Question

Why nuclei invariably have mass less than the sum of the masses of the individual nucleons in them?

Answer 1

It is found that the rest mass of the nucleus is always slightly less than the sum of rest mass of neutrons and protons composing the nucleus. The mass different between the sum of nucleus consistuent and original nucleus mass is responsible for the nuclear binding energy to form the nucleus which is liberated, when the protons nucleons combine the nuclear.