A very unstable radioactive transuranic element that is thought to be a noble gas; a.n. 118; r.a.m. 294. Oganesson has the highest atomic number and highest mass number of any element so far discovered or synthesized. As very few atoms of oganesson have been detected, its physical and chemical properties have not been experimentally determined, although there have been theoretical predictions concerning these properties. The possibility of an element with atomic number 118 was first considered by Niels Bohr in 1922, in his discussion of the periodic table in terms of the old quantum theory. Oganesson was subsequently discovered (2006) by a team of Russian and US scientists in a nuclear reaction between californium-249 and calcium-48. It is named after the Russian nuclear physicist Yuri Oganessian (1933– ).

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