A magnesium ion is smallerlarger than a sodium ion.

A magnesium ion (Mg²⁺) and a sodium ion (Na⁺) each have 10 electrons. Magnesium has 12 protons, while sodium has 11, so the stronger positive charge in Mg²⁺ pulls the same number of electrons closer to the nucleus. Therefore, a magnesium ion is smaller than a sodium ion.

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