You are right to be confused! The valency of sulfur in both SO and SO₂ is +2. However, the reason why we call it SO₂ and not SO is because the chemical formula reflects the actual composition of the molecule.

Here's why:

* SO would imply only one oxygen atom is bonded to sulfur. This is not the case.

* SO₂ accurately reflects the molecule, which consists of one sulfur atom bonded to *two* oxygen atoms.

While valency describes the combining capacity of an element, it doesn't tell us the exact number of atoms involved in a particular compound.

Here's a helpful analogy: Imagine you have a box that can hold two balls (valency = 2). You could put two small balls in the box, or one big ball. The number of balls inside the box (atoms in the molecule) defines the actual composition, not just the box's capacity (valency).

So, even though sulfur has a valency of +2 in both SO and SO₂, the difference in the number of oxygen atoms leads to two distinct molecules with different properties.