oganesson ōˈgənĕsˌən [key], artificially produced radioactive chemical element; symbol Og; at. no. 118; mass number of most stable isotope 294; m.p., b.p., sp. gr., and valence unknown. Situated in Group 18 of the periodic table, it is classed as an inert gas and is expected to have properties similar to those of radon and xenon, but its electrons may be more nebulously associated with discrete shells, and it may as a result be chemically reactive. Scientists from the Joint Institute for Nuclear Research in Dubna, Russia, and Lawrence Livermore National Laboratory in California collaborated in the discovery of oganesson in experiments conducted in 2002 and 2005; the discovery was confirmed by the International Union of Pure and Applied Chemistry (IUPAC) in 2015. They bombarded atoms of californium-249 with ions of calcium-48. Among the products of the bombardments were atoms of oganesson-294 (one in 2002, two in 2005), each of which decayed into an atom of livermorium by emitting an alpha particle. Og-294 has a half-life of approximately 0.89 msec. The name oganesson, in honor of Russian physicist Yuri Oganessian, was proposed for the element by its discoverers and approved by IUPAC in 2016.

In 1999 a research team at the Lawrence Berkeley National Laboratory in Calif. bombarded lead-208 atoms with high-energy krypton-86 ions to create what an analysis showed to be three atoms of element 118 with mass number 293 and a half-life of less than a millisecond. In 2001, however, the team retracted its claim to have produced the element after other laboratories failed to reproduce their results and after a reanalysis of the original data did not show the production of it. A subsequent investigation suggested that the original finding was the result of fraud on the part of one of the team scientists.

See also synthetic elements; transactinide elements; transuranium elements.

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