* Element: An element is a pure substance made up of only one type of atom (like gold, oxygen, or carbon).

* Compound: A compound is formed when two or more different elements chemically combine in a fixed ratio. Sulfur trioxide is made of sulfur (S) and oxygen (O) atoms in a 1:3 ratio.

* Heterogeneous Mixture: A heterogeneous mixture has components that are not evenly distributed and can be easily separated.

* Homogeneous Mixture: A homogeneous mixture has components that are evenly distributed and cannot be easily separated.

Since sulfur trioxide is made of different elements chemically bonded together, it fits the definition of a compound.