A chemical element, symbol Am, atomic number 95. The isotope 241 Am is an alpha emitter with a half-life of 433 years. Other isotopes of americium range in mass from 232 to 247, but only the isotopes of mass 241 and 243 are important. The isotope 241Am is routinely separated from “old” plutonium and sold for a variety of industrial uses, such as 59-keV gamma sources and as a component in neutron sources. The longer-lived 243Am (half-life 7400 years) is a precursor in 244Cm production.

In its most prominent aqueous oxidation state, 3+, americium closely resembles the tripositive rare earths. The formal analogy to the rare earths is also marked in anhydrous compounds of both tripositive and tetrapositive americium. Americium is different in that it is possible to oxidize Am3+ to both the 5+ and 6+ states.

Americium metal has a vapor pressure markedly higher than that of its neighboring elements and can be purified by distillation. The metal is nonmagnetic and super­conducting at 0.79 K. Under high pressure the metal has been compressed to 80% of its room-temperature volume and displays the a-uranium structure.

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