Explain why Silicon Dioxide has a higher melting point than Sulfur Trioxide.

Silicon Dioxide has a macromolecular/giant covalent structure which means it has covalent bonds between all atoms in its structure. Sulfur Trioxide has a simple molecular structure meaning it has Van der Waals forces between molecules.
Covalent bonds are much stronger than Van der Waals forces and so require much more energy to overcome, and this leads to the higher melting point observed in silicon dioxide as melting requires the overcoming of forces between atoms/molecules.