Semi-empirical mass formula (Bethe-Weizsacker)

What you are seeing: the binding energy per nucleon $B/A$ along the valley of stability $(A, Z^*(A))$, computed from the Bethe-Weizsacker formula $$B = a_V A - a_S A^{2/3} - a_C Z(Z-1)/A^{1/3} - a_A (N-Z)^2/A + \delta.$$ The peak near $A \approx 60$ at $B/A \approx 8.8$ MeV is iron-peak territory; fusion releases energy below it, fission releases above.

Hover (slide) the mass number $A$ slider to see the four terms individually. The Coulomb term grows like $Z^2/A^{1/3}$ and dominates the fall-off above iron; the asymmetry term grows quadratically with $N-Z$ (neutron excess); the pairing term oscillates by parity.