What is the electron configuration of chlorine (CI)

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 1s ² 2s ² 2p ⁶ 3s ² 3p ⁶ achieving the stable electronic configuration of its nearest Noble gas, Argon (Z=18).