Question
The electronic configuration of $Cl^-$ ion is:

Answer

Chlorine atom $(Cl)$ has atomic number $17.$ It contains $17$ protons and $17$ electrons.
Chlorine ion $(Cl^-)$ is formed when $Cl$ gains one electron.
So, $Cl^-$ has $18$ electrons and $17$ protons.
Therefore, the electronic configuration of $Cl^- = 2, 8, 8.$

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