96
  Cm  
247.000000
Curium

Name: Curium
Symbol: Cm
Atomic Number: 96
Atomic Weight: 247.000000
Family:  Rare Earth Elements
CAS RN: 7440-51-9
Description: A silver colored radioactive rare earth metal.
State (25C): Solid
Oxidation states: +3, +4

Molar Volume: 18.29 cm3/mole
Valence Electrons: 5f76d17s2

Boiling Point: 3383K, 3110C, 5630F
Melting Point: 1340K, 1067C, 1953F

Electron Energy Level: 2, 8, 18, 32, 25, 9, 2
Isotopes: 21 + None Stable
Heat of Vaporization: unknown
Heat of Fusion : 15.0 kJ/mol

Density : 13.51 g/cm3 at 20oC
Specific Gravity: unknown
Atomic Radius: 174 pm
Ionic Radius: 0.97
Electronegativity: 1.3 (Pauling); 1.2 (Allrod Rochow)
89
Ac
227.0
90
Th
232.0
91
Pa
231.0
92
U
238.0
93
Np
237.0
94
Pu
(244)
95
Am
(243)
96
Cm
(247)
97
Bk
(247)
98
Cf
(251)
99
Es
(252)
100
Fm
(257)
101
Md
(258)
102
No
(259)
103
Lr
(260)

1s2 2s2p6 3s2p6d10 4s2p6d10f14 5s2p6d10f7 6s2p6d1 7s2

History

Curium was first synthesized at the University of California, Berkeley by Glenn T. Seaborg, Ralph A.James, and Albert Ghiorso in 1944.  The team named the new element after Marie Curie and her husband Pierre who are famous for discovering radium and for their work in radioactivity.  It was chemically identified at the Metallurgical Laboratory (now Argonne national Laboratory at the University of Chicago.  They performed a careful chemical fractionation on a sample of plutonium-239, an isotope of plutonium, which had been irradiated with 32 MeV helium ions(alpha particles) at the University of California.  A new radioactive isotope was found, which emitted 4.7 MeV alpha particles and was chemically separable from neptunium and plutonium.  Both nuclear and chemical evidence indicated that the activity could be ascribed to an isotope of a new element with atomic number 96.  It was actually the third transuranium element to be discovered even though it is the second in the series.  This produced atoms of curium-242 and one free neutron.  Curium-242 has a half-life of about 163 days and decays into plutonium-238 through alpha decay or decays through spontaneous fission.

mpcurie.jpg (53047 bytes)

Marie & Pierre Curie

Visible amounts (30Mg) of 242Cm, in the form of the hydroxide [Cm(OH)3], were first isolated by  Louis Werner and  Isadore Perlman of the University of California in 1947 by bombarding americium-241 with neutrons.  In 1950, Crane, Wallmann, and Cunningham found that the magnetic susceptibility of microgram samples of CmF3 was of the same magnitude as that of GdF3.  This provided direct experimental evidence for assigning an electronic configuration to Cm+3.   In 1951, the same workers prepared curium in its elemental form for the first time.

The crystal structure and melting point of curium metal were determined in 1964 by B.B. Cunningham and J.C. Wallmann.

Characteristics

Minute amounts of curium probably exist in natural deposits of uranium, as a result of a sequence of neutron captures and beta decays sustained by the very low flux of neutrons naturally present in uranium ores. The presence of natural curium, however, has never been detected. 242Cm and 244Cm are available in multigram quantities. 248Cm has been produced only in milligram amounts.   242Cm generates about three watts of thermal energy per gram. This compares to one-half watt per gram of 238pU. This suggests use for curium as a power source. 244Cm is now offered for sale at $160/mg plus packing charces. 248Cm is available at a cost of $160/mg, plus packing charges, from the O.R.N.L. Curium absorbed into the body accumulates in the bones, and is therefore very toxic as its radiation destroys the red-cell forming mechanism.  The maximum permissible total body burden of 244Cm (soluble) in a human being is 0.3 uCi (microcurie).  

1s2
2s2 2p6
3s2 3p6 3d10
4s2 4p6 4d10 4f14
5s2 5p6 5d10 5f7
6s2 6p6 6d1
7s2

Electron Shell Configuration

The isotope curium-248 has been synthesized only in milligram quantities, but curium-242 and curium-244 are made in multigram amounts, which allows for the determination of some of the element's properties. Curium-244 can be made in quantity by subjecting plutonium to neutron bombardment. Curium does not occur in nature. Curium bio-accumulates in bone tissue where its radiation destroys bone marrow and thus stops red blood cell creation.

A rare earth homolog, curium is somewhat chemically similar to gadolinium but with a more complex crystal structure.  Chemically reactive, its metal is silvery-white in color and the element is more electropositive than aluminum (most trivalent curium compounds are slightly yellow).

Curium has been studied greatly as a potential fuel for Radioisotope Thermoelectric Generators (RTGs).  Curium-242 can generate up to 120 watts of thermal energy per gram (W/g); its very short half-life though makes it undesirable as a power source for long-term use. Curium-242 is the precursor to plutonium-238 which is the most common fuel for RTGs. Curium-244 has also been studied as an energy source for RTGs having a maximum energy density ~3 W/g, but produces a large amount of neutron radiation from spontaneous fission.  Curium-243 with a ~30 year half-life and good energy density of ~1.6 W/g would seem to make an ideal fuel, but it produces significant amounts of gamma and beta radiation from radioactive decay products.

Compounds

  • Curium Dioxide (CmO2)
  • Curium Trioxide (Cm2O3)
  • Curium Bromide (CmBr3)
  • Curium Chloride (CmCl3)
  • Curium Tetrafluoride (CmF4)
  • Curium Iodide (CmI3)

Since only milligram amounts of curium have ever been produced, there are currently no commercial applications for it, although it might be used in radioisotope thermoelectric generators in the future.  Curium is primarily used for basic scientific research.   Curium-242 and curium-244 are used in the space program as a heat source for compact thermionic and thermoelectric power generation. Being an alfa-emitter, its radiation can be easily shielded against.

Isotopes

21 radioisotopes of curium have been characterized, with the most stable being Cm-247 with a half-life of 1.56 107 years, Cm-248 with a half-life of 3.40 105 years, Cm-250 with a half-life of 9000 years, and Cm-245 with a half-life of 8500 years. Cm-247 decays into plutonium-243 through alpha decay.  All of the remaining radioactive isotopes have half-lifes that are less than 30 years, and the majority of these have half lifes that are less than 33 days.  This element also has 4 meta states, with the most stable being Cm-244m (t 34 ms). The isotopes of curium range in atomic weight from 233.051 u (Cm-233) to 252.085 u (Cm-252).

atom.gif (700 bytes)

Isotope  
Atomic Mass
 
Half-Life
232Cm   1 minutes
233Cm 233.05077 ~1 minutes
234Cm 234.05016 51 seconds
235Cm 235.05143 ~5 minutes
236Cm 236.05141 ~10 minutes
237Cm 237.05290 ~20 minutes
238Cm 238.05303 2.4 hours
239Cm 239.05496 ~2.9 hours
240Cm 240.0555295 27 days
241Cm 241.0576530 32.8 days
242Cm 242.0588358 162.8 days
243Cm 243.0613891 29.1 years
244Cm 244.0627526 18.10 years
245Cm 245.0654912 8.5 x 103 years
246Cm 246.0672237 4.76 x 103 years
247Cm 247.070354 1.56 x 107 years
248Cm 248.072349 3.48 x 105 years
249Cm 249.075953 64.15 minutes
250Cm 250.078357 8300 years
251Cm 251.082285 16.8 minutes
252Cm 252.08487 <1 days

Precautions

Nuclear Fuel Cycle

The MOX which is to be used in power reactors should contain little or no curium as the neutron activation of this element will create californium which is a strong neutron emitter. The californium would pollute the back end of the fuel cycle and increase the dose to workers.  Hence if the Minor Actinides are to be used as fuel in a thermal neutron reactor the curium should be excluded from the fuel or placed in special fuel rods where it is the only actinide present.

Curium Data

 

Atomic Structure

Atomic Radius (): unknown
Atomic Volume: 18.28 cm3/mol
Covalent Radius: unknown
Crystal Structure: Hexagonal
Ionic Radius: 0.97

Chemical Properties

Electrochemical Equivalents: 3.0727g/amp-hr
Electron Work Function: unknown
Electronegativity: 1.3 (Pauling); 1.2 (Allrod Rochow)
Heat of Fusion: 15kJ/mol
First Ionization Potential: 6.02
Second Ionization Potential: unknown
Third Ionization Potential: unknown
Valence Electron Potential (-eV): 44.5
Ionization Energy (eV): 6.02 eV

Physical Properties

Atomic Mass Average: 247
Boiling Point: 3383K, 3110C, 5630F
Melting Point: 1340K, 1067C, 1953F
Heat of Vaporization: unknown
Coefficient of Lineal Thermal Expansion/K-1: N/A
Electrical Conductivity: unknown
Thermal Conductivity: 0.1 w/cm-K
Density: 13.5 g/cm3 @ 300K
Enthalpy of Atomization: unknown
Enthalpy of Fusion: 15 kJ/mole
Enthalpy of Vaporization: unknown
Molar Volume: 18.29 cm3/mole
Specific Heat: unknown
Vapor Pressure: unknown
Estimated Crustal Abundance: unknown
Estimated Oceanic Abundance: unknown

Miscellaneous

Polarizability: 23 3
Effective Nuclear Charge: Slater: 1.8
Radius: Cm 2 +: 119 pm
Radius: Cm 3 +: 99 pm
Radius: Cm 4 +: 88 pm
1st. Ionization Energy: 581 kJ/mole