any of four independent sets of unstable heavy atomic nuclei that decay through a sequence of alpha and beta decays until a stable nucleus is achieved. These four chains of consecutive parent and daughter nuclei (shown in the figure-->) begin and end among elements with atomic numbers higher than 81, which is the atomic weight of thallium; the members of each set are genetically related by alpha and beta decay. Three of the sets, the thorium series, uranium series, and actinium series, called natural or classical series, are headed by naturally occurring species of unstable nuclei that have half-lives comparable to the age of the elements. By 1935 these three radioactive series had been fully delineated. The fourth set, the neptunium series, is headed by neptunium-237, which has a half-life of 2,000,000 years. Its members are produced artificially by nuclear reactions and do not occur naturally; all their half-lives are short compared with the age of the elements.

alpha decay, symbolized by a larger arrow in the accompanying diagrams, involves the ejection from an unstable nucleus of a particle composed of two protons and two neutrons. Thus alpha emission lowers the atomic number (number of protons) by two units, the neutron number by two units, and the mass number (total of neutrons and protons) by four units. At the head of the thorium series, for example, thorium-232 undergoes alpha decay to radium-228.

branching (the decay of a given species in more than one way) occurs in all four of the radioactive series. For example, in the actinium series, bismuth-211 decays partially by negative beta emission to polonium-211 and partially by alpha emission to thallium-207.