If we compare the mass of a nucleus with the sum of the masses of its constituent protons and neutrons, we find that the mass of the nucleus is smaller than the sum of the masses of its constituents.

This difference is called the mass defect. In general, where m is the mass of the nucleus, Z is the number of protons, N is the number of neutrons, m   is the mass of the protons, and m   is the mass of the neutrons.