Americium

americium (ămərĭ`shēəm), 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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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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americium

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.)

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americium [‚am·ə′ris·ē·əm]

(chemistry)

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

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Americium 

(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 [T_1/2] 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 (T_1/2 = 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, T_1/2 = 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.

REFERENCE

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

S. S. BERDONOSOV