Sulfur trioxide (SO3) is a covalent compound. This can be understood by examining its molecular structure and the types of bonds formed between the sulfur and oxygen atoms.
In SO3, sulfur is the central atom surrounded by three oxygen atoms. Each sulfur-oxygen bond in sulfur trioxide involves the sharing of electrons. The difference in electronegativity between sulfur (approximately 2.58) and oxygen (about 3.44) is not large enough to result in the complete transfer of electrons, which is characteristic of ionic bonds. Instead, the electrons are shared, resulting in covalent bonds.
Additionally, sulfur trioxide has a trigonal planar molecular geometry, which further supports the presence of covalent bonding as opposed to ionic. When considering the formation of SO3, the sulfur atom promotes one of its electrons to form double bonds with each of the three oxygen atoms, again indicating the covalent nature.
In summary, due to the sharing of electrons and the molecular geometry, sulfur trioxide is classified as a covalent compound.