americium

Radioactive metallic element of the actinide series, atomic number 95, relative atomic mass 243.13; it was first synthesized in 1944. It occurs in nature in minute quantities in pitchblende and other uranium ores, where it is produced from the decay of neutron-bombarded plutonium, and is the element with the highest atomic number that occurs in nature. It is synthesized in quantity only in nuclear reactors by the bombardment of plutonium with neutrons. Its longest-lived isotope is Am-243, with a half-life of 7,650 years.

The element was named by US nuclear chemist Glenn Seaborg, one of the team who first synthesized it, by analogy with europium, the corresponding lanthanide. Ten isotopes are known.