Question

what is inert gas core

Answer 1

Definition: A noble gas core is an abbreviation in an atom's electron configuration where the previous noble gas's electron configuration is replaced with the noble gas's element symbol in brackets.

Examples: Sodium has an electron configuration of 1s22s2p63s1.

The previous noble gas on the periodic table is neon with an electron configuration of 1s22s2p6. If this configuration is replaced by [Ne] in sodium's electron configuration it becomes [Ne]3s1.

This is the noble gas core notation of sodium.