The nuclear mass M is always less than the total mass of its constituents. The difference in mass of a nucleus and its constituents ΔM is called the mass defect,
ΔM = (Z mp + (A – Z)mn) – M
Using Einstein’* mass energy relation, we express this mass difference in terms of energy as
ΔEb = ΔM c2
The energy ΔEb represents the binding energy of the nucleus. In the mass number range A = 30 to 170, the binding energy per nucleon is nearly constant, about 8 MeV/nucleon.
Example :
The atomic mass of nucleus of 168O found from mass spectroscopy experiments is seen to be 15.99053 u. The expected mass of 168O nucleus is by adding 8 protons mass and 8 neutrons mass = 16.12744 u.
The difference between these two masses is mass defect = 0.13691 u.
