radioactivity

Definition: The distance between theoretical calculated mass and experimentally measured mass of nucleus is called mass defect. It is denoted by Δm. It can be calculated as follows:

Mass defect = (Theoretical calculated mass) - (measured mass of nucleus)

i.e, (sum of masses of protons and neutrons) - (measured mass of nucleus)

In nuclear reactions, the energy that must be radiated or otherwise removed as binding energy may be in the form of electromagnetic waves, such as gamma radiation, or as heat.
Again, however, no mass deficit can in theory appear until this radiation has been emitted and is no longer part of the system.

The energy given off during either nuclear fusion or nuclear fission is the difference between the binding energies of the fuel and the fusion or fission products.
In practice, this energy may also be calculated from the substantial mass differences between the fuel and products, once evolved heat and radiation have been removed.

When the nucleons are grouped together to form a nucleus, they lose a small amount of mass, i.e., there is mass defect.
This mass defect is released as (often radiant) energy according to the relation E = mc2; thus binding energy = mass defect · c2 .
This energy holds the nucleons together and is known as binding energy. In fact, mass defect is a measure of the binding energy of the nucleus. The greater the mass defect, the greater is the binding energy of the nucleus.