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copernicium, artificially produced radioactive chemical element; symbol Cn; at. no. 112; mass number of most stable isotope 285; m.p., b.p., sp. gr., and valence unknown. Situated in Group 12 of the periodic table, it is expected to have properties similar to those of zinc, cadmium, and mercury.

In 1996 an international research team led by Peter Armbruster and Sigurd Hofmann at the Institute for Heavy Ion Research at Darmstadt, Germany bombarded lead-208 atoms with high-energy zinc-70 ions. In a two-week experiment, one of the resultant atoms was unambiguously identified as an isotope of element 112 with mass number 277 and a half-life of 280 msec. Copernicium was initially called ununbium, from the Latin roots un for one and bi for two, under a convention for neutral temporary names proposed by the International Union of Pure and Applied Chemistry (IUPAC) in 1980. In 2009 the name copernicium, for Copernicus, was proposed for the element by its discoverers, and IUPAC approved the name in 2010. The most stable isotope, copernicium-285, has a half-life of approximately 29 sec.

See also synthetic elements; transactinide elements; transuranium elements.

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