Isotopes are atoms with a different number of neutrons, many elements have a few different isotopes. E.g. there are two common isotopes of carbon, carbon-14 has two more neutrons than normal carbon
Usually each element only has one or two stable isotopes, the other isotopes tend to be radioactive. The nucleus is unstable so it decays and emits radiation. Carbon-14 is an unstable isotope of carbon.
The nuclei of unstable isotopes break down…