → Fe<sub>3</sub>O<sub>4</sub> + 4 H<sub>2</sub>
 
Many metals such as [[zirconium]] undergo a similar reaction with water leading to the production of hydrogen.
 
Hydrogen was [[Liquid hydrogen|liquefied]] for the first time by [[James Dewar]] in 1898 by using [[regenerative cooling]] and his invention, the [[vacuum flask]].<ref name="nbb" /> He produced [[solid hydrogen]] the next year.<ref name="nbb" /> [[Deuterium]] was discovered in December 1931 by [[Harold Urey]], and [[tritium]] was prepared in 1934 by [[Ernest Rutherford]], [[Mark Oliphant]], and [[Paul Harteck]].<ref name="Nostrand" /> [[Heavy water]], which consists of deuterium in the place of regular hydrogen, was discovered by Urey's group in 1932.<ref name="nbb" /> François Isaac de Rivaz built the first internal combustion engine powered by a mixture of hydrogen and oxygen in 1806. [[Edward Daniel Clarke]] invented the hydrogen gas blowpipe in 1819. The [[Döbereiner's lamp]] and [[limelight]] were invented in 1823.<ref name="nbb" />
 
The first hydrogen-filled [[balloon]] was invented by [[Jacques Charles]] in 1783.<ref name="nbb" /> Hydrogen provided the lift for the first reliable form of air-travel following the 1852 invention of the first hydrogen-lifted airship by [[Henri Giffard]].<ref name="nbb" /> German count [[Ferdinand von Zeppelin]] promoted the idea of rigid airships lifted by hydrogen that later were called [[Zeppelin]]s; the first of which had its maiden flight in 1900.<ref name="nbb" /> Regularly scheduled flights started in 1910 and by the outbreak of World War I in August 1914, they had carried 35,000 passengers without a serious incident. Hydrogen-lifted airships were used as observation platforms and bombers during the war.
 
The first non-stop transatlantic crossing was made by the British airship ''[[R34 (airship)|R34]]'' in 1919. Regular passenger service resumed in the 1920s and the discovery of [[helium]] reserves in the United States promised increased safety, but the U.S. government refused to sell the gas for this purpose. Therefore, H<sub>2</sub> was used in the [[LZ 129 Hindenburg|''Hindenburg'']] airship, which was destroyed in a midair fire over [[New Jersey]] on May 6, 1937.<ref name="nbb" /> The incident was broadcast live on radio and filmed. Ignition of leaking hydrogen is widely assumed to be the cause, but later investigations pointed to the ignition of the [[aluminium|aluminized]] fabric coating by [[static electricity]]. But the damage to hydrogen's reputation as a [[lifting gas]] was already done.
 
In the same year the first [[hydrogen-cooled turbogenerator]] went into service with gaseous hydrogen as a [[coolant]] in the rotor and the stator in 1937 at [[Dayton, Ohio|Dayton]], Ohio, by the Dayton Power & Light Co,<ref>{{cite web|url=http://www.archive.org/stream/chronologicalhis00natirich/chronologicalhis00natirich_djvu.txt|title=A chronological history of electrical development from 600 B.C|publisher=Archive.org|accessdate=2009-04-06}}</ref> because of the thermal conductivity of hydrogen gas this is the most common type in its field today.
 
The [[nickel hydrogen battery]] was used for the first time in 1977 aboard the U.S. Navy's Navigation technology satellite-2 (NTS-2).<ref>{{cite web|url=http://www.aiaa.org/content.cfm?pageid=406&gTable=japaperimportPre97&gID=57704|title=NTS-2 Nickel-Hydrogen Battery Performance 31|publisher=Aiaa.org|accessdate=2009-04-06}}</ref> For example, the [[ISS]],<ref>{{cite journal|url=http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20020070612_2002115777.pdf|title=IECEC '02. 2002 37th Intersociety Energy Conversion Engineering Conference, 2002|pages=45–50|year=2004 (2002)|accessdate=2011-11-11|doi=10.1109/IECEC.2002.1391972|chapter=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 }}</ref> [[2001 Mars Odyssey|Mars Odyssey]]<ref>{{cite journal|doi=10.1109/AERO.2002.1035418 |title=A lightweight high reliability single battery power system for interplanetary spacecraft|chapter=A lightweight, high reliability, single battery power system for interplanetary spacecraft|year=2002|last1=Anderson|first1=P.M.|last2=Coyne|first2=J.W.|isbn=0-7803-7231-X|volume=5|pages=5–2433}}</ref> and the [[Mars Global Surveyor]]<ref>{{cite web|url=http://www.astronautix.com/craft/marveyor.htm|title=Mars Global Surveyor|publisher=Astronautix.com|accessdate=2009-04-06}}</ref> are equipped with nickel-hydrogen batteries.
In the dark part of its orbit, the [[Hubble Space Telescope]] is also powered by nickel-hydrogen batteries, which were finally replaced in May 2009, more than 19 years after launch, and 13 years over their design life.
 
===Role in quantum theory===
[[File:Emission spectrum-H.svg|500px|thumb|Hydrogen emission spectrum lines in the visible range. These are the four visible lines of the [[Balmer series]]|alt=A line spectrum showing black background with narrow lines superimposed on it: two violet, one blue and one red.]]
Because of its relatively simple atomic structure, consisting only of a proton and an electron, the [[hydrogen atom]], together with the spectrum of light produced from it or absorbed by it, has been central to the development of the theory of [[atom]]ic structure.<ref>{{cite book
|last=Crepeau
|first=Bob
|title=Niels Bohr: The Atomic Model
|journal=Great Scientific Minds
|publisher=Great Neck Publishing
|date=2006-01-01
|isbn=1-4298-0723-7}}</ref> Furthermore, the corresponding simplicity of the hydrogen molecule and the corresponding cation [[H2+|H<sub>2</sub><sup>+</sup>]] allowed fuller understanding of the nature of the [[chemical bond]], which followed shortly after the quantum mechanical treatment of the hydrogen atom had been developed in the mid-1920s.
 
One of the first quantum effects to be explicitly noticed (but not understood at the time) was a Maxwell observation involving hydrogen, half a century before full [[Quantum mechanics|quantum mechanical theory]] arrived. Maxwell observed that the [[specific heat capacity]] of H<sub>2</sub> unaccountably departs from that of a [[diatomic]] gas below room temperature and begins to increasingly resemble that of a monatomic gas at cryogenic temperatures. According to quantum theory, this behavior arises from the spacing of the (quantized) rotational energy levels, which are particularly wide-spaced in H<sub>2</sub> 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
|last=Berman|first=R.|coauthors=Cooke, A. H.; Hill, R. W.
|title=Cryogenics|journal=Annual Review of Physical Chemistry
|year=1956|volume=7|pages=1–20
|doi=10.1146/annurev.pc.07.100156.000245|bibcode = 1956ARPC....7....1B }}</ref>
 
==Natural occurrence==
[[Image:Nursery of New Stars - GPN-2000-000972.jpg|left|thumb|[[NGC 604]], a giant [[H II region|region of ionized hydrogen]] in the [[Triangulum Galaxy]]|alt=A white-green cotton-like clog on black background.]]
Hydrogen, as atomic H, is the most [[Natural abundance|abundant]] [[chemical element]] in the universe, making up 75% of [[Baryon|normal matter]] by [[mass]] and over 90% by number of atoms (most of the mass of the universe, however, is not in the form of chemical-element type matter, but rather is postulated to occur as yet-undetected forms of mass such as [[dark matter]] and [[dark energy]]).<ref>{{cite web
|first=Steve|last=Gagnon
|url=http://education.jlab.org/itselemental/ele001.html
|title=Hydrogen|publisher=Jefferson Lab
|accessdate=2008-02-05}}</ref> This element is found in great abundance in stars and [[gas giant]] planets. [[Molecular cloud]]s of H<sub>2</sub> are associated with [[star formation]]. Hydrogen plays a vital role in powering [[star]]s through [[proton-proton reaction]] and [[CNO cycle]] [[nuclear fusion]].<ref>{{cite web
|last=Haubold|first=Hans|coauthors=Mathai, A. M.
|date=November 15, 2007
|url=http://liveweb.archive.org/http://neutrino.aquaphoenix.com/un-esa/sun/sun-chapter4.html
|title=Solar Thermonuclear Energy Generation
|publisher=[[Columbia University]]|accessdate=2008-02-12
}}</ref>
 
Throughout the universe, hydrogen is mostly found in the [[atom]]ic and [[Plasma (physics)|plasma]] states whose properties are quite different from molecular hydrogen. As a plasma, hydrogen's electron and proton are not bound together, resulting in very high electrical conductivity and high emissivity (producing the light from the Sun and other stars). The charged particles are highly influenced by magnetic and electric fields. For example, in the [[solar wind]] they interact with the Earth's [[magnetosphere]] giving rise to [[Birkeland current]]s and the [[Aurora (phenomenon)|aurora]]. Hydrogen is found in the neutral atomic state in the [[Interstellar medium]]. The large amount of neutral hydrogen found in the damped Lyman-alpha systems is thought to dominate the cosmological baryonic density of the [[Universe]] up to [[redshift]] ''z''=4.<ref>{{cite journal
|last=Storrie-Lombardi|first=Lisa J.
|coauthors=Wolfe, Arthur M.
|title=Surveys for z > 3 Damped Lyman-alpha Absorption Systems: the Evolution of Neutral Gas
|journal=Astrophysical Journal
|year=2000|volume=543|pages=552–576
|arxiv=astro-ph/0006044
|doi=10.1086/317138
|bibcode=2000ApJ...543..552S
|issue=2}}</ref>
 
Under ordinary conditions on Earth, elemental hydrogen exists as the diatomic gas, H<sub>2</sub> (for data see table). However, hydrogen gas is very rare in the Earth's atmosphere (1 [[part per million|ppm]] by volume) because of its light weight, which enables it to [[atmospheric escape|escape from Earth's gravity]] more easily than heavier gases. However, hydrogen is the third most abundant element on the Earth's surface,<ref name="ArgonneBasic">{{cite web
|author=Dresselhaus, Mildred et al.|date=May 15, 2003
|url=http://www.sc.doe.gov/bes/hydrogen.pdf|format=PDF
|title=Basic Research Needs for the Hydrogen Economy
|publisher=Argonne National Laboratory, U.S. Department of Energy, Office of Science Laboratory
|accessdate=2008-02-05
}}</ref> mostly in the form of [[chemical compound]]s such as [[hydrocarbon]]s and water.<ref name="Miessler">{{cite book
|first=Gary L.|last=Miessler|coauthors=Tarr, Donald A.
|year=2003|title=Inorganic Chemistry|edition=3rd
|publisher=Prentice Hall|isbn=0-13-035471-6}}</ref> Hydrogen gas is produced by some bacteria and [[algae]] and is a natural component of [[flatus]], as is [[methane]], itself a hydrogen source of increasing importance.<ref>{{cite web
|first=Wolfgang H.|last=Berger
|date=November 15, 2007
|url=http://earthguide.ucsd.edu/virtualmuseum/climatechange2/11_3.shtml
|title=The Future of Methane
|publisher=University of California, San Diego
|accessdate=2008-02-12}}</ref>
 
A molecular form called [[protonated molecular hydrogen]] ({{chem|H|3}}<sup><span style="font-size:98%">{{{p|+}}}</span></sup>) is found in the [[interstellar medium]] (ISM), where it is generated by ionization of molecular hydrogen from [[cosmic ray]]s. This charged ion has also been observed in the upper atmosphere of the planet [[Jupiter]]. The ion is relatively stable in the environment of outer space due to the low temperature and density. {{chem|H|3}}<sup><span style="font-size:98%">{{{p|+}}}</span></sup> 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, Oka Group|date=April 22, 2005|url=http://h3plus.uiuc.edu/|title=H3+ Resource Center|publisher=Universities of Illinois and Chicago|accessdate=2008-02-05}}</ref>
Neutral [[triatomic hydrogen]] H<sub>3</sub> can only exist in an excited form and is unstable.<ref name=couple>{{cite web|url=http://frhewww.physik.uni-freiburg.de/H3/guber4.pdf|title=Coupling of Bound States to Continuum States in Neutral Triatomic Hydrogen|publisher=Department of Molecular and Optical Physics, University of Freiburg, Germany|author=Helm, H. ''et al.''|accessdate=2009-11-25}}</ref> By contrast, the positive [[hydrogen molecular ion]] ({{chem|H|2}}<sup><span style="font-size:98%">{{{p|+}}}</span></sup>) is a rare molecule in the universe.
 
==Production==
{{Details|Hydrogen production}}
H<sub>2</sub> is produced in chemistry and biology laboratories, often as a by-product of other reactions; in industry for the [[hydrogenation]] of [[Saturation (chemistry)|unsaturated]] substrates; and in nature as a means of expelling [[redox|reducing]] equivalents in biochemical reactions.
 
===Laboratory===
In the [[laboratory]], H<sub>2</sub> is usually prepared by the reaction of dilute [[oxidizing acid|non-oxidizing acids]] on some reactive metals such as [[zinc]] with [[Kipp's apparatus]].
:Zn + 2 {{chem|H|+}} → {{chem|Zn|2+}} + {{chem|H|2}}
 
[[Aluminium]] can also produce {{chem|H|2}} upon treatment with bases:
:2 Al + 6 {{chem|H|2|O}} + 2 {{chem|OH|-}} → 2 {{chem|Al(OH)|4|-}} + 3 {{chem|H|2}}
 
The [[electrolysis of water]] is a simple method of producing hydrogen. A low voltage current is run through the water, and gaseous oxygen forms at the [[anode]] while gaseous hydrogen forms at the [[cathode]]. Typically the cathode is made from platinum or another inert metal when producing hydrogen for storage. If, however, the gas is to be burnt on site, oxygen is desirable to assist the combustion, and so both electrodes would be made from inert metals. (Iron, for instance, would oxidize, and thus decrease the amount of oxygen given off.) The theoretical maximum efficiency (electricity used vs. energetic value of hydrogen produced) is in the range 80–94%.<ref>{{cite web
|last=Kruse|first=B.
|coauthors=Grinna, S.; Buch, C.
|year=2002
|url=http://bellona.org/filearchive/fil_Hydrogen_6-2002.pdf
|format=PDF
|title=Hydrogen Status og Muligheter
|publisher=Bellona
|accessdate=2008-02-12
}}</ref>
 
:2 {{chem|H|2|O}}(l) → 2 {{chem|H|2}}(g) + {{chem|O|2}}(g)
 
In 2007, it was discovered that an alloy of aluminium and [[gallium]] in pellet form added to water could be used to generate hydrogen. The process also creates [[alumina]], but the expensive gallium, which prevents the formation of an oxide skin on the pellets, can be re-used. This has important potential implications for a hydrogen economy, as hydrogen can be produced on-site and does not need to be transported.<ref>{{cite web| last=Venere|first=Emil|date=May 15, 2007| url=http://news.uns.purdue.edu/x/2007a/070515WoodallHydrogen.html| title=New process generates hydrogen from aluminum alloy to run engines, fuel cells| publisher=Purdue University|accessdate=2008-02-05}}</ref>
 
===Industrial===
{{Main|Hydrogen production}}
Hydrogen can be prepared in several different ways, but economically the most important processes involve removal of hydrogen from hydrocarbons. Commercial bulk hydrogen is usually produced by the [[steam reforming]] of [[natural gas]].<ref name="Oxtoby">{{cite book
|first=D. W.|last=Oxtoby|year=2002
|title=Principles of Modern Chemistry
|edition=5th|publisher=Thomson Brooks/Cole
|isbn=0-03-035373-4}}</ref> At high temperatures (1000–1400&nbsp;K, 700–1100&nbsp;°C or 1300–2000&nbsp;°F), steam (water vapor) reacts with [[methane]] to yield [[carbon monoxide]] and {{chem|H|2}}.
:{{chem|CH|4}} + {{chem|H|2|O}} → CO + 3 {{chem|H|2}}
 
This reaction is favored at low pressures but is nonetheless conducted at high pressures (2.0 &nbsp;MPa, 20&nbsp;atm or 600&nbsp;[[inHg]]). This is because high-pressure {{chem|H|2}} is the most marketable product and [[Pressure Swing Adsorption]] (PSA) purification systems work better at higher pressures. The product mixture is known as "[[synthesis gas]]" because it is often used directly for the production of [[methanol]] and related compounds. [[Hydrocarbon]]s other than methane can be used to produce synthesis gas with varying product ratios. One of the many complications to this highly optimized technology is the formation of coke or carbon:
:{{chem|CH|4}} → C + 2 H<sub>2</sub>
 
Consequently, steam reforming typically employs an excess of {{chem|H|2|O}}. Additional hydrogen can be recovered from the steam by use of carbon monoxide through the [[water gas shift reaction]], especially with an [[iron oxide]] catalyst. This reaction is also a common industrial source of [[carbon dioxide]]:<ref name="Oxtoby" />
:CO + {{chem|H|2|O}} → {{chem|CO|2}} + {{chem|H|2}}
 
Other important methods for {{chem|H|2}} production include partial oxidation of hydrocarbons:<ref>{{cite web| title=Hydrogen Properties, Uses, Applications| publisher=Universal Industrial Gases, Inc.| year=2007| url=http://www.uigi.com/hydrogen.html| accessdate=2008-03-11}}</ref>
:2 {{chem|CH|4}} + {{chem|O|2}} → 2 CO + 4 {{chem|H|2}}
 
and the coal reaction, which can serve as a prelude to the shift reaction above:<ref name="Oxtoby" />
:C + {{chem|H|2|O}} → CO + {{chem|H|2}}
 
Hydrogen is sometimes produced and consumed in the same industrial process, without being separated. In the [[Haber process]] for the [[Ammonia production|production of ammonia]], hydrogen is generated from natural gas.<ref>{{cite web| last=Funderburg| first=Eddie| title=Why Are Nitrogen Prices So High?| publisher=The Samuel Roberts Noble Foundation|year=2008| url=http://www.noble.org/Ag/Soils/NitrogenPrices/Index.htm| accessdate=2008-03-11}}</ref> [[Electrolysis]] of [[brine]] to yield [[chlorine]] also produces hydrogen as a co-product.<ref>{{cite web| last=Lees| first=Andrew| title=Chemicals from salt| publisher=BBC|year=2007|url=http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsrocks/chemicals_saltrev3.shtml|accessdate=2008-03-11|archiveurl = http://web.archive.org/web/20071026052022/http://www.bbc.co.uk/schools/gcsebitesize/chemistry/usefulproductsrocks/chemicals_saltrev3.shtml |archivedate = October 26, 2007|deadurl=yes}}</ref>
 
===Thermochemical===
There are more than 200 thermochemical cycles which can be used for [[water splitting]], around a dozen of these cycles such as the [[iron oxide cycle]], [[cerium(IV) oxide-cerium(III) oxide cycle]], [[zinc zinc-oxide cycle]], [[sulfur-iodine cycle]], [[copper-chlorine cycle]] and [[hybrid sulfur cycle]] are under research and in testing phase 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|format=PDF}}</ref> A number of laboratories (including in France, Germany, Greece, Japan, and the USA) 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, Robert|accessdate=2008-05-17|format=PDF}}</ref>
 
===Anaerobic corrosion===
Under anaerobic conditions, [[iron]] and [[steel alloy]]s are slowly oxidized by the protons of water concomitantly reduced in molecular hydrogen (H<sub>2</sub>). The [[anaerobic corrosion]] of iron leads first to the formation of [[ferrous hydroxide]] (green rust) and can be described by the following reaction:
 
:Fe + 2 H<sub>2</sub>O → Fe(OH)<sub>2</sub> + H<sub>2</sub>
 
In its turn, under anaerobic conditions, the [[ferrous hydroxide]] (Fe(OH)<sub>2</sub> ) can be oxidized by the protons of water to form [[magnetite]] and molecular hydrogen.
This process is described by the [[Schikorr reaction]]:
 
:3 Fe(OH)<sub>2</sub> → Fe<sub>3</sub>O<sub>4</sub> + 2 H<sub>2</sub>O + H<sub>2</sub>
:''ferrous hydroxide → magnetite + water + hydrogen''
 
The well crystallized magnetite (Fe<sub>3</sub>O<sub>4</sub>) is thermodynamically more stable than the ferrous hydroxide (Fe(OH)<sub>2</sub> ).
 
This process occurs during the anaerobic corrosion of [[iron]] and [[steel]] in [[Anoxic waters|oxygen-free]] [[groundwater]] and in reducing [[soil]]s below the [[water table]].
 
===Geological occurrence: the serpentinization reaction===
In the absence of atmospheric oxygen (O<sub>2</sub>), in deep geological conditions prevailing far away from Earth atmosphere, hydrogen (H<sub>2</sub>) is produced during the process of [[Serpentinization#Hydrogen production by anaerobic oxidation of fayalite ferrous ions|serpentinization]] by the anaerobic oxidation by the water protons (H<sup>+</sup>) of the ferrous (Fe<sup>2+</sup>) silicate present in the crystal lattice of the [[fayalite]] (Fe<sub>2</sub>SiO<sub>4</sub>, the [[olivine]] iron-endmember). The corresponding reaction leading to the formation of [[magnetite]] (Fe<sub>3</sub>O<sub>4</sub>), [[quartz]] (SiO<sub>2</sub>) and hydrogen (H<sub>2</sub>) is the following:
 
:3 Fe<sub>2</sub>SiO<sub>4</sub> + 2 H<sub>2</sub>O → 2 Fe<sub>3</sub>O<sub>4</sub> + 3 SiO<sub>2</sub> + 3 H<sub>2</sub>
:''fayalite + water → magnetite + quartz + hydrogen''
 
This reaction closely resembles the [[Schikorr reaction]] observed in the anaerobic oxidation of the [[ferrous hydroxide]] in contact with water.
 
===Formation in transformers===
From all the fault gases formed in power [[transformer]]s, hydrogen is the most common and is generated under most fault conditions; thus, formation of hydrogen is an early indication of serious problems in the transformer's life cycle.<ref>{{cite book|author=Hirschler, M. M.|title=Electrical Insulating Materials: International Issues|url=http://books.google.com/books?id=bmxcV_TlsV8C&pg=PA89|accessdate=13 July 2012|date=2000|publisher=ASTM International|isbn=978-0-8031-2613-8|pages=89–}}</ref>
 
==Applications==
===Consumption in processes===
Large quantities of {{chem|H|2}} are needed in the petroleum and chemical industries. The largest application of {{chem|H|2}} is for the processing ("upgrading") of fossil fuels, and in the production of [[ammonia]]. The key consumers of {{chem|H|2}} in the petrochemical plant include [[hydrodealkylation]], [[hydrodesulfurization]], and [[hydrocracking]]. {{chem|H|2}} has several other important uses. {{chem|H|2}} is used as a hydrogenating agent, particularly in increasing the level of saturation of unsaturated fats and [[Vegetable oil|oils]] (found in items such as margarine), and in the production of [[methanol]]. It is similarly the source of hydrogen in the manufacture of [[hydrochloric acid]]. {{chem|H|2}} is also used as a [[reducing agent]] of metallic [[ore]]s.<ref>{{cite web
|author=Chemistry Operations|date=2003-12-15
|url=http://periodic.lanl.gov/1.shtml
|title=Hydrogen|publisher=Los Alamos National Laboratory
|accessdate=2008-02-05}}</ref>
 
Hydrogen is highly soluble in many [[Rare earth element|rare earth]] and [[transition metal]]s<ref name="Takeshita">
{{cite journal
|last=Takeshita|first=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
|year=1974
|doi=10.1021/ic50139a050
}}</ref> and is soluble in both nanocrystalline and [[amorphous metal]]s.<ref name="Kirchheim1">
{{cite journal
|last=Kirchheim|first=R.
|last2=Mutschele|first2=T.
|last3=Kieninger|first3=W.
|title=Hydrogen in amorphous and nanocrystalline metals
|journal=Materials Science and Engineering
|year=1988|volume=99|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
|year=1988
|doi=10.1016/0079-6425(88)90010-2
}}</ref> These properties may be useful when hydrogen is purified by passage through hot [[palladium]] disks, but the gas's high solubility is a metallurgical problem, contributing to the [[hydrogen embrittlement|embrittlement]] of many metals,<ref name="Rogers 1999 1057–1064" /> complicating the design of pipelines and storage tanks.<ref name="Christensen" />
 
Apart from its use as a reactant, {{chem|H|2}} has wide applications in physics and engineering. It is used as a [[shielding gas]] in [[welding]] methods such as [[atomic hydrogen welding]].<ref>{{cite journal
|last=Durgutlu| first=Ahmet
|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
|year=2003
|doi=10.1016/j.matdes.2003.07.004}}</ref><ref>{{cite web
|title=Atomic Hydrogen Welding| publisher=Specialty Welds
|year=2007
|url=http://web.archive.org/web/20110716115120/http://www.specialwelds.com/underwater-welding/atomic-hydrogen-welding.htm}}</ref> H<sub>2</sub> is used as the rotor coolant in [[electrical generator]]s at [[power station]]s, because it has the highest [[thermal conductivity]] of any gas. Liquid H<sub>2</sub> is used in [[cryogenic]] research, including [[superconductivity]] studies.<ref>{{cite journal
|last=Hardy
|first=Walter 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
|year=2003
|doi=10.1016/S0921-4534(02)02591-1|bibcode = 2003PhyC..388....1H }}</ref> Because {{chem|H|2}} is lighter than air, having a little more than {{frac|15}} of the density of air, it was once widely used as a [[lifting gas]] in balloons and [[airship]]s.<ref name="zeppelins">{{cite web
|last=Barnes
|first=Matthew
|title=LZ-129, Hindenburg
|work=The Great Zeppelins| year=2004
|url=http://www.ciderpresspottery.com/ZLA/greatzeps/german/Hindenburg.html
|accessdate=2008-03-18}}</ref>
 
In more recent applications, hydrogen is used pure or mixed with nitrogen (sometimes called [[forming gas]]) as a tracer gas for minute leak detection. Applications can be found in the automotive, chemical, power generation, aerospace, and telecommunications industries.<ref>{{cite conference| first=Matthias
|last=Block| title=Hydrogen as Tracer Gas for Leak Detection
|booktitle=16th WCNDT 2004
|publisher=Sensistor Technologies
|date=2004-09-03
|location=Montreal, Canada
|url=http://www.ndt.net/abstract/wcndt2004/523.htm
|accessdate=2008-03-25}}</ref> Hydrogen is an authorized food additive (E 949) that allows food package leak testing among other anti-oxidizing properties.<ref>{{cite web
|url=http://ec.europa.eu/food/fs/sfp/addit_flavor/flav15_en.pdf
|format=PDF| title=Report from the Commission on Dietary Food Additive Intake
|publisher=[[European Union]]
|accessdate=2008-02-05}}</ref>
 
Hydrogen's rarer isotopes also each have specific applications. [[Deuterium]] (hydrogen-2) is used in [[CANDU reactor|nuclear fission applications]] as a [[neutron moderator|moderator]] to slow [[neutron]]s, and in [[nuclear fusion]] reactions.<ref name="nbb" /> Deuterium compounds have applications in chemistry and biology in studies of reaction [[Kinetic isotope effect|isotope effect]]s.<ref>{{cite journal|last=Reinsch| first=J|coauthors=A Katz, J Wean, G Aprahamian, JT 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|year=1980|pmid=7410413}}</ref> [[Tritium]] (hydrogen-3), produced in [[nuclear reactor]]s, is used in the production of [[hydrogen bomb]]s,<ref>{{cite journal| last=Bergeron| first=Kenneth D.| title=The Death of no-dual-use| journal=Bulletin of the Atomic Scientists| volume=60| issue=1| page=15| publisher=Educational Foundation for Nuclear Science, Inc.|year=2004|url=http://find.galegroup.com/itx/start.do?prodId=SPJ.SP06|doi=10.2968/060001004}}</ref> as an isotopic label in the biosciences,<ref name="holte" /> and as a [[Beta radiation|radiation]] source in luminous paints.<ref>{{cite journal| last=Quigg| first=Catherine T.| title=Tritium Warning| journal=Bulletin of the Atomic Scientists| volume=40|issue=3| pages=56–57|month=March|year=1984 }}</ref>
 
The [[triple point]] temperature of equilibrium hydrogen is a defining fixed point on the [[International Temperature Scale of 1990|ITS-90]] temperature scale at 13.8033&nbsp;[[kelvin]]s.<ref>{{cite conference| title=International Temperature Scale of 1990
|booktitle=Procès-Verbaux du Comité International des Poids et Mesures
|pages=T23–T42
|year=1989
|url=http://www.bipm.org/utils/common/pdf/its-90/ITS-90.pdf
|accessdate=2008-03-25|format=PDF}}</ref>
 
===Coolant===
{{Main|Hydrogen-cooled turbo generator}}
Hydrogen is commonly used in power stations as a coolant in generators due to a number of favorable properties that are a direct result of its light diatomic molecules. These include low [[density]], low [[viscosity]], and the highest [[Specific heat capacity|specific heat]] and [[thermal conductivity]] of all gases.
 
===Energy carrier===
{{See also|Hydrogen economy|Hydrogen infrastructure}}
Hydrogen is not an energy resource,<ref name="sustain">{{cite web
|last=McCarthy| first=John| title=Hydrogen
|publisher=[[Stanford University]]
|date=1995-12-31
|url=http://www-formal.stanford.edu/jmc/progress/hydrogen.html
|accessdate=2008-03-14}}</ref> except in the hypothetical context of commercial [[nuclear fusion]] power plants using [[deuterium]] or [[tritium]], a technology presently far from development.<ref>{{cite web
|title=Nuclear Fusion Power
|publisher=World Nuclear Association
|month=May|year=2007
|url=http://www.world-nuclear.org/info/inf66.html
|accessdate=2008-03-16}}</ref> The Sun's energy comes from nuclear fusion of hydrogen, but this process is difficult to achieve controllably on Earth.<ref>{{cite web
|title=Chapter 13: Nuclear Energy&nbsp;— Fission and Fusion
|work=Energy Story
|publisher=California Energy Commission
|year=2006
|url=http://www.energyquest.ca.gov/story/chapter13.html
|accessdate=2008-03-14}}</ref> Elemental hydrogen from solar, biological, or electrical sources require more energy to make it than is obtained by burning it, so in these cases hydrogen functions as an energy carrier, like a battery. Hydrogen may be obtained from fossil sources (such as methane), but these sources are unsustainable.<ref name="sustain" />
 
The [[energy density]] per unit ''volume'' of both [[liquid hydrogen]] and [[compressed hydrogen]] gas at any practicable pressure is significantly less than that of traditional fuel sources, although the energy density per unit fuel ''mass'' is higher.<ref name="sustain" /> Nevertheless, elemental hydrogen has been widely discussed in the context of energy, as a possible future ''carrier'' of energy on an economy-wide scale.<ref>{{cite press release
|title=DOE Seeks Applicants for Solicitation on the Employment Effects of a Transition to a Hydrogen Economy
|work=Hydrogen Program
|publisher=US Department of Energy
|date=2006-03-22
|url=http://web.archive.org/web/20110719105413/http://www.hydrogen.energy.gov/news_transition.html
|accessdate=2008-03-16}}</ref> For example, {{chem|CO|2}} [[CO2 sequestration|sequestration]] followed by [[carbon capture and storage]] could be conducted at the point of {{chem|H|2}} production from fossil fuels.<ref name="GATech" /> Hydrogen used in transportation would burn relatively cleanly, with some [[NOx|NO<sub>x</sub>]] emissions,<ref>{{cite journal
|last=Heffel| first=James W.
|title=NOx emission and performance data for a [[hydrogen fuel]]ed internal combustion engine at 1500&nbsp;rpm using exhaust gas recirculation
|journal=International Journal of Hydrogen Energy
|volume=28
|issue=8
|pages=901–908
|year=2002
|doi=10.1016/S0360-3199(02)00157-X}}</ref> but without carbon emissions.<ref name="GATech">{{cite press release
|title=Carbon Capture Strategy Could Lead to Emission-Free Cars
|publisher=Georgia Tech
|date=2008-02-11
|url=http://www.gatech.edu/newsroom/release.html?id=1707
|accessdate=2008-03-16}}</ref> However, the infrastructure costs associated with full conversion to a hydrogen economy would be substantial.<ref>{{cite book
|first=Joseph J.|last=Romm|authorlink=Joseph J. Romm
|year=2004
|title=[[The Hype About Hydrogen]]: Fact And Fiction In The Race To Save The Climate
|edition=1st
|publisher=Island Press
|isbn=1-55963-703-X}}</ref>
 
===Semiconductor industry===
Hydrogen is employed to saturate broken ("dangling") bonds of [[amorphous silicon]] and [[amorphous carbon]] that helps stabilizing material properties.<ref>{{cite journal
|last=Le Comber| first= 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
|year=1977
|last2=Jones
|first2=D. I.
|last3=Spear
|first3=W. E.|bibcode = 1977PMag...35.1173C }}</ref> It is also a potential [[electron donor]] in various oxide materials, including [[zinc oxide|ZnO]],<ref>{{cite journal
|last=Van de Walle| first= Chris 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|year=2000
|pmid=10991462|bibcode=2000PhRvL..85.1012V}}</ref><ref>{{cite journal
|last=Janotti|first= Anderson
|title=Hydrogen multicentre bonds|doi=10.1038/nmat1795
|journal=Nature Materials
|volume=6|pages=44–47
|year=2007
|pmid=17143265
|last2=Van De Walle
|first2=CG
|issue=1|bibcode = 2007NatMa...6...44J }}</ref> [[Tin dioxide|SnO<sub>2</sub>]], [[Cadmium oxide|CdO]], [[Magnesium oxide|MgO]],<ref>{{cite journal|last=Kilic| first= Cetin|title=n-type doping of oxides by hydrogen|doi=10.1063/1.1482783|journal=Applied Physics Letters|volume=81
|issue=1|pages=73–75|year=2002|last2=Zunger|first2=Alex|bibcode = 2002ApPhL..81...73K }}</ref> [[Zirconium dioxide|ZrO<sub>2</sub>]], [[Hafnium(IV) oxide|HfO<sub>2</sub>]], [[Lanthanum(III) oxide|La<sub>2</sub>O<sub>3</sub>]], [[Yttrium(III) oxide|Y<sub>2</sub>O<sub>3</sub>]], [[Titanium dioxide|TiO<sub>2</sub>]], [[Strontium titanate|SrTiO<sub>3</sub>]], LaAlO<sub>3</sub>, [[Silicon dioxide|SiO<sub>2</sub>]], [[Aluminium oxide|Al<sub>2</sub>O<sub>3</sub>]], ZrSiO<sub>4</sub>, HfSiO<sub>4</sub>, and SrZrO<sub>3</sub>.<ref>{{cite journal
|last=Peacock| first= 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
|year=2003
|last2=Robertson
|first2=J.
|bibcode = 2003ApPhL..83.2025P }}</ref>
 
==Biological reactions==
{{See|Biohydrogen|Biological hydrogen production}}
H<sub>2</sub> is a product of some types of [[Fermentation (biochemistry)|anaerobic metabolism]] and is produced by several [[microorganism]]s, usually via reactions [[catalysis|catalyzed]] by [[iron]]- or [[nickel]]-containing [[enzyme]]s called [[hydrogenase]]s. These enzymes catalyze the reversible [[redox]] reaction between H<sub>2</sub> and its component two protons and two electrons. Creation of hydrogen gas occurs in the transfer of reducing equivalents produced during [[pyruvate]] [[fermentation (biochemistry)|fermentation]] to water.<ref>{{cite book
|first=Richard|last=Cammack|url=http://books.google.com/?id=GTzajKoBoNwC&pg=PA202
|coauthors=Robson, R. L.|year=2001|pages=202–203
|title=Hydrogen as a Fuel: Learning from Nature
|publisher=Taylor & Francis Ltd
|isbn=0-415-24242-8}}</ref>
 
[[Water splitting]], in which water is decomposed into its component protons, electrons, and oxygen, occurs in the [[Light-dependent reactions|light reactions]] in all [[photosynthetic]] organisms. Some such organisms, including the alga ''[[Chlamydomonas reinhardtii]]'' and [[cyanobacteria]], have evolved a second step in the [[dark reaction]]s in which protons and electrons are reduced to form H<sub>2</sub> gas by specialized hydrogenases in the [[chloroplast]].<ref>{{cite journal
|last=Kruse|first=O.
|coauthors=Rupprecht, J.; Bader, K.-P.; Thomas-Hall, S.; Schenk, P. M.; Finazzi, G.; Hankamer, B
|title=Improved photobiological H<sub>2</sub> production in engineered green algal cells
|journal=The Journal of Biological Chemistry
|year=2005|volume=280|issue=40|pages=34170–7
|doi=10.1074/jbc.M503840200
|pmid=16100118}}</ref> Efforts have been undertaken to genetically modify cyanobacterial hydrogenases to efficiently synthesize H<sub>2</sub> gas even in the presence of oxygen.<ref>{{cite web
|first=H. O.|last=Smith|coauthors=Xu, Q|year=2005
|url=http://ec.europa.eu/food/fs/sfp/addit_flavor/flav15_en.pdf
|format=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
|accessdate=2008-02-05}}</ref> Efforts have also been undertaken with genetically modified [[Biological hydrogen production (Algae)|alga in a bioreactor]].<ref>{{cite news|last=Williams| first=Chris| title=Pond life: the future of energy| work=Science| publisher=The Register| date=2006-02-24| url=http://www.theregister.co.uk/2006/02/24/pond_scum_breakthrough/| accessdate=2008-03-24}}</ref>
 
==Safety and precautions==
{{Main|Hydrogen safety}}
Hydrogen poses a number of hazards to human safety, from potential [[detonation]]s and fires when mixed with air to being an [[asphyxia]]nt in its pure, [[oxygen]]-free form.<ref name=NASAH2>{{cite web
|author=Brown, W. J. et al.
|first=H. O.|last=Smith|coauthors=Xu, Q
|url=http://www.hq.nasa.gov/office/codeq/doctree/canceled/871916.pdf
|format=PDF|year=1997
|title=Safety Standard for Hydrogen and Hydrogen Systems
|publisher=[[NASA]]|accessdate=2008-02-05}}</ref> In addition, liquid hydrogen is a [[cryogen]] and presents dangers (such as [[frostbite]]) associated with very cold liquids.<ref>{{cite web| title=Liquid Hydrogen MSDS| publisher=Praxair, Inc.| month=September|year=2004| url=http://www.hydrogenandfuelcellsafety.info/resources/mdss/Praxair-LH2.pdf| format=PDF| accessdate=2008-04-16}}</ref> Hydrogen dissolves in many metals, and, in addition to leaking out, may have adverse effects on them, such as [[hydrogen embrittlement]],<ref>{{cite journal| title='Bugs' and hydrogen embrittlement| journal=Science News| volume=128| issue=3| page=41| location=Washington, D.C.| date=1985-07-20|doi=10.2307/3970088| jstor=3970088}}</ref> leading to cracks and explosions.<ref>{{cite web|url=http://www.twi.co.uk/content/oilgas_casedown29.html|title=Union Oil Amine Absorber Tower|last=Hayes|first=B.|publisher=TWI|accessdate=29 January 2010}}</ref> Hydrogen gas leaking into external air may spontaneously ignite. Moreover, hydrogen fire, while being extremely hot, is almost invisible, and thus can lead to accidental burns.<ref>{{cite web| title=Hydrogen Safety| publisher=[[Humboldt State University]]| url=http://www.schatzlab.org/education/h2safety.html| accessdate=2010-04-14}}</ref>
 
Even interpreting the hydrogen data (including safety data) is confounded by a number of phenomena. Many physical and chemical properties of hydrogen depend on the [[Spin isomers of hydrogen|parahydrogen/orthohydrogen]] ratio (it often takes days or weeks at a given temperature to reach the equilibrium ratio, for which the data is usually given). Hydrogen detonation parameters, such as critical detonation pressure and temperature, strongly depend on the container geometry.<ref name=NASAH2/>
 
==See also==
https://dv.wikipedia.org/wiki/ހައިޑްރަޖަން އިން