Resonance structures

Resonance

Movement of electrons through the p-orbital pathway is described, as conjugation, mesomeric effect and even as resonance.
Resonance structures are a result of flow of electrons through the p-orbital pathway.
There is a minor difference between resonance and mesomeric effect which is not considered in this article.
There are molecules which cannot be represented by a single conventional valence structure that is a Lewis structure.
Such molecules are represented as a combination of several Lewis structures called Resonance structures or canonical forms.
A simple example is 1,3-Butadiene.

The molecule can be represented with two alternate double bonds.
Close examination of the p-orbital picture reveals that this will not sufficiently explain its structure.

The structure with two alternate double bonds is with the understanding, that the electrons in the p-orbitals of the carbons 1 & 2 and 3 & 4 are involved in formation of the Pi-bond as in scheme A.
Since the four p-orbitals are adjacent to one another it is quite possible that there can be a -bond between 2 & 3 as well which leads to another structure with different distribution of electrons. As shown in B.

Note: These arrangements which differ in distribution of electrons are termed resonance structures.
It should be noted that the two structures differ only in distribution of electrons and the atoms are in the same relative positions.
In scheme A each p orbital has one electron while in B p-orbital of C1 has no electrons hence it is positively charged . In C4 there is an extra electron hence it is negatively charged.
Molecules can have more than two resonance structures.
The actual molecule is considered as a hybrid of all canonical forms or resonance structures.

The two structures are separated by a double headed arrow and not a reversible arrow (shown above)
They are are not in equilibrium. They do not exist sometimes in one form and sometimes in another. They cannot be separated.
Though resonance structures are imaginary they are very useful in explaining certain features of reaction mechanism.