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(ămərĭ`shēəm), artificially produced radioactive chemical element; symbol Am; at. no. 95; mass no. of most stable isotopeisotope
, in chemistry and physics, one of two or more atoms having the same atomic number but differing in atomic weight and mass number. The concept of isotope was introduced by F.
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 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-lifehalf-life,
measure of the average lifetime of a radioactive substance (see radioactivity) or an unstable subatomic particle. One half-life is the time required for one half of any given quantity of the substance to decay.
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 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 elementtransuranium elements,
in chemistry, radioactive elements with atomic numbers greater than that of uranium (at. no. 92). All the transuranium elements of the actinide series were discovered as synthetic radioactive isotopes at the Univ.
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to be synthesized, americium is a member of the actinide seriesactinide series,
a series of radioactive metallic elements in Group 3 of the periodic table. Members of the series are often called actinides, although actinium (at. no. 89) is not always considered a member of the series.
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 in Group 3 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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. It was discovered in 1944 by Glenn T. SeaborgSeaborg, Glenn Theodore
, 1912–99, American chemist, b. Ishpeming, Mich., grad. Univ. of California at Los Angeles, 1934, Ph.D. Univ. of California at Berkeley, 1937.
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, 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.



(Am), a synthetic radioactive chemical element belonging to the actinide series; atomic number 95. Americium has no stable isotopes. It was synthesized at the end of 1944 and beginning of 1945 by a group of American scientists—G. Seaborg, R. James, L. Morgan, and A. Ghiorso—by neutron bombardment of 239Pu. The name “americium” derives from the word “America” by analogy with americium’s homologue in the lanthanide series, europium, which occupies the same place (sixth) after lanthanium as americium after actinium.

Americium has known isotopes with mass numbers from 237 through 246, nuclear isomers 242mAm and 244mAm, and the so-called fissionable nuclear isomers 238mfAm, 240mfAm, 242mfAm, and 244mfAm. The rapid radioactive disintegration of these isomers (the half-life time [T1/2] is between 60 microseconds to 14 milliseconds) proceeds by spontaneous fission; the spontaneous fission of such induced nuclear isomers was revealed using 243mfAm by a group of Soviet physicists in Dubna. The longest-lived isotope, 243Am, is α-active (T1/2 = 7,950 years). The production of amercium in milligram quantities is extremely difficult but was accomplished after 1960. Another isotope of americium, 241Am (α-decay, T1/2 = 458 years), is formed in atomic reactors as a byproduct of 239 Pu and can be extracted from spent nuclear fuel elements in weighable amounts. 241 Am is extracted by coprecipitation with lanthanium salts and by chromatographic and extraction methods.

The element americium is a silvery metal with a hexagonal lattice. It has a density of 13.67 g/m3, a melting point of 995±4°C, and a boiling point of 2607°C. Almost all the chemical data have been gained from the observation of 241Am. As a result of its own α-radiation, americium and its solid compounds glow in the dark.

In compounds americium has a valence from three to six. Americium is the first actinide which in solutions has three as its most stable valence. The solutions of Am3+ are bright pink. Three solid compounds are known in which americium is tetravalent: AmO2, AmF4, and KAmF5. In an aqueous medium, Am4+ exists only as a complex fluoride ion; under other conditions it is quickly reduced by water to Am3+. Oxidation of Am3+ in a weakly alkaline medium produces pentavalent americium in the form of an ion, AmO2+. In strongly acid solutions, the yellow ion AmO2+ disproportions, producing Am3+ and a yellowish brown ion Am022+ in which americium is hexavalent. The latter ion is also formed by oxidation of acid solutions of AmO2+ or by the action of certain strong oxidizing agents on Am3+. Because of their own strong α-radiation, both 241AmO2+ and 241AmO 22+ energetically reduce in aqueous solutions.

A mixture of 241 Am and beryllium is used in the preparation of neutron sources.


Penneman, R., and T. Keenan. Radiokhimiia ameritsiia i kiuriia. Moscow, 1961. (Translated from English.)



A chemical element, symbol Am, atomic number 95; the mass number of the isotope with the longest half-life is 243.


a white metallic transuranic element artificially produced from plutonium. It is used as an alpha-particle source. Symbol: Am; atomic no.: 95; half-life of most stable isotope, 243Am: 7.4 × 103 years; valency: 2,3,4,5, or 6; relative density: 13.67; melting pt.: 1176°C; boiling pt.: 2607°C (est.)