The stability of isotope is determined by the ratio of neutrons to protons. Elements with (Z<20) are lighter and the nuclei of these elements have a 1:1 ratio and tend to have the same proton and neutron number. Elements with atomic numbers between 20 and 83 are heavy elements, so the ratio is different. The ratio is 1.5:1.
Stable isotope does not emit radiation but if it does its half-life is too long to be measure while unstable isotope emits radiation.
