What is the highest inert gas?

Answer 1

Although there are many common gases that inert under most common conditions, the only ones that are inert under nearly all conditions are the noble gases. As an example - nitrogen is usually inert, but when subject to the conditions in an internal combustion engine, it will react to form a range of nitrogen-oxides.

The noble gases - helium, neon, argon, krypton, xenon, and radon - are extremely non-reactive. Until the 1960's it was thought to be impossible to get them to react with anything because their oxidation number is 0. In the past 50 years a few compounds have been formed under very specially crafted conditions using highly oxidizing atoms like fluorine, but most of them are not even very stable.

With that said - the highest atomic number for an inert gas is that for radon - which has an atomic number of 86 and an average atomic weight of 222 amu.

Ununoctium (proposed name Oganesson) - with an atomic number of 118 and standard atomic weight of 294 is in Group 18 with the noble gases, but is predicted to be a solid due to relativistic effects: most of its properties can only be predictedbecause only 3 and possibly 4 atoms of this highly radioactive man-made element have ever been synthesized.