The semi-empirical mass formula is a function of two variables, as A = N + Z. It gives the binding energy of the ground state of any nucleus, i.e. any values of Z and N. ... For even A, then as Z changes by one, then Z (and N, given that A is fixed and even) goes from even to odd or vice versa.
The main contribution to the binding energy comes from the volume energy term arising from the short- range nature of nuclear forces leading to their saturation. The LDM is not valid for nuclei with very small mass number values below A =12 where the surfaces can be considered as diffuse. The LDM is a classical model.