Memory is a bit rusty on this (30 years since I did nuclear physics) but elements have stable and unstable isotopes (with the same number of protons and a different number of neutrons). E.g. Carbon 12 and 13 are stable and Carbon 14 is unstable (half-life 5730 years). An atom will keep decaying until it reaches a stable form. Then it will stay like that until some nuclear fusion or fission event occurs.