The valency of aluminum in its oxide, Al₂O₃, is +3.

Here's why:

* Aluminum (Al) is a metal in Group 13 of the periodic table, which means it typically forms a +3 ion (Al³⁺) to achieve a stable octet electron configuration.

* Oxygen (O) is a nonmetal in Group 16 of the periodic table, and it usually forms a -2 ion (O²⁻).

* To balance the charges, two aluminum ions (2 x +3 = +6) combine with three oxygen ions (3 x -2 = -6).

Therefore, the chemical formula for aluminum oxide is Al₂O₃, and the valency of aluminum in this compound is +3.