The Invisible Column

For most of chemistry's history, an entire column of the periodic table was missing. Noble gases — helium, neon, argon, krypton, xenon, and radon — are colorless, odorless, and almost completely unreactive.

Noble gases have full outer electron shells — helium has 2 electrons filling its first shell, and the others have 8 in their outermost shell. Since bonding is driven by atoms seeking full shells, noble gases have no motivation to bond. They are already complete.