Hello aspirant

The rest mass of the stable nucleus of a stable atom is always less than the sum of the masses of constituent nucleons.

The difference is called the mass defect Δm (i.e., Δm.c ² ) which is utilised in keeping the nucleons bound together. This energy is known as the binding energy .

In order to break the nucleus into its constituent nucleons an amount of energy equal to its binding energy has to be supplied to the nucleus.

Binding energy per nucleon is obtained by dividing the binding energy with mass number and is the measure of the stability of nucleus.

The binding energy per nucleon is less for lighter nuclides and increase with the mass number.

Thus, the binding energy per nucleon decreases with increase in the mass number.

Hope this helps