americium

/am'euh rish"ee euhm/, n. Chem.

a transuranic element, one of the products of high-energy helium bombardment of uranium and plutonium. Symbol: Am; at. no.: 95.

[1945-50, Amer.; AMERIC(A) + -IUM]

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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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▪ chemical element

 (Am), 

 synthetic chemical element (atomic number 95) of the actinoid series of the periodic table. Undetected in nature, americium (as the isotope americium-241) was artificially produced from plutonium-239 (atomic number 94) in 1944 by Glenn T. Seaborg, Ralph A. James, Leon O. Morgan, and Albert Ghiorso in a nuclear reactor. It was the fourth transuranium element to be discovered (curium, atomic number 96, was discovered a few months previously). The metal is silvery white and tarnishes slowly in dry air at room temperature. The isotope americium-241 is the most important because of its availability; it has been prepared in kilogram amounts from plutonium and has been used industrially in fluid-density gauges, thickness gauges, aircraft fuel gauges, and distance-sensing devices, all of which utilize its gamma radiation. The isotope's alpha-particle emission is exploited in smoke detectors. All isotopes of americium are radioactive; the stablest isotope, americium-243, has proved more convenient for chemical investigations in view of its longer half-life (7,370 years as compared with 458 years for americium-241).

      Americium reacts with oxygen to form the dioxide AmO₂ and with hydrogen to form the hydride AmH₂. There is some evidence that the ion Am²⁺ has been prepared in trace amounts, its existence suggesting that americium is similar to its lanthanoid homologue, europium, which can be reduced to its +2 oxidation state. Americium has four oxidation states, from +3 to +6, in acidic aqueous solution with the following ionic species: Am³⁺, pink; Am⁴⁺, rose (very unstable); AmO₂ ⁺, yellow; and AmO₂²⁺, light tan. In the common +3 state, americium is very similar to the other actinoid and lanthanoid elements.

atomic number

95

stablest isotope

243

melting point

above 850° C (1,550° F)

specific gravity

13.67 (20° C)

oxidation states

+2, +3, +4, +5, +6

electronic config.

[Rn]5f ⁷7s²

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