Oganesson

Oganesson completes period 7 and the periodic table as it currently exists: a filled 7p⁶ shell, five or six atoms ever made, and a half-life near 0.7 milliseconds. It is placed in group 18 and shown here as a gas by convention, but both of those are inherited assumptions rather than measurements. Relativistic calculations indicate the electron shells are so smeared by spin-orbit effects that oganesson has no meaningful shell structure at all, giving it a substantial electron affinity, real chemical reactivity and — most modern estimates agree — a solid state at room temperature, possibly a semiconductor. It is one of only two elements named for a living person, honouring Yuri Oganessian, who invented the hot-fusion method that produced most of the seventh row.