On the basis of Einstein's theory of mass energy equivalent it was found that the rest mass of a nucleus is always slightly less than the sum of the free neutrons and protons comprising the nucleus. This indicates that some mass disappears when a nucleus is formed. This difference in the masses is called the mass defect. It is this mass defect, which appears in the form of binding energy.
where Dm is the mass defect, mp is mass of a proton, mn is mass of a neutron, mN is mass of nucleus ZXA.
B.E = (DmC2) = [Zmp + (A-Z)mn]C2