Average masses

Although the proton and neutron on the atomic mass scale have masses of "1", their exact masses differ a little and the sum of the individual masses of particles in an atom is greater than the measured mass of the assembled atom. For those reasons, the atomic masses found on the periodic table which indicate average isotopic mass adjusted for abundance in nature cannot be determined with simple integer sums of the protons and neutrons in a nucleus.

This "missing" mass is one of the first puzzles we encounter when we try to understand nuclear processes. Understanding its significance sheds some light on the whole question of why there is radioactivity in the first place, i.e., why are some nuclei unstable?

On the simplest level, a nucleus is unstable if the repulsive forces in the nucleus are greater than the attractive forces in the nucleus. The nature of both of these forces is different. The strong nuclear force exists between all nucleons. But this force only acts at very small distances. The weaker electrostatic force exists among the protons with their like charges.