Chemical Properties
Americium readily reacts with oxygen and dissolves well in acids. The most common oxidation state for americium is +3, in which americium compounds are rather stable against oxidation and reduction. In this sense, americium is chemically similar to most lanthanides. The trivalent americium forms insoluble fluoride, oxalate, iodate, hydroxide, phosphate and other salts. Other oxidation states have been observed between +2 and +7, which is the widest range among the actinide elements. Their color in aqueous solutions varies as follows: Am3+ (colorless to yellow-reddish), Am4+ (yellow-reddish), AmVO+
2; (yellow), AmVIO2+
2 (brown) and AmVIIO5−
6 (dark green). All oxidation states have their characteristic optical absorption spectra, with a few sharp peaks in the visible and mid-infrared regions, and the position and intensity of these peaks can be converted into the concentrations of the corresponding oxidation states. For example, Am(III) has two sharp peaks at 504 and 811 nm, Am(V) at 514 and 715 nm, and Am(VI) at 666 and 992 nm.
Americium compounds with oxidation state +4 and higher are strong oxidizing agents, comparable in strength to the permanganate ion (MnO−
4) in acidic solutions. Whereas the Am4+ ions are unstable in solutions and readily convert to Am3+, the +4 oxidation state occurs well in solids, such as americium dioxide (AmO2) and americium(IV) fluoride (AmF4).
All pentavalent and hexavalent americium compounds are complex salts such as KAmO2F2, Li3AmO4 and Li6AmO6, Ba3AmO6, AmO2F2. These high oxidation states Am(IV), Am(V) and Am(VI) can be prepared from Am(III) by oxidation with ammonium persulfate in dilute nitric acid, with silver(I) oxide in perchloric acid, or with ozone or sodium persulfate in sodium carbonate solutions. The pentavalent oxidation state of americium was first observed in 1951. It is present in aqueous solution in the form of AmO+
2 ions (acidic) or AmO−
3 ions (alkaline) which are however unstable and subject to several rapid disproportionation reactions:
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