Discovered | Name | Characteristics | Prepared | Uses

The element is a member of the actinide series, which stretches from actinium (Ac) to lawrencium (Lr). Americium is a synthetic element; none occurs naturally on earth.

Americium was the fourth transuranium element to be discovered (after neptunium, plutonium, and curium). The isotope 241Am was identified by Seaborg, R.A. James, L.O. Morgan and A. Ghiorso late in 1944 at what is now the Argonne National Laboratory. It was produced by neutron bombardment of plutonium--239 in a nuclear reactor to form 241Pu, which then decays to 241Am by beta emission.

239Pu + 1n 240Pu
240Pu + 1n 241Pu
241Pu 241Am + b

Kilogram quantities of 241Am can be obtained by processing the plutonium produced in reactors. (For more on the discovery, see curium.)

The name is derived from the Americas, since that is where it was discovered; the symbol Am is derived from the name. The naming followed the practice in the lanthanide series. The lanthanide element immediately above Am is Eu, europium. Hence the new transuranium element was named for its continent of discovery.

At least 11 isotopes of Am are known, ranging from 237Am to 247Am. The most stable isotope is 243Am, which has longest half-life (7400 years as compared to 432.2 years for 241Am, for example).

Americium metal can be obtained by reduction of americium trifluoride with barium vapor at 1000-1200°C.

3 Ba + 2 AmF3 3 BaF2 + 2 Am

It is whiter and more silvery than plutonium or neptunium prepared in the same manner. It appears to be more malleable than uranium or neptunium and tarnishes slowly in dry air at room temperature.

The element may exist in five oxidation states in aqueous solution, at least three of which are moderately stable.

+2 Am2+ (existence not confirmed)
+3 Am3+ (light salmon)
+4 Am4+ (pink) (very unstable)
+5 [AmO2]+ (yellow)
+6 [AmO2]2+ (light tan).

The trivalent state is highly stable and difficult to oxidize. However, the ion [AmO2]+, like the comparable plutonium-based ion, is unstable with respect to disproportionation into Am3+ and [AmO2]2+.

3 [AmO2]+(aq) Am3+(aq) + [2 AmO2]2+(aq)

Americium(IV) ion is so unstable in solution that it has not yet been detected, although tetravalent solid compounds are well known. Finally, there is some evidence that Am2+ has been prepared at very low concentrations which would be similar to the chemistry of the analogous lanthanide, europium.

Dark brown americium dioxide, AmO2, is the important oxide, but Am2O3 and AmO2 are also known. The halides AmF4, AmX3 (X = F, Cl, Br, and I), have also been prepared. And finally, americium is the only actinide for which dihalides (AmX2, X = Cl, Br, and I) have been prepared.

Americium is used in smoke detectors and as a neutron source for neutron activation analysis, which can detect trace quantities of many elements.