Question

why neutron to proton ratio tells about stability

Answer 1

The neutron–proton ratio (N/Z ratio ornuclear ratio) of an atomic nucleus is the ratioof its number of neutrons to its number ofprotons. Among stable nuclei and naturally occurring nuclei, this ratio generally increases with increasing atomic number. This is because electrical repulsive forces between protons scale with distance differently thanstrong nuclear force attractions. In particular, most pairs of protons in large nuclei are not far enough apart, such that electrical repulsion dominates over the strong nuclear force, and thus proton density in stable larger nuclei must be lower than in stable smaller nuclei where more pairs of protons have appreciable short-range nuclear force attractions.