AMERICIUM

Introduction

Atomic Number: 95
Group: None
Atomic Weight: 243
Period: 7
CAS Number: 7440-35-9

Classification

Chalcogen
Halogen
Noble Gas
Lanthanoid
Actinoid
Rare Earth Element
Platinum Group Metal
Transuranium
No Stable Isotopes
Solid
Liquid
Gas
Solid (Predicted)

Description • Uses/Function

Americium was the fourth transuranium element to be discovered; the isotope americium-241 was identified by Seaborg, James, Morgan, and Ghiorso late in 1944 at the wartime Metallurgical Laboratory of the University of Chicago as the result of successive neutron capture reactions by plutonium isotopes in a nuclear reactor. Plutonium-239 is bombarded by two neutrons, which is followed by beta decay, leaving americium-241. Since the isotope americium-241 can be prepared in relatively pure form by extraction as a decay product over a period of years from strongly neutron bombarded plutonium-241, this isotope is used for much of the chemical investigation of this element. Better suited is the isotope americium-243 due to its longer half-life (7.37 X 10^3 years as compared to 432.2 years for americium-241). A mixture of the isotopes americium-241, americium-242, and americium-243 can be prepared by intense neutron irradiation of americium-241. Nearly isotopically pure 243Am can be prepared by a sequence of neutron bombardments and chemical separations as follows: neutron bombardment of americium-241 yields plutonium-242, after chemical separation the plutonium-242 can be transformed to americium-243 via the neutron bombardment of plutonium-242 and the americium-243 can be chemically separated. Fairly pure plutonium-242 can be prepared more simply by very intense neutron irradiation of plutonium-239 as the result of successive neutroncapture reactions. Sixteen radioactive isotopes and isomers are now recognized. Americium metal has been prepared by reducing the trifluoride with barium vapor at 1000 to 1200°C or the dioxide by lanthanum metal. The luster of freshly prepared americium metal is white and more silvery than plutonium or neptunium prepared in the same manner. It appears to be more malleable than uranium or neptunium and tarnishes slowly in dry air at room temperature. 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 microcuries of americium-241 is the maximum permissible total body burden. The alpha activity from americium-241 is about three times that of radium. When gram quantities of americium-241 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 americium-241 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. 1

Physical Properties

Melting Point:2*  1176 °C = 1449.15 K = 2148.8 °F
Boiling Point:2* 2011 °C = 2284.15 K = 3651.8 °F
Sublimation Point:2 
Triple Point:2 
Critical Point:2 
Density:3  12 g/cm3

* - at 1 atm

Electron Configuration

Electron Configuration:  *[Rn] 7s2 5f7
Block: f
Highest Occupied Energy Level: 7
Valence Electrons: 2

Quantum Numbers:

n = 5
ℓ = 3
m = 3
ms = +½

Bonding

Ionization Potential   eV 4  kJ/mol  
Ionization Potential   eV 4  kJ/mol  
Ionization Potential   eV 4  kJ/mol  
1 5.9738    576.4

Thermochemistry

Specific Heat: 
Thermal Conductivity: 10 (W/m)/K, 27°C 5
Heat of Fusion: 14.4 kJ/mol 6 = 59.3 J/g
Heat of Vaporization: 
State of Matter Enthalpy of Formation (ΔHf°)7 Entropy (S°)7 Gibbs Free Energy (ΔGf°)7
(kcal/mol) (kJ/mol) (cal/K) (J/K) (kcal/mol) (kJ/mol)
(c) 0 0 15.0 62.76 0 0

Isotopes

Nuclide Mass 8 Half-Life 8 Nuclear Spin 8 Binding Energy
231Am 231.04556(32)# 30# s 1,752.89 MeV
232Am 232.04659(32)# 79(2) s 1,760.96 MeV
233Am 233.04635(11)# 3.2(8) min 1,769.03 MeV
234Am 234.04781(22)# 2.32(8) min 1,777.10 MeV
235Am 235.04795(13)# 9.9(5) min 5/2-# 1,785.17 MeV
236Am 236.04958(11)# 3.6(1) min 1,793.25 MeV
237Am 237.05000(6)# 73.0(10) min 5/2(-) 1,792.00 MeV
238Am 238.05198(5) 98(2) min 1+ 1,800.07 MeV
239Am 239.0530245(26) 11.9(1) h (5/2)- 1,808.14 MeV
240Am 240.055300(15) 50.8(3) h (3-) 1,816.22 MeV
241Am 241.0568291(20) 432.2(7) a 5/2- 1,824.29 MeV
242Am 242.0595492(20) 16.02(2) h 1- 1,832.36 MeV
243Am 243.0613811(25) 7.37(4)E+3 a 5/2- 1,831.11 MeV
244Am 244.0642848(22) 10.1(1) h (6-)# 1,839.19 MeV
245Am 245.066452(4) 2.05(1) h (5/2)+ 1,847.26 MeV
246Am 246.069775(20) 39(3) min (7-) 1,855.33 MeV
247Am 247.07209(11)# 23.0(13) min (5/2)# 1,854.09 MeV
248Am 248.07575(22)# 3# min 1,862.16 MeV
249Am 249.07848(32)# 1# min 1,870.23 MeV
Values marked # are not purely derived from experimental data, but at least partly from systematic trends. Spins with weak assignment arguments are enclosed in parentheses. 8

Abundance

Safety Information


Material Safety Data Sheet - ACI Alloys, Inc.

For More Information

External Links:

Sources

(1) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 4:3-4:4.
(2) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 4:132.
(3) - Lide, David R. CRC Handbook of Chemistry and Physics, 84th ed.; CRC Press: Boca Raton, FL, 2002; p 4:39-4:96.
(4) - Dean, John A. Lange's Handbook of Chemistry, 11th ed.; McGraw-Hill Book Company: New York, NY, 1973; p 4:8-4:149.
(5) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; p 10:178 - 10:180.
(6) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; pp 6:193, 12:219-220.
(7) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; pp 6:123-6:137.
(8) - Lide, David R. CRC Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Boca Raton, FL, 2002; pp 6:107-6:122.
(9) - Dean, John A. Lange's Handbook of Chemistry, 12th ed.; McGraw-Hill Book Company: New York, NY, 1979; p 9:4-9:94.
(10) - Atomic Mass Data Center. http://amdc.in2p3.fr/web/nubase_en.html (accessed July 14, 2009).