Transuranium Elements

Elements with atomic numbers greater than 92 (uranium) are called transuranium elements. All transuranium elements are radioactive, and they are generally not found in nature. Scientists can synthesize a transuranium element by the artificial transmutation of a lighter element.

Neptunium was the first transuranium element synthesized. In 1940, scientists at the University of California, Berkeley, bombarded uranium-238 with neutrons, producing uranium-239. The uranium-239 underwent beta decay to form neptunium-239.

${}_{92}{}^{239}\mathrm{U}\to {}_{93}{}^{239}\mathrm{Np}+{}_{-1}{}^0\mathrm{e}$

Although most transuranium elements have only been produced for research, some are synthesized for industrial or consumer use. For example, americium-241 is a transuranium element used in smoke detectors. As americium-241 decays, it emits alpha radiation. This radiation ionizes the air inside a smoke detector to allow an electric current to flow. When smoke enters the smoke detector, it disrupts the current and the alarm goes off. Another useful transuranium element is plutonium-238. Figure 13 shows a space probe that runs on electrical energy generated by the decay of plutonium-238.