Nascent hydrogen
1504792
219997735
2008-06-17T20:29:48Z
Abhishek191288
5471367
/* Making atomic hydrogen */
H atoms, also called '''''nascent hydrogen''''' {{Fact|date=April 2008}} or ''atomic hydrogen'', are claimed to exist transiently but long enough to effect chemical reactions. According to one claim, nascent hydrogen is generated ''[[in situ]]'' usually by the reaction of [[zinc]] with an [[acid]], or by [[electrolysis]] at the [[cathode]].{{Fact|date=April 2008}} Being monoatomic, H atoms are much more reactive and thus a much more effective [[reducing agent]] than ordinary diatomic H<sub>2</sub>, but again the key question is whether H atoms exist in any chemically meaningful way under the conditions claimed. The concept is more popular in engineering and in older literature on catalysis.{{Fact|date=April 2008}} '''Atomic hydrogen''' is made of individual [[hydrogen]] [[atoms]] which are not bound together like ordinary hydrogen into [[molecules]].
==Making atomic hydrogen==
It takes 4.476 [[eV]] to disassociate ordinary H<sub>2</sub> hydrogen molecules. When they recombine, they liberate this energy. An [[electric arc]] or [[ultraviolet]] [[photon]] can generate atomic hydrogen.
Atomic hydrogen can be formed at temperatures high enough (> 2000 K) {{Fact|date=April 2008}} to thermally dissociate the molecule, or equivalent excitation in an [[electric discharge]]. Also, electromagnetic radiation above about 11 [[electronvolt|ev]] {{Fact|date=April 2008}} can be absorbed by H<sub>2</sub> and lead to its dissociation.
==Uses of atomic hydrogen==
The [[atomic hydrogen torch]] uses it to generate very high temperatures near 4,000°C for [[welding]]. Hydrogen is a powerful [[reducing agent]] which eliminates the need for [[Flux (metallurgy)|flux]] to prevent oxidation of the weld.
Atomic hydrogen determines the frequency of [[hydrogen maser]]s which are used as precise [[frequency standard]]s. They operate at the 1420 MHz [[frequency]] corresponding to an [[absorption line]] in atomic hydrogen.
[[NASA]] has investigated the use of atomic hydrogen as a [[rocket propellant]]. It could be stored in liquid helium to prevent it from recombining into molecular hydrogen. When the helium is vaporized, the atomic hydrogen would be released and combine back to molecular hydrogen. The result would be a intensely hot stream of hydrogen and helium gas. The liftoff weight of rockets could be reduced by 50% by this method.<ref>[http://gltrs.grc.nasa.gov/reports/2002/TM-2002-211915.pdf NASA/TM—2002-211915 : Solid Hydrogen Experiments for Atomic Propellants]</ref>
Nascent hydrogen is claimed to reduce [[nitrite]]s to [[ammonia]], or [[arsenic]] to [[arsine]] even under mild conditions{{Fact|date=April 2008}}. Detailed scrutiny of such claims usually points to alternative pathways, not H atoms.
==In nature==
Most [[interstellar]] hydrogen is in the form of atomic hydrogen because the atoms can seldom collide and combine. They are the source of the important 21cm [[hydrogen line]] in [[astronomy]] at 1420 MHz. <ref>[http://mysite.du.edu/~jcalvert/phys/hydrogen.htm 21 cm Line]</ref>
==Another meaning==
Occasionally, hydrogen chemisorbed on metal surfaces is referred to as "nascent", although this terminology is fading with time.{{Fact|date=April 2008}} Other views hold that such chemisorbed hydrogen is "a bit less reactive than nascent hydrogen because of the bonds provided by the catalyst metal surface".{{Fact|date=April 2008}} Also, such catalyst provided atoms are not called nascent hydrogen, because they do not need to be captured and reacted in their instanteneous, temporary, "just generated" state, because the catalyst is able to reversibly generate them from the hydrogen gas supply at any time.{{Fact|date=April 2008}}
==References==
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