A radioactive series is a chain of radioactive decays which results transforms an unstable nucleus into a stable nucleus. Unlike simple alpha, beta, or gamma decay, there are multiple decays involved in a radioactive series. The decays of the series occur along the nuclear stability curve, an experimentally determined line on a graph of neutron number versus proton number (atomic number). For proton numbers less than about 20, the stability curve traces the line of equal numbers of protons and neutrons. Above proton number 20, there normally are more neutrons than protons in stable nuclei.

Each nucleus in the series that results from a previous decay either has excessive neutrons or excessive protons to lie on the stability curve, hence it will also decay. The decay mode is usually determined by whether the nucleus has too many protons or neutrons. If the nucleus has excessive protons, the nucleus...