Final answer:
The charge on the oxygen ion in aluminum oxide is -2. To maintain electrical neutrality in Al2O3, a 2:3 ratio balances out the +3 charge on aluminum ions with the -2 charge on oxygen ions.
Step-by-step explanation:
The charge on the oxygen ion in aluminum oxide is -2. Since aluminum has an oxidation number of +3, to maintain electrical neutrality, the compound aluminum oxide (Al2O3) must have an equal amount of positive and negative charges. This is achieved by having two aluminum ions (2 x +3 = +6) and three oxygen ions (3 x -2 = -6), resulting in a 2 to 3 ratio of aluminum to oxygen. The 2:3 ratio ensures that the total positive charge from the aluminum ions balances out the total negative charge from the oxygen ions, making the compound electrically neutral.