The valency of sulfur in SO₂ (sulfur dioxide) is +4.

Here's how we figure that out:

* Oxygen's Valency: Oxygen typically has a valency of -2.

* Total Valency of Oxygen: In SO₂, there are two oxygen atoms, so their total valency is 2 x (-2) = -4.

* Balancing the Valencies: To make the molecule neutral, sulfur's valency must balance out the -4 valency from the oxygen atoms. Therefore, sulfur's valency in SO₂ is +4.