Final answer:
The correct formula for the compound formed between magnesium and phosphate ion is Mg₃(PO₄)₂, which balances the charges of Mg²⁺ and PO₄⁻³.
Step-by-step explanation:
The correct formula for the compound formed between magnesium and the phosphate ion is Mg₃(PO₄)₂. Magnesium has a 2+ charge (Mg²⁺) and the phosphate ion has a 3- charge (PO₄⁻³). To balance the charges, we need three magnesium ions for every two phosphate ions, resulting in the chemical formula Mg₃(PO₄)₂.