Identify the formula for sulfur trioxide.

The formula for sulfur trioxide is correctly identified as SO3. In chemical nomenclature, the prefix "tri-" indicates that there are three oxygen atoms bonded to one sulfur atom.

To break it down further, sulfur has the ability to form compounds with oxygen, and when it combines with three oxygen atoms, the resulting molecular formula is SO3. The subscript "3" denotes the three oxygen atoms, while the absence of a subscript next to sulfur signifies a single sulfur atom.

Understanding this nomenclature is crucial, as each prefix indicates the number of atoms of each element within the compound. In contrast, SO2, which would imply sulfur dioxide, has only two oxygen atoms and does not correspond to trioxide. Similarly, SO4 would refer to sulfur tetraoxide, which is not the correct representation of the compound in question. Lastly, S3O does not accurately represent the composition of sulfur trioxide either, as it suggests a different structure with three sulfur atoms and one oxygen atom, which does not align with the definition of sulfur trioxide.