Here's why:

* Compound: A compound is formed when two or more different elements chemically combine in a fixed ratio.

* Mixture: A mixture is a combination of two or more substances where each substance retains its own properties.

Sulfur dioxide (SO₂) is formed by the chemical combination of sulfur (S) and oxygen (O) in a fixed ratio of 1:2. This chemical bonding results in a new substance with properties distinct from either sulfur or oxygen.

Therefore, sulfur dioxide is a compound.