|Boiling Point: 2284oK, 2011oC, or 3652oF
Melting Point: 1449oK, 1176oC, or 2149oF
Elec. Energy Level: 2, 8, 18, 32, 25, 8, 2
Isotopes: 19 + None Stable + 3 Meta States
Heat of Vaporization: 238.5 kJ/mol
Heat of Fusion: 14.4 kJ/mol
Density : 13.67 g/cm3 @ 300°K
Specific Heat: 0.11 J/g°K
Atomic Radius: 184 pm
Ionic Radius: 0.982Å
Electronegativity: 1.3 (Pauling), 1.2 (Allrod Rochow)
1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10f7 6s2p6 7s2
Americium was first synthesized by Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and Albert Ghiorso in lat 1944 at the wartime Metallurgical Laboratory at the University of Chicago (now known as the 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.
The result of successive neutron capture reactions by plutonium isotopes in a nuclear reactor:
239Pu(n,y) 240Pu(n,y) 241Pu ---B---> 241Am
Since the isotope 241Am can be prepared in relatively pure form by extraction as a decay product over a period of years from strongly neutron- bombarded plutonium, 241Pu, this isotope is used for much of the chemical investigation of this element. Better suited is the isotope 243Am due to its longer half-life (7370 years as compared to 432.2 years for 241Am). A mixture of the isotopes 241Am, 242Am, and 243Am can be prepared by intense neutron irradiation of 241Am according to the reactions:
241Am (n,y) 242Am (n,y) 243Am
Nearly isotopically pure 243Am can be prepared by a sequence of neutron bombardments and chemical separations. Neutron bombardment of 241Am yields 242Pu by the reactions:
241Am(n,y) 242Am 242Pu
after chemical separation the 242Pu can be transformed to 243Am via the reactions:
242Pu(n,y) 243Pu 243Am
and the 243Am can be chemically separated. Fairly pure 242Pu can be prepared more simply by very intense neutron irradiation of 239Pu as the result of successive neutron-capture reactions. Americium metal has been prepared by reducing the trifluoride with barium vapor at 1000oC to 1200oC or the dioxide by lanthanum metal.
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.
Pure americium has a silvery and white 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.
Americium is thought to exist in two forms: an alpha form which has a double hexagonal close-packed structure and a loose-packed cubic beta form. Americium must be handled with great care to avoid personal contamination. As little as 0.03 uCi of 241Am is the maximum permissible total body burden. The alpha activity from 241Am is about three times that of radium. When gram quantities of 241Am are handled, the intense gamma activity makes exposure a serious problem. Americium dioxide, AmO2, is the most important oxide. AmF3, AmF4, AmCl3, AmBr3, AmI3, and other compounds have been prepared. The isotope 241Am has been used as a portable source for gamma radiography. It has also been used as a radioactive glass thickness gage for the flat glass industry, and as a source of ionization for smoke detectors. Americum-243 is available from the Oak Ridge National Laboratory at a cost of $160/mg plus packing charges.
Americium is also fissile; the critical mass for an unreflected sphere of 241Am is approximately 60 kilograms. It is unlikely that Americium would be used as a weapons material, as its minimum critical mass is considerably larger than more readily obtained plutonium or uranium isotopes.
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 microgram) 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. This isotope is, however, extremely expensive to produce in usable quantities.
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 oxidise Pu (III) to Pu (IV).
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.
Americium, unlike uranium, does not readily form a dioxide americyl core (AmO2). This is because americium is very hard to oxidise above 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 moieties (OH-, NO2-, NO3-, and SO4-2) to form charged complexes which tend to be readily mobile with low affinities to soil.
19 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 (t½ 141 years). The isotopes of americium range in atomic weight from 231.046 amu (231Am) to 249.078 amu (249Am).
|243Am||243.0613811||7.37 x 103 years|
Atomic Radius (Å): unknown
Electrochemical Equivalents: 3.0229 g/amp-hr
Atomic Mass Average: 243.0614
Critical Temperature: 10527°C