Which element has the greatest average atomic mass?

The element with the greatest average atomic mass is Oganesson (Og), with an atomic number of 118. Oganesson is a synthetic element and is known to be very unstable, with its most stable isotopes having an average atomic mass of approximately 294 atomic mass units (amu).

While other elements exist with heavy isotopes, Oganesson is notable as it occupies the last position in the periodic table and is considered to have the highest atomic mass currently known. Typically, as you go down the periodic table, elements increase in atomic mass, but Oganesson's unique position and properties make it particularly significant in discussions around atomic mass.