Oganesson is the chemical element with the chemical symbol Og and atomic number 118. Until 2016, it was known as Ununoctium, symbol Uuo, from the Latin uni (1) and oct (8). It is named in honour of Yuri Oganessian.

Only one isotope, 294Og has been observed. Oganesson is predicted to from isotopes with A ranging from the neutron dripline, somewhere in the vicinity of 420Og down to 281Og(1),(2),(3),(4). Of these, the most stable are predicted to occur in the band between 304Og and 294Og, with half-lives peaking below 1 second. No isotope of Og either has a long life or is the descendant of long-lived nuclides (types of atomic nuclei). If the distinction between "present" and extinct is set [Og] = 1.5E-30 (w.r.t. the sample of which it is a part) - equal to [218At] in earth as a whole - all isotopes of Og have become extinct within 100 sec of the event which led to their formation. At this point, local temperature exceeds 5E06 K, equivalent to mean kinetic energy of 430 eV. Og will exist only as positive ions, with minimum ionization probably in the +12 to +14 range(a).

Oganesson has no chemistry. Properties such as MP, BP, crystal structure, group, block, or oxidation state do not refer to something which exists in this universe.


1. "Decay Modes and a Limit of Existence of Nuclei"; H. Koura; 4th Int. Conf. on the Chemistry and Physics of Transactinide Elements; Sept. 2011.

2. “Systematic Study of Decay Properties of Heaviest Elements.”; Y. M. Palenzuelaa, L. F. Ruiza, A. Karpov, and W. Greiner; Bulletin of the Russian Academy of Sciences, Physics.  Vol . 76, No.11, pp 1165 – 1177; 2012

3. "Chart of the Nuclides, 2014", Japan Atomic Energy Agency; website available using "chart of nuclides" and "JAEA" as internet search terms.

4. "Nuclear Properties for Astrophysical Applications"; P. Moller & J. R. Nix; Los Alamos National Laboratory website; search by "LANL, T2", then "Nuclear Properties for Astrophysical Applications".


a. Except for one special case: a low-mass, population I (metal-rich) star, with a planet large enough to have an atmosphere and in an orbit which makes liquid water present in contact with both atmosphere and sunlight, several billion years to allow physicists and chemists to evolve, and a cultural willing-ness to spend vast sums of money to study Oganesson.