* Composition: It's composed of sulfur (S), a nonmetal, and oxygen (O), also a nonmetal.

* Properties: SO3 is a gas at room temperature, has a low melting point, and does not conduct electricity well, all characteristics of nonmetals.

* Chemical Bonding: The bonding in SO3 is covalent, where atoms share electrons, another typical characteristic of nonmetals.

While some elements can exist in both metallic and nonmetallic forms (like carbon), sulfur trioxide is a definite nonmetal compound.