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Comment by ridgeguy | Hacker News Reader

I’m curious why the helium hydride (HeH) would form before H2 (molecular hydrogen). Hydrogen (single atomic form) should have formed a larger fraction of post-bang elements than helium (and much more than lithium). I didn’t see anything in the article about this.

HeH has a binding energy of 25.1 kJ/mole [1]. H2 has a dissociation energy of 104 kcal/mole [2]. Expressing these energies in eV per molecule, it’s ~0.26eV for HeH and ~ 4.5eV for H2. So H2 would seem to be stable at higher temperatures and should form first in a cooling gas mixture whose particle kinetic energies are declining.

Or is “binding energy” different from “dissociation energy”?

Would like to understand a little of the thermodynamics that allow HeH rather than H2 to be the primordial molecule.

[1] https://en.wikipedia.org/wiki/Helium_hydride_ion [2] https://en.wikipedia.org/wiki/Bond-dissociation_energy

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That was also my first reaction.

But TFA says that H+ reacted with neutral He. That gives you HeH+, which is what they detected. And I see that the ionization energies of He are 2372.3 kJ/mol for the first electron, and 5250.5 kJ/mol for the second. But the ionization energy of H is just 1312.0 kJ/mol. So neutral He would have formed long before neutral H, and He plus H+ was the first possible combination.

Also, HeH+ is an extremely strong acid, so it would grab an electron from just about anything else that it encountered. And neutral HeH is not stable.

Thanks, I forgot this is an ionized species (the + charge). That can change kinetics a lot.

In the post-bang environment, you can flip those numbers around to see the order in which the hot nuclei capture the hot electrons. A [He+2] nucleus can grab hotter electrons than a [He+1] ion, which in turn can grab hotter electrons than a [H+] proton.

So at a temperature cool enough for neutral He to exist and still hot enough for [H+] ions, they were pretty much the only massive things around that could react with each other.

If an [HeH+] caught a hot electron, it would fall apart into He and H, with no further need to share electrons. A [H+] could glom onto that neutral H to make [H2+] (which also forms when cosmic rays blast through an H2 cloud). If an [H2+] caught another electron, the H2 and another [H2+] will react to make [H3+] and H. So it turns out [H3+] is a pretty common hot cloud molecule, too.

Plain old H2 doesn't exist by itself until the temperature drops enough for those positive ions to catch electrons and stop being an acidic plasma.

I don't know anything about HeH, but molecular hydrogen is actually quite hard to form. It's easily dissociated by UV radiation so can't form until a protostar has become concentrated enough so as to be self-shielding. A planetary nebula like the one they found the HeH in is not an environment conducive to formation of molecular hydrogen.

Yes. H2 is easily dissociated by UV. (Re)combination of 2H0 (two neutral hydrogen atoms) can't happen in the gas phase without a third body to take away the recombination energy. Quantum mechanics again. So there has to be a third thing around (another atom, molecule, or surface) to make H2 out of 2H0.

BDE (Bond dissociation energy) is the energy require to split the chemical bond in half. This would lead to two radicals through homolysis.

Binding energy is a bit more vague in my opinion. For atoms it means disassembling the atom into free electrons and a nucleus. In chemistry it is the average energy required to break all bonds in a molecule.

Source: Chemist and wiki

https://en.m.wikipedia.org/wiki/Bond_energy

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