What is characterized as a radionuclide?

A radionuclide is defined as an unstable isotope of an element that undergoes radioactive decay, emitting radiation in the process. This emission can take the form of alpha particles, beta particles, or gamma rays, depending on the nature of the decay. Unlike stable isotopes, which do not change over time and do not emit radiation, radionuclides are inherently unstable and will eventually transform into different elements or isotopes while releasing energy in the form of radiation.

This characteristic of emitting radiation is what distinguishes radionuclides from stable isotopes and non-radioactive elements. Also, a byproduct of chemical reactions does not specifically define a radionuclide, as many byproducts can be stable or non-radioactive. Thus, the understanding of what constitutes a radionuclide is centered on its unstable nature and the radiation it emits.