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americium, 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 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.

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Periodic Table of the Elements: Americium

Atomic Number:	Atomic Number: 95
Atomic Symbol:	Atomic Symbol: Am
	Name of Element: Americium
Atomic Weight:	Atomic Weight: (243)
Electron Configuration:	Electron Configuration: 2 · 8 · 1832 · 258 · 2

The Columbia Electronic Encyclopedia Copyright © 2004.
Licensed from Columbia University Press

americium

Synthetic radioactive chemical element, chemical symbol Am, atomic number 95. The fourth transuranium element discovered, it was first produced in 1944 from plutonium-239 in a nuclear reactor. The isotope americium-241 has been prepared in kilogram quantities and is used in a variety of measuring applications that utilize its gamma radiation. Its most familiar use is in household smoke detectors.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.