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americium[amuri´shEum] Pronunciation Key, artificially produced radioactive chemical element; symbol Am; at. no. 95; mass no. of most stable isotope 243; m.p. about 1,175°C; b.p. about 2,600°C; sp. gr. 13.67 at 20°C; 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°C. It tarnishes slowly in dry air. All 16 known isotopes are radioactive. Americium-243, the most stable isotope, has a half-life of over 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 IIIb 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.
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