Another feature of nuclei not shown in the table is that thespin-spin force very much favors
nucleons which are paired. So nuclear isotopes with odd numbers of protons or odd numbers of
neutrons have less binding energy and nuclei with odd numbers of both protons and neutrons are
unstable (with one exception).
26.6 Examples
26.6.1 Boron Ground State
Boron, withZ= 5 has the 1S and 2S levels filled. They add up toj= 0 as do all closed shells. The
valence electron is in the 2P state and hence hasℓ= 1 ands=^12. Since the shell is not half full we
couple to the the lowestj=|ℓ−s|=^12. So the ground state is^2 P 12.