##1″s”^2 2″s”^2 2″p”^6 3″s”^2 3″p”^6##
OR
##”[Ne]” 3″s”^2 3″p”^6##
The of a chlorine atom (##”Cl”##) is as follows:
##1″s”^2 2″s”^2 2″p”^6 3″s”^2 3″p”^5##
But a chlorine ion (##”Cl”^-##) has acquired a ##”(1)-“## charge as a result of gaining 1 electron. Thus, its electron configuration is:
##1″s”^2 2″s”^2 2″p”^6 3″s”^2 3″p”^6##
This may also be expressed as:
##”[Ne]” 3″s”^2 3″p”^6##