Americium

Related Topics:
Americium

Americium is a synthetic element in the periodic table that has the symbol Am and atomic number 95. A radioactive metallic element, americium is an actinide that was obtained by bombarding plutonium with neutrons and was the fourth transuranic element to be discovered. It was named for the Americas, by analogy with europium.

Notable characteristics

Freshly prepared americium metal has a white and silvery lustre, at room temperatures it slowly tarnishes in dry air. It is more silvery than plutonium or neptunium and apparently more malleable than neptunium or uranium. Alpha emission from ²⁴¹Am is approximately three times that of radium. Gram quantities of ²⁴¹Am emit intense gamma rays which creates a serious exposure problem for anyone handling the element.

Please see Actinides in the environment for details of the environmental aspects of this element.

Applications

This element can be produced in kilogram amounts and has some uses (mostly ²⁴¹Am since it is easier to produce relatively pure samples of this isotope). Americium has found its way into the household, where one type of smoke detector contains a tiny amount (about 0.2 milligrams) of ²⁴¹Am as a source of ionizing radiation. ²⁴¹Am has been used as a portable gamma ray source for use in radiography. The element has also been employed to gauge glass thickness to help create flat glass. ²⁴²Am is a neutron emitter and has found uses in neutron radiography. However this isotope is extremely expensive to produce in usable quantities.

History

Americium was first synthesized by Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and Albert Ghiorso in late 1944 at the wartime Metallurgical Laboratory at the University of Chicago (now known as Argonne National Laboratory). The team created the isotope ²⁴¹Am by subjecting ²³⁹Pu to successive neutron capture reactions in a nuclear reactor. This created ²⁴⁰Pu and then ²⁴¹Pu which in turn decayed into ²⁴¹Am via beta decay. Seaborg was granted patent 3,156,523 for "Element 95 and Method of Producing Said Element". The discovery of americium and curium was first announced informally on a children's quiz show in 1945. ^*

Isotopes

18 radioisotopes of americium have been characterized, with the most stable being ²⁴³Am with a half-life of 7370 years, and ²⁴¹Am with a half-life of 432.2 years. All of the remaining radioactive isotopes have half-lives that are less than 51 hours, and the majority of these have half-lives that are less than 100 minutes. This element also has 8 meta states, with the most stable being ^242mAm (t_½ 141 years). The isotopes of americium range in atomic weight from 231.046 amu (²³¹Am) to 249.078 amu (²⁴⁹Am).

Chemistry

In aqueous systems the most common oxidation state is +3, it is very much harder to oxidise Am(III) to Am(IV) than it is to do the same oxidation for Pu(III).

Currently the solvent extraction chemistry of americium is important as in several areas of the world scientists are working on reducing the medium term radiotoxicity of the waste from the reprocessing of used nuclear fuel.

See liquid-liquid extraction for some examples of the solvent extraction of americium.

Americium unlike uranium does not readily form a dioxide americyl core (AmO₂)^*, this is because americium is very hard to oxidise about the +3 oxidation state when it is in an aqeuous solution. In the environment, this americyl core could complex with carbonate as well as other oxygen moeities (OH^-, NO₂^-, NO₃^-, and SO₄^-2) to form charged complexes which tend to be readily mobile with low affinities to soil.

AmO₂(OH)⁺¹
AmO₂(OH)₂⁺²
AmO₂(CO₃)₁⁺¹
AmO₂(CO₃)₂^-1
AmO₂(CO₃)₃^-3

References

Los Alamos National Laboratory - Americium
Guide to the Elements - Revised Edition, Albert Stwertka, (Oxford University Press; 1998) ISBN 0-19-508083-1
It's Elemental - Americium

External links

WebElements.com - Americium (also used as a reference)

Chemical elements | Actinides | Americium compounds

Gourt Home::Gourt :: Americium