A magnesium ion is than a sodium ion.

A magnesium ion (Mg²⁺) and a sodium ion (Na⁺) each have 10 electrons. Mg²⁺ has 12 protons, while Na⁺ has 11, so its stronger pull draws the same number of electrons into a smaller space. Therefore, a magnesium ion is smaller than a sodium ion.

AI-written and checked against the verified answer.