nihonium nĭhōˈnēəm [key], artificially produced radioactive chemical element; symbol Nh; 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 element 113. They bombarded atoms of americium-243 with ions of calcium-48. Among the products of the bombardment were four atoms of moscovium (element 115), which in less than one tenth of a second decayed into atoms of element 113 by emitting an alpha particle. Two isotopes of element 113 were identified, the longer-lived (Nh-284) having a half-life of about 48 msec, and the other (Nh-283) a half-life of about 46.6 msec. The same team later reported creating nihonium-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 nihonium-279 (2004) and nihonium-278 (2012). In 2015, after review, the International Union of Pure and Applied Chemistry (IUPAC) awarded credit for the discovery of element 113 to the RIKEN group. The name nihonium, to honor Japan, was suggested by the element's discoverers in 2016 and approved by IUPAC later that year.

See also synthetic elements; transactinide elements; transuranium elements.

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