RESONANCE STRUCTURES
The language of organic chemistry does not permit the description of the exact
electronic configuration of molecules in terms of a single Lewis structure. To illustrate
this point, imagine a language which has words for blue and yellow but none for green.
To describe green, one could do it in terms of various proportions of blue and yellow.
Similarly, organic chemists describe molecules in terms of imaginary extreme electronic
structures termed resonance structures.
1. Resonance structures are written for compounds that cannot be described by a
single Lewis structure.
2. Resonance structures are imaginary structures.
3. Resonance structures are described by using the double-headed arrow formalism.
This formalism does not indicate that the molecule is resonating between the two
extreme Lewis structures. It is the easiest way that chemists can describe different
contributions to the total picture of the hybrid molecule. In the same way that
green does not change between blue and yellow, a molecule does not shift from
one resonance structure to the other. Green always looks green and an electronic
structure of a molecule always is the same. It just may be difficult for us to
describe it using Lewis structure language.
4. The hybrid molecule is the only one that has real existence and a resonance hybrid
is always characterized by having lower energy and greater thermodynamic
stability than any of its resonance contributors.
5. Resonance contributors cannot involve any changes in the position of the nuclei.
6. Resonance contributors involve only electron reorganization. As such, lone pairs
and multiple bond electrons are usually the ones involved.
7. Resonance structures must have the same number of paired or unpaired electrons,
but not necessarily the same number of bonds.
