Final answer:
Each of the three identical ions combined with an ion of oxidation number +3 must have an oxidation number of -1 to ensure the ionic compound is electrically neutral.
Step-by-step explanation:
An ionic compound is formed when an ion with an oxidation number of 3 combines with three identical ions each having an oxidation number that balances the total positive charge to zero. In this case, each of these three identical ions must have an oxidation number of -1.
Because the ionic compound must be electrically neutral, it must have the same number of positive and negative charges. For instance, aluminum oxide, Al2O3, is an ionic compound where two aluminum ions, each with a charge of 3+ (oxidation number +3), combine with three oxide ions, each with a charge of 2- (oxidation number -2), resulting in an oxidation state balance: (2 x +3) + (3 x -2) = 0.
Simple ionic compounds such as sodium chloride, NaCl, have sodium ions (Na+) with an oxidation number of +1 and chloride ions (Cl-) with an oxidation number of -1, demonstrating how the charges of ions correspond to their oxidation numbers to maintain neutrality.