Answer
SO$_{3}$ - Sulfur Trioxide
Work Step by Step
1. First, begin by writing the name of the first element. S -Sulfur.
2. Since, SO$_{3}$ is a molecular compound (two non-metals joined together), we have to use the appropriate prefix for the second element.
3. Note that the second element, oxygen, has three atoms. This means we have to use the prefix "tri" and change oxygen's ending to oxide.
4. Combine the two names to get: $$Sulfur Trioxide$$
