Sulfur trioxide (alternative spelling sulphur trioxide) is the chemical compound with the formula SO₃. In the gaseous form, this species is a significant pollutant, being the primary agent in acid rain. It is prepared on an industrial scale as a precursor to sulfuric acid.

Gaseous SO₃ is a trigonal planar molecule of D_3h symmetry, as predicted by VSEPR theory. SO₃ belongs to the D_3hpoint group.

In terms of electron-counting formalism, the sulfur atom has an oxidation state of +6 and a formal charge of +2. The Lewis structure consists of an S=O double bond and two S–O dative bonds without utilizing d-orbitals.

The electrical dipole moment of gaseous sulfur trioxide is zero. This is a consequence of the 120° angle between the S-O bonds.

SO₃ is the anhydride of H₂SO₄. Thus, the following reaction occurs:

The reaction occurs both rapidly and exothermically, too violently to be used in large-scale manufacturing. At or above 340 °C, sulfuric acid, sulfur trioxide, and water coexist in significant equilibrium concentrations.

Sulfur trioxide also reacts with sulfur dichloride to yield the useful reagent, thionyl chloride.

SO₃ is a strong Lewis acid readily forming crystalline complexes with pyridine, dioxane, and trimethylamine. These adducts be used as sulfonating agents.

Sulfur trioxide can be prepared in the laboratory by the two-stage pyrolysis of sodium bisulfate. Sodium pyrosulfate is an intermediate product:

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