Outer shell ‘octet’ configuration is observed for the elements of the group

The octet rule dictates that atoms are most stable when their valence shells are filled with eight electrons. It is based on the observation that the atoms of the main group elements have a tendency to participate in chemical bonding in such a way that each atom of the resulting molecule has eight electrons in the valence shell. The octet rule is only applicable to the main group elements.

Group-18 elements consists of noble gas elements that have a complete octet, so usually they do not participate in bond formation. This is the reason that these elements also called inert gas elements.