Group 18 (Inert gases) elements

Group 18, the noble (inert) gases, comprises helium, neon, argon, krypton, xenon and radon. Because every member has a completely filled outermost electron shell (ns2 np6, except helium which has just 1s2), they are exceptionally stable, chemically almost entirely unreactive, and were long believed to form no compounds at all — a belief overturned only in 196 …