The noble gases (argonides)..
..are generally found as isolated atoms. They don't bind to other atoms forming molecules or lattices. Their electron structure must be particularly stable.
An electron octet (8 e- on the last -outer- layer) is stable. (..except for the first period (line) of the periodic table, where the stable structure of helium consists of 2 electrons).
Nature tends generally to a maximum of stability:
All atoms tend to gain the stable octet structure