Problem

When aluminum and oxygen combine, they form an ionic compound where aluminum loses electrons and oxygen gains them. The key is figuring out how many electrons transfer: aluminum has 3 valence electrons it wants to lose (forming $\text{Al}^{3+}$), while each oxygen atom needs 2 electrons to fill its outer shell (forming $\text{O}^{2-}$). Use the criss-cross method—swap the charges as subscripts—to balance the positive and negative charges in the final compound.

When aluminum and oxygen combine, they form an ionic compound where aluminum loses electrons and oxygen gains them. The key is figuring out how many electrons transfer: aluminum has 3 valence electrons it wants to lose (forming $\text{Al}^{3+}$), while each oxygen atom needs 2 electrons to fill its outer shell (forming $\text{O}^{2-}$). Use the criss-cross method—swap the charges as subscripts—to balance the positive and negative charges in the final compound.