Name: Americium
Symbol: Am
Atomic Number: 95
Atomic Weight: 243.000000
Family: Rare Earth Elements
CAS RN: 7440-35-9
Description: A silvery white radioactive rare earth metal.
State (25 C): Solid
Oxidation states: +3, +4, +5, +6

Molar Volume: 17.78 cm3/mole
Valence Electrons: 5f77s2

Boiling Point: 2284oK, 2011oC, or 3652oF
Melting Point: 1449oK, 1176oC, or 2149oF

Elec. Energy Level: 2, 8, 18, 32, 25, 8, 2 
Isotopes: 19 + None Stable + 3 Meta States
Heat of Vaporization: 238.5 kJ/mol
Heat of Fusion: 14.4 kJ/mol

Density : 13.67 g/cm3 @ 300K 
Specific Heat: 0.11 J/gK 
Atomic Radius: 184 pm  
Ionic Radius: 0.982
Electronegativity: 1.3 (Pauling), 1.2 (Allrod Rochow) 

1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10f7 6s2p6 7s2


Americium was first synthesized by Glenn T. Seaborg, Leon O. Morgan, Ralph A. James, and Albert Ghiorso in lat 1944 at the wartime Metallurgical Laboratory at the University of Chicago (now known as the 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. 

The result of successive neutron capture reactions by plutonium isotopes in a nuclear reactor: 

239Pu(n,y) rarrow.gif (63 bytes) 240Pu(n,y) rarrow.gif (63 bytes) 241Pu ---B---> 241Am

Since the isotope 241Am can be prepared in relatively pure form by extraction as a decay product over a period of years from strongly neutron- bombarded plutonium, 241Pu, this isotope is used for much of the chemical investigation of this element. Better suited is the isotope 243Am due to its longer half-life (7370 years as compared to 432.2 years for 241Am).  A mixture of the isotopes 241Am, 242Am, and 243Am can be prepared by intense neutron irradiation of 241Am according to the reactions:

241Am (n,y) rarrow.gif (63 bytes) 242Am (n,y) rarrow.gif (63 bytes) 243Am

Nearly isotopically pure 243Am can be prepared by a sequence of neutron bombardments and chemical separations.  Neutron bombardment of 241Am yields 242Pu by the reactions:

241Am(n,y) rarrow.gif (63 bytes) 242Am rarrow.gif (63 bytes) 242Pu

after chemical separation the 242Pu can be transformed to 243Am via the reactions:

242Pu(n,y) rarrow.gif (63 bytes) 243Pu rarrow.gif (63 bytes) 243Am

and the 243Am can be chemically separated.  Fairly pure 242Pu can be prepared more simply by very intense neutron irradiation of 239Pu as the result of successive neutron-capture reactions.  Americium metal has been prepared by reducing the trifluoride with barium vapor at 1000oC to 1200oC or the dioxide by lanthanum metal. 

Seaborg was granted patent 3,156,523 for "Element 95 and Method of Producing Said Element".   The discovery of americium and curium was first announced informally on a children's quiz show in 1945.


Pure americium has a silvery and white lustre.  At room temperatures 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 three times that of radium.  Gram quantities of 241Am emit intense gamma rays which creates a serious exposure problem for anyone handling the element.

2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d10 5f7
6s2 6p6

Americium is thought to exist in two forms: an alpha form which has a double hexagonal close-packed structure and a loose-packed cubic beta form.  Americium must be handled with great care to avoid personal contamination.  As little as 0.03 uCi of 241Am is the maximum permissible total body burden.  The alpha activity from 241Am is about three times that of radium.  When gram quantities of 241Am are handled, the intense gamma activity makes exposure a serious problem.  Americium dioxide, AmO2, is the most important oxide.  AmF3, AmF4, AmCl3, AmBr3, AmI3, and other compounds have been prepared.  The isotope 241Am has been used as a portable source for gamma radiography.  It has also been used as a radioactive glass thickness gage for the flat glass industry, and as a source of ionization for smoke detectors.  Americum-243 is available from the Oak Ridge National Laboratory at a cost of $160/mg plus packing charges.

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 more readily obtained plutonium or uranium isotopes.


This element can be produced in kilogram amounts and has some uses (mostly 241Am 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 (about 0.2 microgram) of 241Am as a source of ionizing radiation.  241Am 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. 242Am is a neutron emitter and has found uses in Neutron radiography.  This isotope is, however, extremely expensive to produce in usable quantities.


In aqueous systems the most common oxidation state is +3. It is very much harder to oxidise Am (III) to Am (IV) than it is to oxidise Pu (III) to Pu (IV).

Currently the solvent extraction chemistry of americium is important as in several areas of the world scientists are working on reducing the medium term radiotoxicity of the waste from the reprocessing of used nuclear fuel.

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 aqeuous solution. In the environment, this americyl core could complex with carbonate as well as other oxygen moieties (OH-, NO2-, NO3-, and SO4-2) to form charged complexes which tend to be readily mobile with low affinities to soil.

  • AmO2(OH)+1
  • AmO2(OH)2+2
  • AmO2(CO3)1+1
  • AmO2(CO3)2-1
  • AmO2(CO3)3-3


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

atom.gif (700 bytes)

Atomic Mass
231Am 231.04556 ~30 seconds
232Am 232.04659 79 seconds
233Am 233.04635 3.2 minutes
234Am 234.04781 2.32 minutes
235Am 235.04795 9.9 minutes
236Am 236.04958 3.6 minutes
237Am 237.05000 73.0 minutes
238Am 238.05198 98 minutes
239Am 239.0530245 11.9 hours
240Am 240.055300 50.8 hours
241Am 241.0568291 432.2 years
242Am 242.0595492 16.02 hours
242m1Am   141 years
243Am 243.0613811 7.37 x 103 years
244Am 244.0642848 10.1 hours
244mAm   26 minutes
245Am 245.066452 2.05 hours
246Am 246.069775 39 minutes
246m1Am   25.0 minutes
247Am 247.07209 23.0 minutes
248Am 248.07575 ~3 minutes
249Am 249.07848 ~1 minutes

atom.gif (700 bytes)

Americium Data


Atomic Structure

Atomic Radius (): unknown
Atomic Volume: 17.86cm3/mol
Covalent Radius: unknown
Crystal Structure: Hexagonal
Ionic Radius: 0.982

Chemical Properties

Electrochemical Equivalents: 3.0229 g/amp-hr
Electron Work Function: unknown
Electronegativity: 1.3 (Pauling); 1.2 (Allrod Rochow)
Heat of Fusion: 14.4 kJ/mol
First Ionization Potential: 5.993
Second Ionization Potential: unknown
Third Ionization Potential: unknown
Valence Electron Potential (-eV): 44
Ionization Energy (eV): 5.993 eV

Physical Properties

Atomic Mass Average: 243.0614
Boiling Point: 2880K, 2607C,  4725F
Melting Point: 1267K,  994C,  1821F
Heat of Vaporization: 238.5 kJ/mol
Coefficient of Lineal Thermal Expansion/K-1: N/A
Electrical Conductivity: 0.022 106/cm
Thermal Conductivity: 0.1 W/cmK
Density: 13.67 g/cm3 @ 300K
Enthalpy of Atomization: 268 kJ/mole @ 25C
Enthalpy of Fusion: 14.39 kJ/mole
Enthalpy of Vaporization: unknown
Molar Volume: 17.78 cm3/mole
Specific Heat: 0.11 J/gK
Vapor Pressure: unknown

Regulatory / Health

  • CAS Number
    • 7440-35-9
  • NFPA 704
    • Health: unknown
    • Fire: unknown
    • Reactivity: unknown
    • Special Hazard: Radioactive<
    • OSHA Permissible Exposure Limit (PEL)
      • No limits set by OSHA
    • OSHA PEL Vacated 1989
      • No limits set by OSHA
    • NIOSH Recommended Exposure Limit (REL)
      • No limits set by NIOSH
    • Levels In Humans:
      Note: this data represents naturally occuring levels of elements in the typical human, it DOES NOT represent recommended daily allowances.
      • Blood/mg dm-3: unknown
      • Boneunknown
      • Liver: unknown
      • Muscle: unknown
      • Daily Dietary Intake: unknown
      • Total Mass In Avg. 70kg human: unknown

    Who / Where / When / How

    • Discoverer: Glen T. Seaborg, R. A. James, L. O. Morgan, A. Ghiorso
    • Discovery Location: Chicago Illinois United States
    • Discovery Year: 1944
    • Name Origin:
      From America by analogy with europium.
    • Abundance:
      • Estimated Crustal: unknown
      • Estimated Oceanic: unknown
      • Atmosphere: unknown
      • Sun (Relative to H=1E12): unknown
    • Sources:
      Produced by bombarding plutonium with neutrons. Total world production is probably around a few kilograms per year.
    • Uses:
      In the past Americium was used as a source of radiation for radiography.
    • Additional Notes:
      Americium is normally never encountered out side of the lab. It is extremely dangerous because of its intense alpha radiation.


Critical Temperature: 10527C
Electrical Resistivity: 68 Ocm