A radioactive species of an atom. The term nuclide implies an atom of specified atomic number and mass number. In the study of biochemical processes, radioactive isotopes are used for labelling compounds that subsequently are used to investigate various aspects of the reactivity or metabolism of proteins, carbohydrates and lipids or as sources of radiation in imaging. The fate of the radionuclide in reactive products or metabolites is determined by following (counting) the emitted radiation. In analytical chemistry radionuclides are used as tracers for the localization of the analyte or in yield experiments.