radioactive isotope
an isotope of a chemical element that emits radiation during its decay to a stable form. Different isotopes of an element all have the same number of nuclear protons but different numbers of neutrons. The radiation emitted by a radioactive isotope consists of alpha or beta particles or gamma rays, which are produced as the isotope decays into simpler atoms and loses energy while gaining stability. Radioactive isotopes are widely used in diagnostic, research, and therapeutic techniques; they affect photographic film, produce an electric charge in the surrounding air, produce fluorescence with certain other substances, and have the ability to destroy or alter cells or microorganisms. Also called radioisotope.