# Absorption of a gas into a solid

When a gas interacts with a (crystalline) solid, some scenarios may happen:

• scattering: gas atoms will not stick or penetrate (do not interact with the solid)
• Adsorption: gas atoms stick to the surface
• Absorption: gas atoms penetrate into the solid and reside between other atoms in the bulk of the matter (like what happens in doped Silicon)

Regarding the 3rd case, I want to know under what circumstances (temperature, pressure, atomic radius of the gas atoms, crystal structure of the solid) a gas can penetrate (absorb) into a crystal (absorption), and what effects does the addition of the gas atoms to the crystal structure have on the solid's statistical/thermodynamic properties.

• Can you be more specific about what you want to know about these effects? Alternatively, if you really only want general references, you should see our resource recommendation policy. – David Z Dec 7 '13 at 22:36
• @DavidZ More specifically, I want to know under what circumstances a gas can interact with a crystal (absorption) and what effects does it have on the statistical/thermodynamic properties of the solid. – user215721 Dec 7 '13 at 22:43
• That would be a perfectly fine question. If you edit your question to ask that, instead of asking for references, I think it would be quite fine to reopen. – David Z Dec 7 '13 at 22:45
• @DavidZ I edited the question. Is it acceptable now? – user215721 Dec 7 '13 at 23:03
• Yeah, I suppose that works. By the way, you don't have to explicitly ask for books and papers; people will always provide relevant references if there is too much information to include in an answer. I edited that part out for you. – David Z Dec 7 '13 at 23:23

The main process whereby absorption happens begins when the $H_2$ bond is cloven by the metal - there is likely to be a pretty quantum mechanical description of this process whereby the atomic electrons interact with the conduction electron sea in the metal, but I shall leave another answer to give a reference as I am not aware of one and I am not a chemist, so this isn't my field. However, such cleaving is well known to happen experimentally, and the Wikipedia article on bond dissociation energy states that the $H_2$ bond is strong (indeed quite comparable to all the other bonds listed there) and only cloven by metals or strong oxidants. So this is the first requirement: most often the target crystal will be a metal or a strong oxidant.
The hydrogen atom is especially small so this helps the penetration of a proton electron pair into a lattice: the small size means that the charges are "well shielded" so the Coulombic potential barrier to penetration is going to be smallish. This characteristic also helps the second stage of the process whereby lone hydrogen atoms recombine in "voids" inside the metal. The $H_2$ molecule is small, thus helping this process too.