resonancia

Resonance
Sometimes more than one Lewis formula can be written for a molecule, especially if the molecule contains a double or triple bond. A simple example is ozone (O3)
The Lewis formula forozone, however, is inconsistent with the experimentally determined structure. On the basis of the Lewis formula, we would expect ozone to have two different O-O bond lengths, one of them similar to theO-O single bond distance of 147 pm in hydrogen peroxide (HO-OH) and the other similar to the 121 pm O-O double bond distance in O2 . In fact, both bond distances are the same (128 pm) somewhat shorterthan a single bond, somewhat longer than a double bond. The structure of ozone requires that the central oxygen must be identically bonded to both terminal oxygens.
An electrostatic potential mapshows the equivalence of the two terminal oxygens. Notice, too, that the central oxygen is blue (positively charged) and both terminal oxygens are red (negatively charged).
To deal with circumstancessuch as the bonding in ozone, yet retain Lewis formulas as a useful tool for representing molecular structure, the notion of resonance was developed. According to the resonance concept, when two or moreLewis structures that differ only in the distribution of electrons can be written for a molecule, no single Lewis structure is sufficient to describe its true electron distribution. The true structureis said to be a resonance hybrid of the various Lewis formulas, called contributing structures, that can be written for the molecule. In the case of ozone, the two Lewis formulas are equivalent andcontribute equally to the resonance hybrid.
We use a double-headed arrow to signify resonance and read it to mean that the Lewis formulas shown contribute to, but do not separately describe, theelectronic structure of the molecule.
Resonance attempts to correct a fundamental defect in Lewis formulas. Lewis formulas show electrons as being localized; they either are shared between two atoms in...