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curium (kyo͝orˈēəm), artificially produced radioactive chemical element; symbol Cm; at. no. 96; mass no. of most stable isotope 247; m.p. about 1,340℃; b.p. 3,110℃; sp. gr. 13.5 (calculated); valence +3, +4. A hard, brittle, silvery metal that tarnishes in air, curium is chemically reactive and resembles gadolinium in its chemical properties, although it has a more complex crystalline structure. Oxides, fluorides, a chloride, a bromide, and an iodide of curium have been prepared. Curium is a member of the actinide series in Group 3 of the periodic table. Sixteen isotopes of curium are known. Curium-242, prepared by neutron bombardment of americium-241, has a half-life of 163 days; curium-247, the most stable isotope, has a half-life of 15.6 million years. Some curium isotopes are available in multigram quantities.
Curium is intensely radioactive; it is about 3,000 times as radioactive as radium. It is also very toxic when absorbed into the body because it accumulates in the bones and disrupts the formation of red blood cells. Curium-242 and curium-244 are used in the space program as a heat source (from the heat they generate as they undergo radioactive decay) for compact thermionic and thermoelectric power generation.
Curium has not been found to occur naturally; it was the third transuranium element to be synthesized. Curium was first produced by the bombardment of plutonium-239 with alpha particles in a cyclotron at the Univ. of California at Berkeley. Identified in 1944 by Glenn T. Seaborg, Ralph A. James, and Albert Ghiorso, it was named for Pierre and Marie Curie, the noted pioneers in the study of radioactivity. The metal was first isolated in visible amounts as the hydroxide by L. B. Werner and I. Perlman in 1947.
Cm, an artificially obtained radioactive chemical element of the actinide series. Atomic number, 96. It has no stable isotopes. Curium was first obtained in 1944 by the American scientists G. Seaborg, R. James, and A. Ghiorso by the nuclear reaction 23992Pu(α,n)24296Cm. It was named in honor of P. Curie and M. Sklodowska Curie, the founders of the science of radioactivity. Isotopes of curium with the mass numbers 238–250 are known. The most long-lived isotope is 247Cm (half-life T1/2 = 1.64 × 107 years). Some isotopes of curium may be accumulated in atomic reactors (244Cm, T1/2 = 17.59 years, and other isotopes) in kilogram quantities by long-term irradiation of either plutonium or uranium with neutrons.
Curium is a shiny silvery metal with a melting point of 1340°C; its calculated density is about 13 g/cm3. The most typical oxidation state of curium is + 3, as is that of the other heavy actinides. The compounds Cm2O3, CmCl3, and others have been synthesized. However, stable compounds of curium with the +4 state are also known (CmO)2, CmF4). Curium may be separated from other actinides by ion-exchange methods.
The intense evolution of heat by compounds of curium is caused by its radioactive disintegration. This property makes it possible to use the isotopes 242Cm, 244Cm, and others in the construction of small-scale sources of electric energy. The work span of such generators may be as high as several months.
S. S. BERDONOSOV