Why is sulfur trioxide compound? pls help i need it asap​

A compound is 2 or more elements and SO3 contain more than 2 elements

Explanation:

Sulfur trioxide (alternative spelling sulphur trioxide) is the chemical compound with the formula SO3. It has been described as "unquestionably the most important economically" sulfur oxide.[5] It is prepared on an industrial scale as a precursor to sulfuric acid.Sulfur trioxide exists in several forms - gaseous monomer, crystalline trimer, and solid polymer. All are colorless forms. with a relatively narrow liquid range. Gaseous SO3 is the primary precursor to acid rain.[6]

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