Final answer:
The formula unit for aluminum oxide is Al₂O₃, achieved by balancing the 3+ charge of two aluminum ions with the 2- charge of three oxygen ions to maintain electrical neutrality.option (3).
Step-by-step explanation:
The formula unit of aluminum oxide is found by balancing the charges of aluminum and oxygen ions to ensure electrical neutrality. Aluminum has a charge of 3+ (Al³⁺) and oxygen has a charge of 2- (O²⁻). To balance the charges, you need two aluminum ions to balance with three oxygen ions, resulting in a neutral compound. Therefore, the correct formula unit for aluminum oxide is Al₂O₃.
This can also be determined by the crisscross method where the numerical charge of the aluminum ion becomes the subscript of the oxygen ion, and vice versa, after removing the charge signs. Hence, we cross the 3+ charge from aluminum to oxygen and the 2- charge from oxygen to aluminum, ending up with Al₂O₃ as the formula for aluminum oxide.