Radionuclide

A radionuclide is an instable nuclide which changes from a high-energy (metastable) state to a lower-energy state or decays into another nuclide spontaneously without extraneous cause emitting high-energy (ionising) radiation. Currently more than 3,300 different nuclides are known which distribute over 118 currently known elements. Of these nuclides more than 3,000 nuclides are instable and radioactive.