Radioactive isotopes (radioisotopes or radionuclides) are isotopes which have an unstable atomic nucleus. Radioisotopes release energy in the form of radiation, which mainly acts in the form of alpha, beta or gamma rays, emit particles (there is a process of radioactive decay) until the nucleus becomes stable.
There are isotopes: stable and radioactive. More than 1000 radioactive isotopes of chemical elements are known. Of these, 50 occur naturally, and most are obtained artificially. In fact, every chemical element has one or more radioactive isotopes.
Each radioactive isotope has its own half-life and own unique rate at which it decays. The half-lifes of radioactive isotopes differ significantly from each other, they can vary from a few minutes to several years.