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ununpentium, artificially produced radioactive chemical element; symbol Uup; at. no. 115; mass number of most stable isotope 288; m.p., b.p., sp. gr., and valence unknown. Situated in Group 15 of the periodic table, it is expected to have properties similar to those of bismuth and antimony.

In a 27-day experiment in 2003, scientists from the Joint Institute for Nuclear Research in Dubna, Russia, and Lawrence Livermore National Laboratory in California collaborated in the discovery of ununpentium. They bombarded atoms of americium-243 with ions of calcium-48. Among the products of the bombardment were one atom of ununpentium-287 and three atoms of ununpentium-288, each of which in less than one tenth of a second decayed into atoms of ununtrium by emitting an alpha particle. Ununpentium-289 and ununpentium-290 have also been produced (2009) at Dubna by alpha-particle decay of ununseptium. Swedish researchers from Lund Univ., in a 2013 experiment at the GSI Helmholtz Center for Heavy Ion Research, Darmstadt, Germany, largely replicated the 2003 results, producing ununpentium-288. No name has yet been adopted for element 115, which is therefore called ununpentium, from the Latin roots un for one and pent for five, under a convention for neutral temporary names proposed by the International Union of Pure and Applied Chemistry (IUPAC) in 1980. The most stable isotope of ununpentium, Uup-289, has a half-life of approximately 200 msec.

See also synthetic elements; transactinide elements; transuranium elements.