Nascent hydrogen

H atoms, also called nascent hydrogen 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. Being monoatomic, H atoms are much more reactive and thus a much more effective reducing agent than ordinary diatomic H₂, 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. 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 H₂ 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) to thermally dissociate the molecule, or equivalent excitation in an electric discharge. Also, electromagnetic radiation above about 11 ev can be absorbed by H₂ 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 to prevent oxidation of the weld.

Atomic hydrogen determines the frequency of hydrogen masers which are used as precise frequency standards. 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.

Nascent hydrogen is claimed to reduce nitrites to ammonia, or arsenic to arsine even under mild conditions. 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.

Another meaning

Occasionally, hydrogen chemisorbed on metal surfaces is referred to as "nascent", although this terminology is fading with time. 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". 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.