What is mass defect?
It is observed that the mass of a nucleus is smaller than the sum of the masses of the constituent nucleons in the free state. The difference between the actual mass of the nucleus and the sum of the masses of constituent nucleons is called mass defect.
The mass defect is
Δm = [Zmp + (A - Z)mn] - M
Z is the atomic number (number of protons),
A is the mass number,
(A - Z) is the number of neutrons,
mp is the mass of a proton,
mn is the mass of a neutron and
M is the measured mass of a nucleus.