The general formula of a compound between any group 13 element (A) and any group 16 element (B), will be A2B3. Let an example of aluminium oxide, Al2O3. Here, A will lose 3 electrons to complete its octet. It will form A3+ ion. B will gain two electrons to complete its octet. It will form B2− ion. Two A3+ ion will combine with three B2− ions. 2A3++3B2−⟶A2B3