Once again we explore the world of valence bond theory but this time only be concerned with molecules that have at least three adjacent p orbitals with electrons located in the pi bonds. What makes these molecules so unique is due to something called pi electron delocalization or resonance. While most valence bond theories require an exact location of electrons, resonance theory allows the pi electrons to exist in any adjacent p orbital.
Let’s jump back to general chemistry and Lewis dot bonding. There were a few molecules that could not be represented by a single Lewis structure, but required the use of resonance structures to adequately describe it. A limitation of these structures is that it localizes or fixes the electrons into one place.
A better way (but beyond the scope of general chemistry) would be to approach these molecules using resonance theory. In this theory, only sigma bonds are fixed with the pi electrons delocalized over any adjacent p orbital. In organic chemistry, molecules that have delocalized electrons are usually more stable. There are even some molecules, called aromatics, that have a very special form of delocalization that make them exceptionally stable.
The physical chemistry textbook (section 12.3) uses ethene (ethylene), butadiene and benzene as examples. Experimental studies have shown that both butadiene and benzene are more stable than expected and do not contain either single bonds or double bonds as required by Lewis dot theory but rather a “hybrid” of the two. Using resonance theory, these observations are predicted.
In ethene’s case, there are only two atoms, so it is just a simple double bond, but butadiene and benzene have extra stability due to the number of atoms that can participate in delocalization. In each case, the molecule has adjacent atoms that have a pi (atomic p) orbital, therefore, any pi electrons are delocalized among any participating atoms.
Why does this arrangement have a greater stability? In simple terms, electrons are a burden for the atom, anything that helps share that burden is favorable and makes the overall molecule more stable. The more atoms to share the burden, generally, the more stable it becomes. Ring systems like benzene have an even greater advantage because it is a circle of endless “helpers”.
Resonance theory helps explain both the increased stability of these molecules and their properties. Molecular orbital theory approaches this topic differently but comes to the same conclusions.