ABSTRACT
A radionuclide is a nuclide with an unbalanced and unstable nucleus. This chapter describes the origin and nature of radionuclides and radioactive decay or radioactivity as the basic properties of radionuclides. It discusses the recent importance of radionuclides, including their use and their health risks. Since the history of radionuclides is immediately connected with our understanding of matter, and thus with the study of atoms and with the discovery of radioactivity, a brief discussion of the history of the atom and radioactivity is presented. Apart from stable chemical elements, very low concentrations of radioactive elements occur naturally in the environment. These natural radionuclides can be divided into three categories according to their origin and formation: primordial radionuclides, secondary radionuclides, and cosmogenic radionuclides. To produce a radioactive nucleus from a stable nucleus, it is necessary to change the number of protons or neutrons so as not to disturb the equilibrium configuration.
