Topic 14.3: Delocalization of electrons

14.3.1 Describe the delocalization of π electrons and explain how this can account for structures of some species.

Instead of being confined to one location, electrons show a tendency to be shared between more than one bonding position, and are said to be delocalized. Free from the constriants of a single bonding position, delocalized electron spread themselves out.

Delocalization is a characteristic of electrons in pi bonds when there is more than one possible position for a double bond within a molecule. For example, if we consider the structure of the nitrate ion.

It can inter-change from any of the different Lewis structures.This suggests that the pi electrons have delocalized and spread themselves equally between all three possible bonding positions. This is the Lewis structure normally drawn.

Resonance is introduced because it can't be represented in a single diagram. The actual structure of the species is a composite or average of the number of Lewis structures that can be drawn, each known as a resonance structure.

Other resonance structures include carbonate, ozone and methanoate.


Benzene is a particular interesting case as all the carbons have made sp^2 bonds.

Because the p orbitals are so close together, they form a huge cloud instead.

This allows the electron to move freely around the benzene hexagon. This is easily portrayed in a Lewis Structure.

Delocalized electrons gives special properties to the structures in which they are found.


Intermediate bond lengths and strengths

By spreading the electrons between more than one bonding position, delocalization causes the affected bonds to be equal each other in length and strength, with values in between those of single and double bonds. The concept of bond order is sometimes used to describe the electron density within the bond. It is calculated by how many bonds divided by number of bonding positions. The higher the bond order, the greater the electron density

Greater Stability

Species with delocalized electrons are more stable than related species with all electrons localized in bonds. This is because delocalization spreads electrons as far apart as possible and so minimizes the repulsion between them. They require extra energy called resonance energy to disrupt the delocalized pi electron cloud.

Electrical conductivity in metal and graphite

Structures that have delocalized electrons spread through the entire structure (graphite). This enables them to move in response to a potential difference applied. In other words, conduct electricity.