Question

what would the chemical formula be if you combine Al^3+ and O^2-?

Answer 1

Aluminum Oxide\[Al_2O_3\] I do believe.

Answer 2

There is a trick to determining the subscripts when combining a cation (+) and anion (-). Simply take the charge from the other species and put it as the subscript. For example, here Al has a charge of 3 and O has a charge of 2. Therefore, the subscript on Al would be 2 (because O has a charge of 2) and the subscript on O would be 3 (because Al has a charge of 3).