Stability and Nuclear Binding Energy

Question

I've read that the cause of Nuclear fission and nuclear fusion lies in the fact that Nuclear Binding Energy confers stability to a nucleus. This is why too heavy or too light nuclei resort to these processes to attain stability, and become nuclei with intermediate atomic weight. Two questions come to my mind here

1. is it an experimental observation that nuclei with intermediate atomic mass are stable, or is there a reason to it? I can understand that too many protons in the nucleus would lead to instability. But what harm can too many neutrons cause?

2. I've always known that everything in nature tries to lose energy to attain stability. That's why bonds are formed in the first place. But why is it that more binding energy per nucleon makes a nucleus more stable?

Answer 1

If you have a collection of nucleons together and they will attempt to minimise their energy density. They will do this by maximising their binding energy per nucleon. For matter at low density this corresponds to nuclei around the "iron-peak" (iron, nickel, cobalt, manganese). This is just a trade-off between the strong nuclear attraction felt by all nucleons and the Coulomb repulsion felt by the protons.

This will only happen if the nuclei are in conditions where equilibration is possible - i.e. where reaction rates are fast enough. For example it happens in the centre of a star prior to a supernova, or in the crust of a neutron star.

Simply adding more and more neutrons doesn't work. Neutrons are indistinguishable fermions and must reside in different quantum states. When increasingly higher energy levels are filled by neutrons they are then unstable to beta decay into a proton and electron.

More neutron-rich nuclei can exist in high density electron-degenerate conditions, such as the crusts of neutron stars, where the electron degeneracy can block beta decay.

More binding energy per nucleon is more stable because binding energy is negative.