Difference Between Localised and Delocalised Chemical Bonds

The key difference between localised and delocalised chemical bonds is that localised chemical bond is a specific bond or a lone electron pair on a specific atom whereas delocalised chemical bond is a specific bond that is not associated with a single atom or a covalent bond.

A chemical bond is a connection between two atoms. This connection occurs due to the overlapping of molecular orbitals. There are two major forms of bonds as localised and delocalised chemical bonds. Localised chemical bonds are the normal molecular orbital overlappings such as sigma bonds and pi bonds. However, delocalised chemical bonds are different. These bonds form when several localised bonds mix with each other. More details are below.

CONTENTS

1. Overview and Key Difference
2. What are Localised Chemical Bonds
3. What are Delocalised Chemical Bonds
4. Side by Side Comparison – Localised vs Delocalised Chemical Bonds in Tabular Form
5. Summary

What are Localised Chemical Bonds?

Localised chemical bonds are normal sigma and pi bonds or lone electron pairs that exist on a single atom. These bonds are concentrated on a limited region of a molecule. These regions have a concentrated electron distribution. In other words, the electron density of this region is very high.

A localised bond forms when two molecular orbitals of two separate atoms overlap with each other. Sigma bonds may form due to the overlap of two s orbitals, two p orbitals or s-p overlap.

What are Delocalised Chemical Bonds?

Delocalised chemical bonds are the chemical bonds that do not associate with only a single atom but with several atoms or other chemical bonds. We call the electrons in these bonds as ‘delocalised electrons’. Delocalization occurs in the conjugated pi system. A conjugated pi system has double bonds and single bonds in an alternating pattern.

For example, the benzene ring has three single bonds and three double bonds in an alternating pattern. Each carbon atom in this ring has a p orbital that does not undergo frontal overlapping. Therefore these p orbitals can have side overlapping. This kind of overlapping is the delocalization. We can indicate this as two circles on the top of the benzene ring and the bottom of the ring. These electrons are free to move throughout the molecule because they do not have a permanent binding to a single atom or a covalent bond.

What is the Difference Between Localised and Delocalised Chemical Bonds?

Localised chemical bonds are normal sigma and pi bonds or lone electron pairs that exists on a single atom. These bonds form due to frontal overlapping between s orbitals, p orbitals or s and p orbitals. Moreover, these electrons are limited to a particular region between two separate atoms. Delocalised chemical bonds are the chemical bonds that do not associate with only a single atom but with several atoms or other chemical bonds. These bonds have electrons spread throughout the molecule that are free to move. These bonds form due to side overlapping of p orbitals. This is the main difference between localised and delocalised chemical bonds.

Summary – Localised vs Delocalised Chemical Bonds

A chemical bond is a connection between two atoms. There are two forms of chemical bonds as localised and delocalised chemical bonds. The difference between localised and delocalised chemical bonds is that a localised chemical bond is a specific bond or a lone electron pair on a specific atom whereas a delocalised chemical bond is a specific bond that is not associated with a single atom or a covalent bond.

Reference:

1. Libretexts. “Delocalization of Electrons.” Chemistry LibreTexts, Libretexts, 20 Dec. 2016. Available here   
2. “Localized Molecular Orbitals.” Wikipedia, Wikimedia Foundation, 19 June 2018. Available here  

Image Courtesy:

1.’Sigma bond’By SVG: ZooFari; Raster: Anselm H. C. Horn (CC BY-SA 3.0) via Commons Wikimedia  
2.’Benzene-6H-delocalized’By An elite – Own work, (CC BY-SA 3.0) via Commons Wikimedia