An atomic nucleus is a stable structure. The nucleus is bound by very strong short range forces called nuclear forces. Certain amount of work has to be done to separate the nucleons to such a distance that there is no interaction. This work done therefore measures binding energy of the nucleus.
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
