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| '''Hydrogen''' ({{IPAc-en|icon|ˈ|h|aɪ|d|r|ɵ|dʒ|ɨ|n}} {{respell|HY|drə-jin}})<ref>{{ cite book | last = Simpson|first=J.A. | last2 = Weiner|first2=E.S.C. | chapter = Hydrogen | title = Oxford English Dictionary | edition = 2nd|volume=7 | publisher = [[Clarendon Press]] | year = 1989 | isbn = 0-19-861219-2 }}</ref> is the [[chemical element]] with [[atomic number]] 1. It is represented by the [[chemical symbol|symbol]] '''H'''. 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 and [[abundance of the chemical elements|most abundant]] chemical element, constituting roughly 75% of the Universe's chemical elemental mass.<ref> | | '''Hydrogen''' ({{IPAc-en|icon|ˈ|h|aɪ|d|r|ɵ|dʒ|ɨ|n}} {{respell|HY|drə-jin}})<ref>{{ cite book | last = Simpson|first=J.A. | last2 = Weiner|first2=E.S.C. | chapter = Hydrogen | title = Oxford English Dictionary | edition = 2nd|volume=7 | publisher = [[Clarendon Press]] | year = 1989 | isbn = 0-19-861219-2 }}</ref> is the [[chemical element]] with [[atomic number]] 1. It is represented by the [[chemical symbol|symbol]] '''H'''. 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 and [[abundance of the chemical elements|most abundant]] chemical element, constituting roughly 75% of the Universe's chemical elemental mass.<ref> |
| {{cite web
| | ==References== |
| |last=Palmer|first=D.
| | {{reflist|colwidth=30em}} |
| |title=Hydrogen in the Universe
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| |url=http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/971113i.html
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| |publisher=[[NASA]]
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| |date=13 September 1997
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| |accessdate=2008-02-05
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| }} Note that most of the universe's mass is not in the form of chemical elements, however. See [[dark matter]] and [[dark energy]]</ref> [[Star]]s in the [[main sequence]] are mainly composed of hydrogen in its [[plasma (physics)|plasma]] state. Naturally occurring elemental hydrogen is relatively rare on Earth.
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| 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. 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. Hydrogen forms compounds with most elements and is present in water and most [[organic compound]]s. It plays a particularly important role in [[acid-base reaction theories|acid-base chemistry]] with many reactions exchanging protons between soluble molecules. 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]].
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| Hydrogen gas (now known to be H<sub>2</sub>) 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>
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| {{Cite episode
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| |title = Discovering the Elements
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| |url = http://www.bbc.co.uk/programmes/b00q2mk5
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| |series = Chemistry: A Volatile History
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| |credits = Presenter: Professor Jim Al-Khalili
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| |network = [[BBC]]
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| |station = [[BBC Four]]
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| |airdate = 2010-01-21
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| |minutes = 25:40
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| }}</ref> and that it produces water when burned, a property which later gave it its name, which in Greek means "water-former." At [[standard temperature and pressure]], hydrogen is a [[Transparency (optics)|colorless]], [[odorless]], [[nonmetal]]lic, [[taste]]less, highly [[combustion|combustible]] [[Diatomic molecule|diatomic]] [[gas]] with the [[molecular formula]] H<sub>2</sub>.
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| 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
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| |title=Hydrogen Basics — Production
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| |url=http://www.fsec.ucf.edu/en/consumer/hydrogen/basics/production.htm
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| |publisher=[[Florida Solar Energy Center]]
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| |year=2007
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| |accessdate=2008-02-05
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| }}</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.
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| Hydrogen is a concern in [[metallurgy]] as it can [[hydrogen embrittlement|embrittle]] many metals,<ref name="Rogers 1999 1057–1064">{{cite journal
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| |last=Rogers|first=H.C.
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| |title=Hydrogen Embrittlement of Metals
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| |journal=[[Science (journal)|Science]]
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| |volume=159|issue=3819|pages=1057–1064
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| |year=1999
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| |doi=10.1126/science.159.3819.1057
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| |pmid=17775040
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| }}</ref> complicating the design of pipelines and storage tanks.<ref name="Christensen">{{cite news
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| |last=Christensen|first=C.H.
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| |coauthors=Nørskov, J.K.; Johannessen, T.
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| |date=9 July 2005
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| |title=Making society independent of fossil fuels — Danish researchers reveal new technology
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| |publisher=[[Technical University of Denmark]]
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| |url=http://www.dtu.dk/English/About_DTU/News.aspx?guid=%7BE6FF7D39-1EDD-41A4-BC9A-20455C2CF1A7%7D
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| |accessdate=2008-03-28
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| }}</ref>
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| ==Properties==
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| ===Combustion===
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| [[Image:Shuttle Main Engine Test Firing cropped edited and reduced.jpg|thumb|upright|left|The [[Space Shuttle Main Engine]] burns 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.]]
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| Hydrogen gas (dihydrogen or molecular hydrogen)<!--[[dihydrogen]] is a redirect to [[hydrogen]]--><ref>
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| {{cite web
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| |url=http://www.usm.maine.edu/~newton/Chy251_253/Lectures/LewisStructures/Dihydrogen.html
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| |title=Dihydrogen
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| |work=O{{=}}CHem Directory
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| |publisher=[[University of Southern Maine]]
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| |accessdate=2009-04-06
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| }}</ref> is highly flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume.<ref>
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| {{cite journal
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| |last=Carcassi|first=M.N.
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| |last2=Fineschi|first2=F.
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| |title=Deflagrations of H<sub>2</sub>–air and CH<sub>4</sub>–air lean mixtures in a vented multi-compartment environment
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| |journal=Energy
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| |volume=30|issue=8|pages=1439–1451
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| |year=2005
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| |doi=10.1016/j.energy.2004.02.012
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| }}</ref> The [[enthalpy of combustion]] for hydrogen is −286 kJ/mol:<ref>
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| {{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
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| |title=The Hydrogen Economy: Opportunities, Costs, Barriers, and R&D Needs
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| |page=240
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| |publisher=[[National Academies Press]]
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| |isbn=0-309-09163-2|url=http://books.google.com/?id=ugniowznToAC&pg=PA240
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| }}</ref>
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| : 2 H<sub>2</sub>(g) + O<sub>2</sub>(g) → 2 H<sub>2</sub>O(l) + 572 kJ (286 kJ/mol)<ref group="note">286 kJ/mol: energy per mole of the combustible material (hydrogen)</ref>
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| 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
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| |url=http://books.google.com/?id=-CRRJBVv5d0C&pg=PA402|page=402
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| |title=A comprehensive guide to the hazardous properties of chemical substances
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| |publisher=Wiley-Interscience|isbn=0-471-71458-5
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| |year=2007
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| |author=Patnaik, P
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| }}</ref> Pure hydrogen-oxygen flames emit [[ultraviolet]] light and are nearly invisible to the naked eye, as
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| 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>
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| {{cite web
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| |last=Dziadecki|first=J.
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| |title=Hindenburg Hydrogen Fire
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| |url=http://spot.colorado.edu/~dziadeck/zf/LZ129fire.htm
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| |year=2005
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| |accessdate=2007-01-16
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| }}</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>
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| {{cite web
| |
| |last=Kelly|first=M.
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| |title=The Hindenburg Disaster
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| |url=http://americanhistory.about.com/od/hindenburg/a/hindenburg.htm
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| |publisher=About.com:American history
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| |accessdate=2009-08-08
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| }}</ref> | |
