What determines binding energy?. We can determine the Binding energy of a nucleus from its rest mass. Consider the nucleus of an atom of atomic number Z and mass number A. The nucleus will contain Z protons and (A-Z) neutrons. If mp and mn are the free state rest masses of the proton and a neutron respectively then the rest mass of the nucleus is [Z(mp) + (A-Z)mn]. It is found that the rest mass M of a nucleus as obtained from experiment is always less than the total mass of the constituent particles. The total energy of the nucleus in the bound state is less than the total energy of the same nucleus in their free state. Hence the binding energy can also be defined as the total energy supplied to break the nucleus completely into its constituent particles.