A TUTORIAL ON RESONANCE THEORY FOR CH610A AND 618A

THE BASIC Terminology

Resonance Stabilization Energy: The energy of a real molecule compared with the energy expected for the best canonical (resonance) structure that can be drawn for the molecule.The expectation energy is based upon the numbers and types of bonds present in the canonical structure.
Canonical or Resonance Structure.: A bond line or other structural representation which fulfills all of the valency rules for each atom involved and does not violate the octet rule.The "best" canonical structure is the one which has the lowest expected energy.
Delocalization.: The sharing of electrons by more than two atoms.

The sharing of electrons by more than two atoms cannot be represented by simple bond line structures.
In real molecules electrons are often shared by many atoms.Bonding in real molecules is thus more complicated than can be represented in a single canonical structure.
In general, electrons are stabilized by delocalization. The stabilization energy engendered by delocalization over more than two atoms is called the resonance stabilization energy or simply the resonance energy.
The greater the extent of electron delocalization the greater the resonance stabilization.

When more than one valid canonical structure is available for a given molecule, the molecule is resonance stabilized, i.e., it is more stable than either of the canonical forms.
The more nearly equal the canonical structures are in energy, the greater the resonance stabilization. When the resonance structures are equivalent (i.e. they are exactly equal in energy) resonance stabilization is maximized.
The larger the number of resonance structures, the greater the resonance stabilization (this correlates with a more extensive electron delocalization).
The greatest resonance stabilization energies of all are achieved when the electrons are delocalized over a cyclic system, as in benzene. In cyclic systems,however, the electron count is vitally important. There must be 2,6,10, etc. electrons in the cyclic system for it to be so highly stabilized (the Huckel 4n+2 rule). Such highly stabilized systems are called "aromatic" systems. One the other hand, if the number of electrons in the cylic system is 4,8,12,etc( i.e. 4n electrons), the molecule is not aromatic.
Molecules for which only one valid resonance structure can be written are not resonance stabilized(e.g. methane).

The geometry (i.e. bond lengths, angles, dihedral angles) of a resonance stabilized molecule is intermediate between the expected geometries of the canonical structures. Such molecules thus have hybrid structures and are called resonance hybrids. In benzene, for example, the canonical structures having alternating single and double bonds are called Kekule structures. The real molecule is intermediate between the two structures, and has six equal C-C bonds which are intermediate between single and double.
When the canonical structures are equivalent in energy, the real molecule is precisely halfway between the two structures.
When the canonical structures are non-equivalent in energy, the real molecule is intermediate between them, but closer to the structure of lower energy.