The difference in mass of a nucleus
and its constituents, DM, is called the mass defect, and is given by
[
(
)
]
p
n
M
Zm
A
Z m
M
∆
=
+
−
−
(13 7)
What is the meaning of the mass defect 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