Here's why:

* Molecular weight: Sulfur dioxide has a molecular weight of 64.07 g/mol, while air has an average molecular weight of approximately 28.97 g/mol. This means that sulfur dioxide molecules are heavier than air molecules.

* Density: Density is a measure of mass per unit volume. Since sulfur dioxide molecules are heavier but occupy roughly the same volume as air molecules, sulfur dioxide gas has a higher density than air.

* Buoyancy: Because sulfur dioxide is less dense than air, it will tend to rise in the atmosphere due to buoyancy.

However, it's important to note that sulfur dioxide gas is heavier than air only at standard temperature and pressure (STP). At higher temperatures, sulfur dioxide gas will expand and become less dense than air.