Answer:
[tex]Cl^-[/tex]
Explanation:
Hello.
In this case, when chlorine gets ionized by gaining one electron it goes:
[tex]Cl^-[/tex]
Whose electron configuration is the same as argon:
[tex]Cl^-=Ar:1s^2,2s^2,2p^2,3s^2,3p^6[/tex]
Since the both of them have 18 electrons, they become isoelectric with the same neutral charge.
Best regards.
