Oganesson is the chemical element with the largest atomic number among the elements officially named as of 2024.
Oganesson has atomic number 118 and the symbol Og. It is a synthetic superheavy element, meaning it is produced in laboratories rather than found naturally in usable quantities. Only a very small number of atoms have been made, and each decays extremely quickly.
Researchers first reported its production in 2002 through a collaboration involving the Joint Institute for Nuclear Research in Dubna and Lawrence Livermore National Laboratory. The element was initially known by the temporary systematic name ununoctium. In 2016, the International Union of Pure and Applied Chemistry approved the name oganesson, honoring physicist Yuri Oganessian.
Oganesson belongs to group 18, the noble-gas group, but its extreme mass and relativistic effects may make its behavior unusual. Predictions are therefore especially important because experiments with longer-lived samples are difficult. Oganesson is not the same as uranium or plutonium, which are naturally occurring actinides.