Promethium

Promethium is the only lanthanide with no stable isotope, and one of just two elements lighter than bismuth (with technetium) that are entirely radioactive. The reason is nuclear rather than chemical: by the Mattauch isobar rule, its neighbours neodymium and samarium already hold stable isotopes at every mass number promethium could use, so any promethium nucleus eventually beta-decays into one of them. The longest-lived isotope, promethium-145, has a half-life of only 17.7 years, so any promethium present when the Earth formed vanished long ago.

Chemically, promethium behaves like a typical lanthanide. Its configuration is [Xe]4f5 6s2, and it gives up the two 6s electrons and one 4f electron to form the Pm3+ ion, whose salts are pink to rose coloured. Most of what chemists know comes from tracer-scale work on promethium-147, a fission product extracted from spent nuclear fuel. Its weak beta radiation, which carries no strong gamma emission, made it useful in luminous paints and miniature nuclear batteries, where it replaced the far more hazardous radium.

Discovery

Element 61 was claimed several times in the 1920s, as 'illinium' in Illinois and 'florentium' in Florence, but neither claim held up. It was finally identified in 1945 by Jacob Marinsky, Lawrence Glendenin and Charles Coryell at Clinton Laboratories (now Oak Ridge National Laboratory), who separated it from uranium fission products using ion-exchange chromatography. The discovery was announced in 1947. Coryell's wife, Grace Mary Coryell, suggested the name after Prometheus, the Titan who stole fire from the gods.