Name, Symbol, Number americium, Am, 95
Chemical series actinides
Group, Period, Block 7, f
Appearance silvery white
Atomic properties
Atomic weight [243] amu
Atomic radius (calc.) 175 pm (no data)
Covalent radius no data
van der Waals radius no data
Electron configuration [Rn]5f7 7s2
e- 's per energy level 2,8,18,32,25,8,2
Oxidation states (Oxide) 6,5,4,3 (amphoteric)
Physical properties
Density 13700 kg/m3
State of matter solid
Melting point 1449 K (2149 ?F)
Boiling point 2880 K (4725 ?F)
Crystal structure hexagonal
Hardness no data
Magnetic ordering no data
Molar volume 17.63 ×10-6 m3/mol
Heat of vaporization no data
Heat of fusion 14

.4 kJ/mol

Vapor pressure no data
Velocity of sound no data
Electronegativity 1.3 (Pauling scale)
Specific heat capacity no data
Electrical conductivity 2.2 106/m ohm
Thermal conductivity 10 W/(m*K)
1st ionization potential 578 kJ/mol
Most stable isotopes
iso NA half-life DM DE MeV DP
241Am {syn.} 432.2 y SF
242mAm {syn.} 141 y IT
243Am {syn.} 7370 y SF
SI units & STP are used except where noted.

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.

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Notable characteristics

Freshly prepared americium metal has a white and silvery luster (more silvery than plutonium or neptunium) and at room temperatures it slowly tarnishes in dry air. 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.


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.


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.


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? 141 years). The isotopes of americium range in atomic weight from 231.046 amu (Am-231) to 249.078 amu (Am-249).


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