The difference between the mass of a nucleus and the sum of the masses of the neutrons and protons that comprise it is known as the mass defect. E B is often divided by the mass number to obtain binding energy per nucleon for comparisons of binding energies between nuclides. Each of the terms in this formula has a theoretical basis. The coefficients a V , a S , a C , a A and a coefficient that appears in the formula for δ δ ( A , Z ) are determined empirically.