Americium is a synthetic radioactive chemical element with the symbol Am and atomic number 95. It is a transuranic member of the actinide series, in the periodic table located under the lanthanide element europium and was thus named after the United States by analogy. 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, as 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 most 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, ranging from +2 to +7, and can be identified by their characteristic optical absorption spectra. The crystal lattices of solid americium and its compounds contain small intrinsic radiogenic defects, due to metamictization 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.

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Atomic properties

Atomic mass(243.061 u)

Atomic radii

Radius (empirical)173 pm
Covalent radius180 ±6 pm

Atomic shell

Electron configurationRn 5f7 7s2
Ionization energy(1st) 5.97381 eV
(2nd) 11.7 eV
(3rd) 21.7 eV
(4th) 36.8 eV
(5th) 50 eV
Shell model

Physical properties

Density13.67 g·cm−3
Molar volume1.778·10-5 m3·mol−1


Melting point1,449 K
Boiling point2,880 K
Liquid range(1,431 K)
Transition temperature1 K

Heat and conductivity

Specific heat capacity0.11 J·kg−1·K−1
Thermal conductivity10 W·m-1·K-1 (300 K)

Electrical properties

Electrical conductivity1.47·108 S·m-1


Magnetic susceptibility0.001 cm3·mol−1

Chemical properties

Oxidation state+2, +3, +4, +5, +6, (+7)
Standard potential-2.07 V (Am3+ + 3e- → Am)


Pauling scale1.3
Allred-Rochow scale1.2
Nagle scale1.1

Other properties

Natural occurrencesynthetic
Crystal structureDouble hexagonal close-packed
Goldschmidt Classificationsynthetic
Superconductorwith transition tempperature (solid body, normal pressure)