What describes the emission of particles from an unstable nucleus?

The emission of particles from an unstable nucleus is best described by radioactivity. This phenomenon occurs when unstable atomic nuclei lose energy by emitting radiation, which can involve alpha particles, beta particles, or gamma rays. Radioactive decay is a natural process that occurs in certain isotopes, leading to the transformation of one element into another or a simpler form of itself.

In contrast, nuclear fission involves the splitting of a large atomic nucleus into smaller nuclei, along with the release of energy, but it specifically pertains to a reaction initiated under certain conditions rather than a general emission from an unstable nucleus. Electromagnetic radiation refers to a form of energy that travels through space as a wave and can be emitted by various processes, but it is not limited to emissions from unstable nuclei. A chemical reaction involves the rearrangement of atoms and the formation or breaking of chemical bonds, which is unrelated to nuclear processes. Hence, radioactivity is the term that accurately characterizes the specific emission of particles from unstable nuclei.