(ămӘrĭ'shēӘm), artificially produced radioactive chemical element; symbol Am; at. no. 95; mass no. of most stable isotope 243; m.p. about 1,175 degrees Celsius; b.p. about 2,600 degrees Celsius; sp. gr. 13.67 at 20 degrees Celsius; valence +2, +3, +4, +5, or +6. Americium is a silver-white metal thought to have either a loose-packed cubic or a close-packed double hexagonal crystalline structure. The pure metal has been prepared by reduction of americium trifluoride with barium vapor at about 1,100 degrees Celsius. It tarnishes slowly in dry air.
All 16 known isotopes are radioactive. Americium-243, the most stable isotope, has a half-life of more than 7,300 years. Americium-241, which has a half-life of about 430 years, is more often used in chemical investigations, since it is easily prepared in a fairly pure form; it is also used in industrial measuring devices, radiology, and household smoke detectors.
The fourth transuranium elementto be synthesized, americium is a member of the actinide series in Group 3 of the periodic table. It was discovered in 1944 by Glenn T. Seaborg, Ralph A. James, Leon O. Morgan, and Albert Ghiorso, who bombarded plutonium-239 with neutrons to form plutonium-241, which decays to form americium-241.
Americium ( atomic number = 95; relative atomic mass = 243; melting point = 994°C; boiling point = 2607°C) is a man-made element...
(kăl'ĭfôr´´nēӘm) [from California], artificially produced, radioactive metallic chemical element; symbol Cf; at. no. 98; mass no. of most stable iso
(bûr'klēӘm) [from Berkeley], artificially produced radioactive chemical element; symbol Bk; at. no. 97; mass no. of most stable isotope 247; m.p. ab