Curium

1 · Identity

Symbol, number

Cm, 96

Oxidation states in water

+3 (book entry); never measured in natural water

Note

Identity from the book entry; nothing measured in water.

Sources

Identity

Name and symbol

Curium, Cm

Atomic number

96 protons

Position

no group (f-block) · period 7 · f-block · actinide

CAS number

7440-51-9

Atomic structure

Atomic mass

247 u

Electron configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹⁴ 5d¹⁰ 6p⁶ 7s² 5f⁷ 6d¹
[Rn] 7s²⁵f⁷⁶d¹

Electrons per shell

2, 8, 18, 32, 25, 9, 2

Valence electrons

10 ns, (n-1)d and (n-2)f

Physical properties

State at room temperature

Solid

Melting point

1,618 K (1,344.85 °C)

Boiling point

3,100 K (2,826.85 °C)

Density

13.51 g/cm3

Appearance

silvery metallic, glows purple in the dark

Thermal conductivity

not in sources

Electrical resistivity

1.25 µΩ·m

Electrical conductivity

800,000 S/m

Crystal structure

double hexagonal close packed

Molar heat capacity

not in sources

Chemical properties

Oxidation states

+3

Electronegativity

1.3 (Pauling Scale)

Ionisation energy

6.02 eV
1st 581 kJ/mol

Electron affinity

not in sources

Atomic radius

empirical 169, covalent 169, van der Waals 245 pm

Ionic radius

Cm³⁺ 97; Cm⁴⁺ 85 pm

Reactivity

A lustrous, silvery, chemically reactive metal, more electropositive than aluminium; the half-filled 5f7 shell makes +3 the dominant state, +4 exists in solids such as CmO2 and CmF4 and only briefly in solution, and the metal resembles its lanthanide homolog gadolinium.

with water

The sources do not describe the bulk metal with water; aqueous Cm(IV) is reduced to Cm(III) by water itself and by radiolysis, and vapor-phase hydrolysis of the trichloride gives the oxychloride: $\ce{CmCl3 + H2O -> CmOCl + 2 HCl}$.

with oxygen, air

The metal surface oxidizes rapidly in air, the film starting as CmO and progressing through Cm2O3 to the stable black CmO2; heating CmO2 in vacuum at 600 to 650 C loses oxygen: $\ce{4 CmO2 -> 2 Cm2O3 + O2}$.

with acids

Dissolves rapidly in dilute acid to give colorless to pale green Cm(III) solutions; Cm(IV) appears in solution only with strong oxidants such as persulfate and is metastable in concentrated fluoride.

with halogens

Colorless CmF3 precipitates from Cm(III) solutions with fluoride and fluorine oxidizes it to brown $\ce{CmF4: 2 CmF3 + F2 -> 2 CmF4}$; CmCl3, CmBr3 and CmI3 are made from the hydroxide or chloride rather than the metal, for example: $\ce{CmCl3 + 3 NH4I -> CmI3 + 3 NH4Cl}$.

Typical compounds

• CmO₂ curium dioxide black, from burning the oxalate or nitrate in oxygen
• Cm₂O₃ curium(III) oxide whitish sesquioxide, from CmO2 by heating or hydrogen
• CmF₃ curium(III) fluoride colorless, precipitated with fluoride ions
• CmF₄ curium(IV) fluoride brown tetravalent solid, from CmF3 and fluorine
• CmCl₃ curium(III) chloride colorless, from the hydroxide and dry HCl gas

Occurrence, production and use

Crustal abundance

Not Applicable

Oceanic abundance

Not Applicable

Occurrence and sources

• synthetic; minute natural amounts undetected made from plutonium in reactors; may exist in uranium deposits but never detected

Extraction, production

Neutron bombardment of plutonium in a reactor; first synthesis by 239Pu (4He, n) 242Cm

nuclear reaction as printed by PubChem

Uses

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.

Scientists have produced several curium compounds. They include: curium dioxide (CmO2), curium trioxide (Cm2O3), curium bromide (CmBr3), curium chloride (CmCl3), curium chloride (CmCl3), curium tetrafluoride (CmF4) and curium iodide (CmI3). As with the element, the compounds currently have no commercial applications and are primarily used for basic scientific research.

Safety, toxicity

not in sources

Discovery and name

Discovered by

Glenn T. Seaborg, Ralph A. James, Albert Ghiorso

Discovered

1944

First isolated

not in sources

Named by

not in sources

Origin of the name

named after Marie Skłodowska-Curie and Pierre Curie

Curium does not occur naturally in the Earth’s crust. It was first synthesized in 1944 by Glenn T. Seaborg and his team at the University of California in Berkeley using the reaction 239Pu (4He, n) 242Cm. The element was named after Pierre and Marie Curie, who discovered radium and polonium.

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. Curium is similar in some regards to gadolinium, its rare earth homolog, but it has a more complex crystal structure. Curium metal is lustrous, malleable, silver in color, chemically reactive, and is more electropositive than aluminum. Curium metal exist in two crystal forms, a double hexagonal close packed (dhcp) and a high temperature face-centered cubic close packed (fcc) structure. Metallic curium dissolves rapidly in dilute acid to form Cm(III) solutions. Curium metal surfaces rapidly oxidize in air to form a thin film possibly starting out as CmO, Oxidation then progressing to Cm2O3, and eventually to form stable CmO2. Note however that the formation of divalent compounds of curium such as CmO have never been observed in bulk form. Most compounds and solutions of trivalent curium are quite stable and are faintly yellow or yellow-green in color. The stability of the trivalent state for curium is attributed to the half-filled 5f7 electron shell configuration. Curium in the tetravalent state is meta-stable in concentrated fluoride solutions but very stable in the solid state, primarily as the oxides and fluorides. Because curium isotopes are available in macro quantities a number of curium compounds have been prepared and characterized with the majority in the trivalent state.

242Cm generates about three watts of thermal energy per gram. This compares to one-half watt per gram of 238Pu. Both 242Cm and 244Cm have been used as power sources for space and medical uses. 244Cm is now offered for sale at $100/mg. 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 microcurie.

This element reviewed and Updated by Dr. David Hobart, 2011