|Name, symbol, number||americium, Am, 95|
|Group, period, block||n/a, 7, f|
|Standard atomic weight||(243) g·mol−1|
|Electron configuration||[Rn] 5f7 7s2|
|Electrons per shell||2, 8, 18, 32, 25, 8, 2 (Image)|
|Density (near r.t.)||12 g·cm−3|
|Melting point||1449 K, 1176 °C, 2149 °F|
|Boiling point||2880 K, 2607 °C, 4725 °F|
|Heat of fusion||14.39 kJ·mol−1|
|Specific heat capacity||(25 °C) 62.7 J·mol−1·K−1|
|Oxidation states||6, 5, 4, 3, 2
|Electronegativity||1.3 (Pauling scale)|
|Ionization energies||1st: 578 kJ·mol−1|
|Atomic radius||173 pm|
|Covalent radius||180±6 pm|
|Magnetic ordering||no data|
|Thermal conductivity||(300 K) 10 W·m−1·K−1|
|CAS registry number||7440-35-9|
|Most stable isotopes|
|Main article: Isotopes of americium|
Americium (pronounced /ˌæməˈrɪsiəm/, AM-ə-RIS-ee-əm) is a synthetic element that has the symbol Am and atomic number 95. A radioactive metallic element, americium is an actinide that was obtained in 1944 by Glenn T. Seaborg who was bombarding plutonium with neutrons and was the fourth transuranic element to be discovered. It was named for the Americas, by analogy with europium. Americium is widely used in commercial ionization chamber smoke detectors, as well as in neutron sources and industrial gauges.
Pure americium has a silvery and white luster. At room temperature 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 3.5 times that of 226radium. 241Am emits low energy gamma rays, creating a serious exposure problem for anyone handling gram quantities of the element.
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 that of more readily obtained plutonium or uranium isotopes.
Americium oxidizes to AmO in air. Similarly, reaction with hydrogen results in AmH2 where Am is divalent. However, the most common oxidation state of Am is +3, especially in solutions which are colored red. It is much harder to oxidize Am(III) to Am(IV) than it is to oxidize Pu(III) to Pu(IV).
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 aqueous solution. In the environment, this americyl core could complex with carbonate as well as other oxygen moieties (OH−, NO−2, NO−3, and SO2−4) to form charged complexes which tend to be readily mobile with low affinities to soil: AmO2(OH)+, AmO2(OH)2+2, AmO2CO+3, AmO2(CO3)−2 and AmO2(CO3)3−3.
Examples of americium +4 compounds are Am(OH)4 and AmF4. All pentavalent and hexavalent americium compounds are complex salts such as KAmO2F2, Li3AmO4 and Li6AmO6, Ba3AmO6, AmO2F2. Hexavalent americium is a strong oxidizing agent and is reduced to AmO2+ in oxidation-reduction reactions.
A large amount of work has been done on the solvent extraction of americium, as americium and other transuranic elements are responsible for much of the long-lived radiotoxicity of spent nuclear fuel. It is thought that by removal of the americium and curium that the used fuel will only need to be isolated from people and the environment for a shorter time than that required for the isolation of untreated used fuel.
Eighteen 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 u (231Am) to 249.078 u (249Am).
Americium was first isolated 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 a patent for "Element 95 and Method of Producing Said Element", whose unusually terse claim number 1 reads simply, "Element 95." The discovery of americium and curium was first announced informally on a children's quiz show in 1945.
Americium can be produced in kilogram amounts and has some uses, mostly involving 241Am since it is easiest to produce relatively pure samples of this isotope. Americium is the only synthetic element to have found its way into the household, where one common type of smoke detector uses 241Am in the form of americium dioxide as its source of ionizing radiation. The amount of americium in a typical smoke detector when new is 1 microcurie or 0.28 microgram. This amount declines slowly as the americium decays into neptunium-237, a different transuranic element with a much longer half-life (about 2.14 million years). With its half-life of 432.2 years, the americium in a smoke detector includes about 3% neptunium after 19 years, and about 5% after 32 years.
241Am has been used as a portable source of both gamma rays and alpha particles for a number of medical and industrial uses. Gamma ray emissions from 241Am can be used for indirect analysis of materials radiography and for quality control in manufacturing fixed gauges. For example, the element has been employed to gauge glass thickness to help create flat glass. 241Am gamma rays were also used to provide passive diagnosis of thyroid function. This medical application is obsolete. 241Am can be combined with lighter elements (e.g., beryllium or lithium) to become a neutron emitter. This application has found uses in neutron radiography as well as a neutron emitting radioactive source. The most widespread use of 241AmBe neutron sources is found in moisture/density gauges used for quality control in highway construction. 241Am neutron sources are also critical for well logging applications. 242mAm has been cited for use as an advanced nuclear rocket propulsion fuel. This isotope is, however, extremely expensive to produce in usable quantities.
Americium emits alpha and gamma radiation. The alpha decay of 241Am is 3.5 times as active as that of radium. It is associated with 5.48 MeV alpha particles and 59 keV gamma emission, which can be a serious health hazard if ingested or inhaled.
[[File:|thumb|Some americium]] Americium is a chemical element. It is a radioactive metal. It has the chemical symbol Am. It has the atomic number 95. In chemistry it is placed in a group of metal elements named the actinides. Americium is a transuranic element. It is a radioactive element that does not exist in nature. Americium has to be made. It has a silver color. Americium is made by bombarding a plutonium target with neutrons. Americium is named after America,
Americium is used in most smoke detectors. The level of radioactivity is not enough to cause cancer, so it is safe to use.