Structure and Bonding
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 +2 (note that in the optimum Lewis structure, two sulfur-oxygen bonds are dative bonds as quantum chemical calculations have shown that d-orbitals are not involved in bonding). Therefore, 3 double bonds shown in the top diagram are misleading. One double and two single (dative) bonds should be shown, resulting in 3 resonance structures. In any case, Sulfur trioxide is a non-polar molecule.
Read more about this topic: Sulfur Trioxide
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