Noble gases
Noble gases close column 18. Their outer shell is saturated (ns² np⁶, except He at 1s²), giving them near-absolute chemical inertness — long considered unreactive.
Helium, neon, argon, krypton, xenon, radon and (synthetic) oganesson. Their inertness stems from their closed-shell configuration. Noble gas compounds were only discovered in the 1960s (XePtF₆, then various xenon fluorides and oxyfluorides), overturning chemistry.
Argon is the most abundant noble gas in the atmosphere (~0.93 %) and the most industrially used: inert welding, incandescent bulbs, cryogenics. Helium is precious but rare: cooling of superconducting magnets (MRI, LHC), weather balloons, deep-sea diving.
Radon, radioactive, accumulates in poorly ventilated basements and is a health risk in certain geological regions. Oganesson, first synthesised in 2002, may not even be a gas at room temperature according to relativistic calculations.