Ununoctium is the temporary IUPAC name for the superheavy element having the atomic number of 118, currently the highest atomic number assigned to a reputedly discovered element. It has the temporary IUPAC element symbol Uuo. Ununoctium probably shares similar properties of its group, the noble gases, resembling radon in its chemical properties, and so some researchers have referred to it as eka-radon. It is probably the second radioactive gaseous element and the first standard semiconductive gas. The element is now named oganesson with the symbol Og. Although oganesson is a member of group 18 (the noble gases) – the first synthetic element to be so – it may be significantly reactive, unlike all the other elements of that group. It was formerly thought to be a gas under normal conditions but is now predicted to be a solid due to relativistic effects.

Noble Gases and Halogens

The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemical reactivity.

The six naturally occurring noble gases are helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and the radioactive radon (Rn).