Unsur kimia, nomor atom 95, lambang Am.
Spectral lines of americium
|Americium in the periodic table|
|Atomic number (Z)||95|
|Group, period||group n/a, period 7|
|Mass number||243 (most stable isotope)|
|Electron configuration||[Rn] 5f7 7s2|
Electrons per shell
|2, 8, 18, 32, 25, 8, 2|
|Melting point||1449 K (1176 °C, 2149 °F)|
|Boiling point||2880 K (2607 °C, 4725 °F) (calculated)|
|Density near r.t.||12 g/cm3|
|Heat of fusion||14.39 kJ/mol|
|Molar heat capacity||62.7 J/(mol·K)|
|Oxidation states||8, 7, 6, 5, 4, 3, 2 (an amphoteric oxide)|
|Electronegativity||Pauling scale: 1.3|
|Ionization energies||1st: 578 kJ/mol|
|Atomic radius||empirical: 173 pm|
|Covalent radius||180±6 pm|
|Crystal structure||double hexagonal close-packed (dhcp)|
|Thermal conductivity||10 W/(m·K)|
|Electrical resistivity||0.69 µΩ·m|
|Magnetic susceptibility (χmol)||+1000.0·10−6 cm3/mol|
|Naming||after the Americas|
|Discovery||Glenn T. Seaborg, Ralph A. James, Leon O. Morgan, Albert Ghiorso (1944)|
|Main isotopes of americium|
Americium is a radioactive transuranic chemical element with symbol Am and atomic number 95. This member of the actinide series is located in the periodic table under the lanthanide element europium, and thus by analogy was named after the Americas.
Americium was first produced in 1944 by the group of Glenn T. Seaborg from Berkeley, California, at the Metallurgical Laboratory of the University of Chicago, a part of the Manhattan Project. Although it is the third element in the transuranic series, it was discovered fourth, after the heavier curium. The discovery was kept secret and only released to the public in November 1945. Most americium is produced by uranium or plutonium being bombarded with neutrons in nuclear reactors – one tonne of spent nuclear fuel contains about 100 grams of americium. It is widely used in commercial ionization chamber smoke detectors, as well as in neutron sources and industrial gauges. Several unusual applications, such as nuclear batteries or fuel for space ships with nuclear propulsion, have been proposed for the isotope 242mAm, but they are as yet hindered by the scarcity and high price of this nuclear isomer.
Americium is a relatively soft radioactive metal with silvery appearance. Its common isotopes are 241Am and 243Am. In chemical compounds, americium usually assumes the oxidation state +3, especially in solutions. Several other oxidation states are known, which range from +2 to +8 and can be identified by their characteristic optical absorption spectra. The crystal lattice of solid americium and its compounds contain small instrinsic radiogenic defects, due to metamicitization induced by self-irradiation with alpha particles, which accumulates with time; this can cause a drift of some material properties over time, more noticeable in older samples.