AMERICIUM

(Am), synthetic chemical element (atomic number 95) of the actinide series in Group IIIb 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. 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 lanthanide homologue, europium, which can be reduced to its divalent 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); AmO+2 , yellow; and AmO22+2, light tan. In the common tripositive state, americium is very similar to the other actinide and lanthanide elements. atomic number 95 stablest isotope 243 melting point above 850 C (1,550 F) specific gravity 13.67 (20 C) valence 2,3,4,5,6 electronic config. 2-8-18-32-25-8-2 or (Rn)5f 77s2