Why is oganesson different from the other noble gases?

Oganesson (Og) is classified as a noble gas based on its position in Group 18 of the periodic table, but it differs significantly from the lighter elements in the group due to relativistic effects.

Because oganesson has a very high nuclear charge, its electrons move at speeds approaching the speed of light. This causes their effective mass to increase, which alters the atom's electronic structure and leads to properties that deviate from the group trend of chemical inertness.12

Key differences include:

• Reactivity: Unlike the lighter, inert noble gases, oganesson is predicted to be more reactive.12
• Physical State: While other noble gases are gaseous at room temperature, oganesson is predicted to be a solid with a melting point of approximately 325 K.12
• Conductivity: In its solid state, oganesson is expected to exhibit semiconductor behavior rather than the insulating properties typical of other noble gases, due to a much smaller band gap.1

Additionally, oganesson is a highly radioactive synthetic element with an extremely short half-life of less than a millisecond.12