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 h

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.