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ununseptium (ōōn´ōōnesĕp´tēəm), artificially produced radioactive chemical element; symbol Uus; at. no. 117; mass number of most stable isotope 294; m.p., b.p., sp. gr., and valence unknown. Ununseptium is a member of Group 17 of the periodic table, the halogens, and may have properties similar to astatine.

In 2010 scientists from the Joint Institute for Nuclear Research in Dubna, Russia, announced that they had bombarded (2009) berkelium-249 atoms with calcium-48 ions to create two forms of ununseptium. The Uus-294 isotope, with a half-life of 78 milliseconds, decayed through alpha-particle emission into dubnium (element 105), while the lighter Uus-293, with a half-life of 14 milliseconds, decayed down to roentgenium (element 111). The Russian researchers collaborated with American scientists at Vanderbilt Univ. and the Oak Ridge National Laboratory, which produced the berkelium used in the experiment. No name has yet been adopted for element 117, which is therefore called ununseptium, from the Latin roots un for one and sept for seven, under a convention for neutral temporary names proposed by the International Union of Pure and Applied Chemistry (IUPAC) in 1980.

See also synthetic elements; transactinide elements; transuranium elements.