An ion of which element has a larger radius than an atom of the same element? (Multiple-choice)

Question

Hello, I'm about to retake a state exam, I was practicing on an older one. This was a question that really stumped me:

14 An ion of which element has a larger radius than

an atom of the same element?

(1) aluminum (3) magnesium

(2) chlorine (4) sodium

Now I know the answer is (2) Chlorine, but why?

Thanks so much!! Responses are much appreciated!

Answer 1

The chlorine ion is the only negative one (it's very electronegative), as the other 3 are metallic and will form positive ions.

Since there are more electrons the electron "cloud" will be larger, and the radius of the ion will be larger than the radius of the base element.

Answer 2

Chlorine is the only element of those four with a negatively charged ion.. negatively charged ions are always bigger than their respective elements because the outermost electrons experience the same effective nuclear charge (positive charge from the nucleus), but since there are more of them, they are not held in as tightly.