Molecule formation when an atomic gas cools down

Question

I have read about the actual formation of interstellar molecules; although interstellar chemistry is very different from solvent-based chemistry on earth, I understand it to be driven by the presence of dust grains, and by the constant ionization by cosmic rays, which facilitates exchange reactions.

Now I'm wondering about this hypothetical case:

• we start with a hot atomic gas with a density similar to the interstellar medium, with the same type of composition we can observe now (74% H, 24% He, O, C, N, Fe, Mg, Si, S,...)
• we let that gas cool down, there isn't any radiation nor any dust.

Without the effects that I mentioned above, covalent bonds could only form when collisions happen between two atoms.

Of course, the more exothermic a formation reaction is, the more likely a collision between two atoms to lead to a covalent bond.

By the time the gas is completely cold, how much of it should I expect to be molecular?

• Would I only see O2, N2 and the likes?
• Would molecules like CH4 ever get to form? I understand its enthalpy of formation is negative, but I don't know if that's the case starting with just atomic C and atomic H?
• Would metals ever bond with anything, or is the probability of formation of Ferric/Magnesium oxides too low given the low density of metals and oxygen?

Answer 1

It depends all on the reaction rates between all the $n$ different atoms and molecules. So you need to know all the $n\times n$ reaction cross sections in order to be able to solve the $n$ equations for the $n$ unknowns. In addition you may have to include the production and loss rates due to the radiation which could result from the covalent bonding and which in turn could dissociate other molecules. Without knowing each of the reaction coefficients it is impossible to say what the eventual outcome will be. Just one particularly strong or weak reaction channel could affect the whole system as everything is coupled to some degree.