Greater binding energy per nucleon implies greater stability. It is most convenient to explore this in the context of a set of isobars, i.e. a set of nuclides with the same A. These can transform into one another by various forms of beta decay.
The masses of the members of a set of isobars can be obtained by rearranging the semiempirical mass formula:
This equation has the form of a parabola for fixed A; we can solve for the value of Z giving the greatest binding energy (smallest mass), i.e. the most stable isobar. Thus
Inserting the values for the coefficients and rearranging,
This then gives the equation for the `valley of stability' on the (N,Z) chart of nuclides. Note that is determined by an interplay between the Coulomb force (makes Z a minimum) and the asymmetry term (makes N=Z).