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The molecules in a sample of SO2

Molecules with multiple bonds

There are further rules in VSEPR theory that simplify the discussion of species with multiple bonds and of species in which resonance must be considered. An analysis of the shapes adopted by species with multiple bonds suggests that each multiple bond can be treated as a single “superpair” of electrons. This rule can be justified by considering the geometric shapes that stem from two atoms sharing two or more pairs of electrons (Figure 9). Thus, the sulfate ion, SO42−, for which a Lewis structure ischemical bondFigure 9: Double bonds. The geometric arrangement of atoms linked by two shared pairs of electrons in a double bond (top) can be simulated by treating the double bond as the result of the sharing of a single superpair of electrons (bottom).Encyclopædia Britannica, Inc.

Lewis structure for the sulfate ion, SO4 2-

can be treated as having the equivalent of four pairs (two ordinary pairs and two superpairs) around the sulfur atom in a tetrahedral arrangement. All four pairs are bonding, so the ion is predicted to be a regular tetrahedron, which it indeed is. The same conclusion about the shape of the molecule would be drawn from another possible Lewis structure, in which each bond is single:

Another possible Lewis structure for the sulfate ion, SO4 2-

The actual molecule is a resonance hybrid of these and related structures; but, as each one corresponds to the same geometry, no particular Lewis structure need be selected before one can make a prediction based on VSEPR theory. In other words, resonance does not affect the shapes of molecules.