ion is more stable than atom 

If the force among the ions is balanced it defines the stability of the atom, if they have two or eight electrons in their valence shell

$C{l}^{-}$ion is more stable because $C{l}^{-}$possesses a stable octet configuration but $Cl$ atom does not have an octet configuration 

Hence, $C{l}^{-}$the ion is more stable than $Cl$an atom