What name is given to the spontaneous breakup of an unstable atomic nucleus?

The spontaneous breakup of an unstable atomic nucleus is called radioactive decay. During decay, a nucleus changes into a more stable configuration, often becoming a different nuclide and releasing radiation or particles.

The main decay modes are alpha decay, beta decay, and gamma decay. Alpha decay ejects a helium-4 nucleus; beta decay changes a neutron into a proton or a proton into a neutron while emitting a beta particle; gamma decay releases excess energy as a high-energy photon. Some nuclei can also undergo spontaneous fission.

Radioactive decay is random for any individual nucleus, so scientists cannot predict exactly when one atom will decay. For a large sample, however, the process follows a precise statistical law described by its half-life—the time required for half the radioactive nuclei in a sample to decay.

Radioactive decay is not the same as nuclear fusion, which joins light nuclei, or chemical oxidation, which changes electron arrangements. It is a nuclear process involving the nucleus itself. “Radioactivity” describes the phenomenon, while “radioactive decay” describes the transformation producing it.