What term describes the process where unstable atoms decay and emit energy in particles or waves?

The process described in the question is known as radioactivity. This occurs when unstable atomic nuclei lose energy by emitting radiation, which can be in the form of particles (such as alpha and beta particles) or electromagnetic waves (such as gamma rays). Radioactivity is a natural phenomenon that results from the instability of certain isotopes, leading them to transform into more stable configurations through the emission of energy.

In contrast, nuclear fission refers specifically to the splitting of a heavy atomic nucleus into two or more lighter nuclei, accompanied by the release of energy; it is not a general term for the decay of unstable atoms. A chemical reaction involves the rearrangement of atoms to form new substances, which does not include the release of particles or waves inherent to radioactivity. Ionization refers to the process where atoms or molecules gain or lose electrons, resulting in the formation of charged particles; while this may be related to radiation, it does not describe the decay and emission of energy from unstable atoms as specifically as radioactivity does.