Periodic table

The periodic table reaches element 138 in the 23rd century. While elements 139 and above have been created, their existence is not terribly meaningful. The nuclei are not just obscenely unstable, but also force their lowest-energy electrons to collide with the nucleus, combining with protons to form neutrons. Incapable of undergoing beta decay, certain isotopes of untrioctium are actually surprisingly stable, though too prone to spontanious fission to be of feasible chemical use.