Americium

95 plutonium ← americium → curium Eu ↑ Am ↓ (Uqp) periodic table
General
Name, Symbol, Number	americium, Am, 95
Chemical series	actinides
Group, Period, Block	n/a, 7, f
Appearance	silvery white
Atomic mass	(243) g/mol
Electron configuration	[Rn] 5f7 7s2
Electrons per shell	2, 8, 18, 32, 25, 8, 2
Physical properties
Phase	solid
Density (near r.t.)	12 g/cm³
Melting point	1449 K (1176 °C, 2149 °F)
Boiling point	2880 K (2607 °C, 4725 °F)
Heat of fusion	14.39 kJ/mol
Heat capacity	(25 °C) 62.7 J/(mol·K)
Vapor pressure P/Pa 1 10 100 1 k 10 k 100 k at T/K 1239 1356
Atomic properties
Crystal structure	hexagonal
Oxidation states	6, 5, 4, 3 (amphoteric oxide)
Electronegativity	1.3 (Pauling scale)
Ionization energies	1st: 578 kJ/mol
Atomic radius	175 pm
Miscellaneous
Magnetic ordering	no data
Thermal conductivity	(300 K) 10 W/(m·K)
CAS registry number	7440-35-9
Notable isotopes
Main article: Isotopes of americium iso NA half-life DM DE (MeV) DP 241Am syn 432.2 y SF - - α 5.638 237Np 242mAm syn 141 y IT 0.049 - α 5.637 238Np SF - - 243Am syn 7370 y SF - - α 5.438 239Np
References

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 luster, 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 Am-241 is approximately three times radium. Gram quantities of Am-241 emit intense gamma rays which creates a serious exposure problem for anyone handling the element.

Applications

This element can be produced in kilogram amounts and has some uses (mostly Am-241 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 of Am-241 as a source of ionizing radiation. Am-241 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. Am-242 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 Am-241 by subjecting plutonium-239 to successive neutron capture reactions in a nuclear reactor. This created Pu-240 and then Pu-241 which in turn decayed into Am-241 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. [1]

Isotopes

18 radioisotopes of americium have been characterized, with the most stable being Am-243 with a half-life of 7370 years, and Am-241 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 Am-242m (t_½ 141 years). The isotopes of americium range in atomic weight from 231.046 amu (Am-231) to 249.078 amu (Am-249).

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 like other actinides readily forms a dioxide americyl core (AmO₂)[2]. In the environment, this americyl core readily complexes 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

Wikimedia Commons has media related to:

Americium

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