Neutral atom chlorine (Z=17), f has 17 electrons, therefore, its electronic configuration can be written as 1s² 2s²2p⁶ 3s² 3p⁵
The chloride ion (Cl⁻), after accepting 1 electron, has an additional electron for a total of 18 electrons. Electronic configuration of Cl⁻is then 1s² 2s² 2p⁶ 3s² 3p⁶ achieving the stable electronic configuration of its nearest Noble gas, Argon (Z=18).