Final answer:
The correct formula for a compound composed of magnesium and an unknown polyatomic ion with a -3 charge is Mg3X2, as three Mg2+ ions combine with two X3- ions to form a neutral compound.
Step-by-step explanation:
To determine the correct formula for a compound composed of magnesium (Mg) and an unknown polyatomic ion with a charge of -3, one must account for the charges of Mg and the polyatomic ion. Magnesium typically forms a Mg2+ ion because it is in group 2 of the periodic table and will lose two electrons to have the same number of electrons as the previous noble gas, neon. Therefore, each Mg2+ ion will require two (-3) charged ions to balance out the total charge to zero. In this case, for every three Mg2+ ions that are present, there would need to be two unknown X3- ions to make the compound electrically neutral. As a result, the correct chemical formula for the compound would be Mg3X2.