Difference Between Molecular Orbital Theory and Hybridization Theory

The key difference between molecular orbital theory and hybridization theory is that molecular orbital theory describes the formation of bonding and anti-bonding orbitals, whereas hybridization theory describes the formation of hybrid orbitals.

There are different theories developed to determine the electronic and orbital structures of molecules. VSEPR theory, Lewis theory, valence bond theory, hybridization theory and molecular orbital theory are such important theories. The most acceptable theory among them is the molecular orbital theory.

CONTENTS

1. Overview and Key Difference
2. What is Molecular Orbital Theory 
3. What is Hybridization Theory
4. Side by Side Comparison – Molecular Orbital Theory vs Hybridization Theory
5. Summary

What is Molecular Orbital Theory?

Molecular orbital theory is a technique of describing the electronic structure of molecules using quantum mechanics. It is the most productive way of explaining chemical bonding in molecules. Let us discuss this theory in detail.

First, we need to know what molecular orbitals are. A chemical bond forms between two atoms when the net attractive force between two atomic nuclei and the electrons in between them exceeds the electrostatic repulsion between two atomic nuclei. Basically, this means, the attractive forces between two atoms should be higher than the repulsive forces between those two atoms. Here, the electrons must exist in a region called “binding region”, to form this chemical bond. If not, the electrons will be in the “anti-binding region” which will help the repulsive force between the atoms.

However, if the requirements are fulfilled and a chemical bond forms between two atoms, then the corresponding orbitals involved in bonding are called molecular orbitals. Here, we can start with two orbitals of two atoms and end up with one orbital (the molecular orbital) which belongs to both atoms.

According to quantum mechanics, atomic orbitals cannot appear or disappear as we wish. When orbitals interact with each other, they tend to change their shapes accordingly. But according to quantum mechanics, they are free to change the shape but need to have the same number of orbitals. Then we need to find the missing orbital. Here, the in-phase combination of the two atomic orbitals makes the bonding orbital while out-of-phase combination forms the anti-bonding orbital.

The bonding electrons occupy the bonding orbital while the electrons in the anti-bonding orbital do not participate in bond formation. Rather, these electrons actively oppose the formation of the chemical bond. The bonding orbital has lower potential energy than the anti-bonding orbital. If we consider a sigma bond, the denotation for bonding orbital is σ, and the anti-bonding orbital is σ*. We can use this theory to describe the structure of complicated molecules to explain why some molecules don’t exist (i.e. He₂) and the bond order of molecules. Thus, this description briefly explains the basis of the molecular orbital theory.

What is Hybridization Theory?

Hybridization theory is a technique we use to describe the orbital structure of a molecule. Hybridization is the formation of hybrid orbitals by mixing two or more atomic orbitals. The orientation of these orbitals determines the geometry of the molecule. It is an expansion of the valence bond theory.

Before the formation of the atomic orbitals, they have different energies, but after the formation, all the orbitals have the same energy. For example, an s atomic orbital, and a p atomic orbital can combine to form two sp orbitals. The s and p atomic orbitals have different energies (energy of s < energy of p). But after the hybridization, it forms two sp orbitals which have the same energy, and this energy lies between the energies of individual s and p atomic orbital energies. Moreover, this sp hybrid orbital has 50% s orbital characteristics and 50% p orbital characteristics.

The idea of hybridization first entered into the discussion because scientists observed that the valence bond theory failed to correctly predict the structure of some molecules such as CH₄. Here, although the carbon atom has only two unpaired electrons according to its electron configuration, it can form four covalent bonds. To form four bonds, there must be four unpaired electrons.

The only way they could explain this phenomenon was to think that s and p orbitals of carbon atom fuse with each other to form new orbitals called hybrid orbitals which have the same energy. Here, one s + three p gives 4 sp³ orbitals. Therefore, the electrons fill these hybrid orbitals evenly (one electron per hybrid orbital), obeying the Hund’s rule. Then there are four electrons for the formation of four covalent bonds with four hydrogen atoms.

What is the Difference Between Molecular Orbital Theory and Hybridization Theory?

The molecular orbital theory is a technique of describing the electronic structure of molecules using quantum mechanics. Hybridization theory is a technique we use to describe the orbital structure of a molecule. So, the key difference between molecular orbital theory and hybridization theory is that molecular orbital theory describes the formation of bonding and anti-bonding orbitals, whereas hybridization theory describes the formation of hybrid orbitals.

Furthermore, according to the molecular orbital theory, new orbital forms from the mixing of atomic orbitals of two atoms while in hybridization theory, new orbital forms form the mixing of atomic orbitals of the same atom. Therefore, this is another difference between molecular orbital theory and hybridization theory.

Summary – Molecular Orbital Theory vs Hybridization Theory

Both molecular orbital theory and hybridization theory are important in determining the structure of a molecule. The key difference between molecular orbital theory and hybridization theory is that molecular orbital theory describes the formation of bonding and anti-bonding orbitals, whereas hybridization theory describes the formation of hybrid orbitals.

Reference:

1. “Hybridization.” Chemistry LibreTexts, Libretexts, 5 June 2019, Available here.

Image Courtesy:

1. “O2MolecularDiagramCR” By TCReuter – Own work (CC BY-SA 4.0) via Commons Wikimedia
2. “Ch4 hybridization” By K. Aainsqatsi at English Wikipedia(Original text: K. Aainsqatsi) – Own work (Original text: self-made) (Public Domain) via Commons Wikimedia