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Definition: americium from Dictionary of Energy

Chemistry. a human-made radioactive element created by the neutron bombardment of plutonium; it is a crystalline silver-white transuranic element of the actinide series, having the symbol Am, atomic number 95, an atomic weight of 243 in its most stable isotope, and a half-life of 475 years. Its longest-lived isotopes, Am 241 and Am 243, are alpha-ray emitters used as radiation sources in research.


Summary Article: americium
from The Columbia Encyclopedia

(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.

The Columbia Encyclopedia, © Columbia University Press 2018

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