Is the mass of an atomic nucleus greater or less than the sum of the masses of the nucleons composing it? why don't the nucleon masses add up to the total nuclear mass?

Explanation:

A nucleus is the central part of an atom on which mass of the atom is concentrated. A nucleus consists of both protons and neutrons.

Whereas a nucleon means a proton or neutron.

It is known that atomic mass of an atom is the sum of total number of protons and neutrons. But the actual mass of an atom will always be less than the  the sum of the masses of the nucleons composing it because whenever a nucleus is formed then there is always release of energy.

This energy contains some amount of mass that is removed from the sum of total number of protons and neutrons. This missing mass is known as mass defect. This is why the nucleon masses do not add up to the total nuclear mass.

This mass defect can be calculated as the difference in observed atomic mass and total atomic mass as the sum of total number of protons and neutrons.