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ununtrium, artificially produced radioactive chemical element; symbol Uut; at. no. 113; mass number of most stable isotope 284; m.p., b.p., sp. gr., and valence unknown. Situated in Group 13 of the periodic table, it is expected to have properties similar to those of thallium and indium.

In an experiment in 2003, scientists from the Joint Institute for Nuclear Research in Dubna, Russia, and Lawrence Livermore National Laboratory in California collaborated in the reported discovery of ununtrium. They bombarded atoms of americium-243 with ions of calcium-48. Among the products of the bombardment were four atoms of ununpentium (element 115), which in less than one tenth of a second decayed into atoms of ununtrium by emitting an alpha particle. Two isotopes of ununtrium were identified, the longer-lived (Uut-284) having a half-life of about 48 msec, and the other (Uut-283) a half-life of about 46.6 msec. The same team later reported creating ununtrium-285 by bombarding neptunium-237 with calcium-48 in 2007. Researchers at Japan's RIKEN Linear Accelerator Facility in Wako also have reported creating the element, with their first results in 2004. They used a bismuth-209 target, which was bombarded with zinc-70 to produce ununtrium-279 (2004) and ununtrium-278 (2012). No name has yet been adopted for element 113, which is therefore called ununtrium, from the Latin roots un for one and tri for three, 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.