* Valency: Valency refers to the combining capacity of an element. It's the number of electrons an atom can gain, lose, or share to achieve a stable electron configuration.

* Oxygen: Oxygen typically has a valency of -2 (it gains two electrons).

* SO₃ (Sulfur Trioxide):

* There are three oxygen atoms, each with a valency of -2, for a total of -6.

* To maintain a neutral molecule, the sulfur atom must have a valency of +6 to balance the negative charge.

Therefore, the valency of sulfur in SO₃ is +6.