It is here that Einstein’s
equivalence  of mass and energy plays a role Since the mass of the oxygen
nucleus is less that the sum of the masses of its constituents (8 protons
and 8 neutrons, in the unbound state), the equivalent energy of the oxygen
nucleus is less than that of the sum of the equivalent energies of its
constituents If one wants to break the oxygen nucleus into 8 protons
and 8 neutrons, this extra energy DM c2, has to supplied This energy
required Eb is related to the mass defect by
Eb = D M c2
(13