Why are chlorine anions larger than chlorine atoms?
Due to the difference in the number of electrons.
Chlorine has a proton number of 17. By writing out the subshell notation, we know that chlorine atom has 7 electrons in the outermost shell.
Chlorine anion, or chloride ion, on the other hand, since it has accepted 1 electron to achieve stable octet arrangement, it has 8 electrons in the outermost shell.
The proton number of both chlorine and chloride ion does not change, but remains at 17. Hence, we can deduce that the forces of attraction exerted on the outermost electron in chloride ion is lesser than chlorine atom, since there are more electrons.
Conclusion, chloride ion has a bigger atomic radius due to an increase in number of electrons.