What factors determine nuclear stability?
The two main factors that determine nuclear stability are the neutron/proton ratio and the total number of nucleons in the nucleus.
The principal factor for determining whether a nucleus is stable is the neutron to proton ratio.
The graph below is a plot of the number of neutrons versus the number of protons in various stable isotopes. Stable nuclei with atomic numbers up to about 20 have an n/p ratio of about 1/1.
Above Z = 20, the number of neutrons always exceeds the number of protons in stable isotopes. The stable nuclei are located in the pink band known as the belt of stability. The belt of stability ends at lead-208.
NUMBER OF NUCLEONS
No nucleus higher than lead-208 is stable. That's because, although the nuclear strong force is about 100 times as strong as the electrostatic repulsions, it operates over only very short distances. When a nucleus reaches a certain size, the strong force is no longer able to hold the nucleus together.