Sulfur trioxide

Sulfur trioxide (alternative spelling sulphur trioxide) is the chemical compound with the formula SO3, with a relatively narrow liquid range. In the gaseous form, this species is a significant pollutant, being the primary agent in acid rain. Sulfur trioxide (alternative spelling sulphur trioxide) is the chemical compound with the formula SO3, with a relatively narrow liquid range. 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. In perfectly dry apparatus, sulfur trioxide vapor is invisible, and the liquid is transparent. However, it fumes profusely even in a relatively dry atmosphere (it has been used as a smoke agent) due to formation of a sulfuric acid mist. This vapor has no odor but is extremely corrosive. Gaseous SO3 is a trigonal planar molecule of D3h symmetry, as predicted by VSEPR theory. SO3 belongs to the D3h point group. In terms of electron-counting formalism, the sulfur atom has an oxidation state of +6 and a formal charge of 0. 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. The nature of solid SO3 is complex because structural changes are caused by traces of water. Upon condensation of the gas, absolutely pure SO3 condenses into a trimer, which is often called γ-SO3. This molecular form is a colorless solid with a melting point of 16.8 °C. It adopts a cyclic structure described as 3. If SO3 is condensed above 27 °C, then α-SO3 forms, which has a melting point of 62.3 °C. α-SO3 is fibrous in appearance. Structurally, it is the polymer n. Each end of the polymer is terminated with OH groups. β-SO3, like the alpha form, is fibrous but of different molecular weight, consisting of an hydroxyl-capped polymer, but melts at 32.5 °C. Both the gamma and the beta forms are metastable, eventually converting to the stable alpha form if left standing for sufficient time. This conversion is caused by traces of water.