...almost sounds as if different atoms could be attributed different levels of neutron-affinity
Neutron affinity wouldn't bring much, because:
- You never have equilibrium situations in a lab. Only brief dynamic situations which end at the first oportunity, not at the best one.
- Under conditions where neutrons could hop from one nucleus to an other, so would protons as well, and alphas... Hence neutron-to-nucleus affinity wouldn't suffice. You'd use the heat of formation of every possible nucleus. That's part of the theory of primordial nucleosynthesis, explaining how much H, D, He, Li, Be result from the big bang - but even the big bang didn't result in an equilibrium.
- Consequently, nuclear people define figures for the dynamic evolution, not for equilibrium situations. They call it "neutron cross-section" (in barns or 10-28
) and use tables for thermal neutrons, fission neutrons, and sometimes fusion (14MeV) neutrons - or they have curves of cross-section versus energy.
Theories about how neutrons and protons are bond... Basically QCD among quarks, yes, but even n-n, p-p, n-p bonds aren't perfectly clear, and complete nuclei even less so...
The latest I've seen suggests that light nuclei are solids while heavy ones are "liquid", with nucleons delocalized over the whole nucleus. As well, it seems that n-p bonds occur more often than others.
Elementary questions like "why no di-neutron" are still open. Very foggy field.