Americium, a radioactive chemical element. In pure form, it is a silvery metal. Americium does not occur in nature; it is artificially created. Its most easily manufactured isotope, americium 241, is produced by bombarding plutonium with neutrons. Americium 241 has a half-life of about 450 years. It is used in radiochemical research and as a source of ionization for smoke detectors. The longest-livedisotope is americium 243, which has a halflife of about 7,400 years.

Americium was first produced in 1944 by Glenn T. Seaborg, Ralph A. James, Leon O. Morgan, and Albert Ghiorso at the University of Chicago. Seaborg named the element americium after the Americas.

Symbol: Am. Atomic number: 95. Atomic weight: 243. Specific gravity: 13.67. Americium has 11 isotopes: Am-237 through Am-247. Melting point: about 1,821 F. (994 C.). Boiling point: 4,725 F. (2,607 C.). Americium is a transuranium element belonging to the actinide series of the Periodic Table and can have a valence of + 2, 3, 4, 5, or 6.