Answer:
Al₂O₃
Step-by-step explanation:
When writing the correct compound, you need to take the charges of both ions into account.
Aluminum (Al) has an ionic charge of +3.
Oxygen (O) has an ionic charge of -2.
The ionic compound should have an overall charge of 0 (neutral). Evidently, the charges of the ions don't balance out if you were to form a compound with just one aluminum and one oxygen (making an overall charge of +3 - 2 = +1).
To fix this, make the charge of each ion the subscript of the other element.
This makes the compound:
Al₂O₃
Now, the aluminum is contributing +6 (2 x +3 = +6) and oxygen is contributing -6 (3 x -2 = -6). This balanced the compound and results in an ionic compound with no overall charge.