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(ə-găn′ə-sŏn′, ô′gə-nĕs′ŏn, -ən)
n. Symbol Og
An artificially produced radioactive element with atomic number 118, whose most stable isotope has a mass number of 294 and a half-life of 1.3 milliseconds.

[oganess- (after Yuri Oganessian (born 1933), Russian nuclear physicist and scientific director at the Joint Institute for Nuclear Research in Dubna, where the element was first synthesized) + -on.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
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In 2016, four new elements were added to it: nihonium, moscovium, tennessine, and oganesson. Their atomic numbers--the number of protons in the nucleus that determines their chemical properties and place in the periodic table--are 113, 115, 117, and 118, respectively.
Oganesson, meanwhile, was named for Russian scientist Yuri Oganessian, who, along with his team, discovered the heaviest of all known elements.