Americium Element Facts

The chemical element americium is classed as an an actinide metal. It was discovered in 1944 by Seaborg, James and Morgan.

Americium-241 under the microscope. Image Ref.(3)

Data Zone

Classification: Americium is an actinide metal
Color: silvery-white
Atomic weight: (243), no stable isotopes
State: solid
Melting point: 1176 oC, 1449 K
Boiling point: 2607 oC, 2880 K
Electrons: 95
Protons: 95
Neutrons in most abundant isotope: 148
Electron shells: 2,8,18,32,25,8,2
Electron configuration: [Rn] 5f7 7s2
Density @ 20oC: 13.67 g/cm3
Show more, including: Heats, Energies, Oxidation, Reactions, Compounds, Radii, Conductivities
Atomic volume: 17.78 cm3/mol
Structure: hcp: hexagonal close-packed
Hardness: mohs
Specific heat capacity 0.11 J g-1 K-1
Heat of fusion 14.40 kJ mol-1
Heat of atomization 266 kJ mol-1
Heat of vaporization 238.5 kJ mol-1
1st ionization energy 578 kJ mol-1
2nd ionization energy
3rd ionization energy
Electron affinity
Minimum oxidation number 0
Min. common oxidation no. 0
Maximum oxidation number 6
Max. common oxidation no. 3
Electronegativity (Pauling Scale) 1.3
Polarizability volume 23.3 Å3
Reaction with air
Reaction with 15 M HNO3
Reaction with 6 M HCl
Reaction with 6 M NaOH
Oxide(s) AmO, AmO2, Am2O3
Hydride(s) AmH2 AmH3
Chloride(s) AmCl2, AmCl3
Atomic radius 173 pm
Ionic radius (1+ ion)
Ionic radius (2+ ion)
Ionic radius (3+ ion) 111.5 pm
Ionic radius (1- ion)
Ionic radius (2- ion)
Ionic radius (3- ion)
Thermal conductivity 10 W m-1 K-1
Electrical conductivity 0.7 x 106 S m-1
Freezing/Melting point: 1176 oC, 1449 K

Discovery of Americium

Dr. Doug Stewart

Americium was the fourth synthetic transuranium element of the actinide series to be discovered.

Americium-241 was first identified in 1944 by Seaborg, James and Morgan at the metallurgical laboratory at the University of Chicago. It was produced by the beta-particle decay of plutonium-241, which had been produced in a nuclear reactor by neutron bombardment of plutonium-239.

The researchers at first referred to americium as “pandemonium” owing to the difficulties they encountered trying to isolate it from another new element with which it was very closely associated,
curium – or “delirium” as it was first called.

Americium was first isolated as a pure compound by Burris Cunningham in 1945, at the University of Chicago.

The element was named after America, because it is located below Europium (element 63) in the periodic table, which was named after Europe.

Americium-241 radiation emissions (preceded by Pitchblend ore ) displayed in a cloud chamber

Americium hydroxide

The triangle in the glass tube contains the world’s first sample of americium. (Photo credit: Berkeley Lab)

Transuranium elements discovery and experiments. 1963 chemistry educational documentary narrated by Glenn Seaborg, Stanley Thompson and Albert Ghiorso.

Appearance and Characteristics

Harmful effects:

Americium is harmful due to its radioactivity.


Americium is a silvery-white highly radioactive metal that has a density similar to lead.

It tarnishes slowly in dry air at room temperature.

Isotope 241Am, the most common isotope, decays to 237Np, emitting alpha and gamma radiation(1).

Uses of Americium

Isotope 241Am is used (in the form of americium dioxide) in very small amounts in ‘ionization chamber’ smoke detectors. One gram of americium dioxide provides enough active material for more than three million household smoke detectors(2).

Americium is used as a portable source of gamma rays and alpha particles for use in medicine, science and industry.

It is also used as a target material in nuclear research to make even heavier elements.

Abundance and Isotopes

Abundance earth’s crust: nil

Abundance solar system: negligible

Cost, pure: $1500 per g

Cost, bulk: per 100g

Source: Americium is obtained as a by-product of plutonium processing.

Isotopes: Americium has 14 isotopes whose half-lives are known, with mass numbers 232 to 247. Americium has no naturally occurring isotopes. Its longest lived isotopes are 243Am, with a half-life of 7370 years, 241Am with a half-life of 432.2 years and 242Am with a half-life of 141 years.


1. James D. Navratil, Wallace W. Schulz, Glenn T. Seaborg., The Most Useful Actinide Isotope: Americium-241., Journal of Chemical Education. 67.1 (1990): p15-16.
2. Smoke Detectors and Americium., World Nuclear Association.
3. Photo: Bionerd

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