Molecular Orbital Theory

What is Molecular Orbital Theory?

The molecular orbital theory is one of the most productive models of chemical bonding. It is the basis of quantitative calculations, including those regarding the computer-generated images.

Molecular orbital theory in overall involves a lot of complicated mathematics. However, the fundamental ideas behind the theory are very easy to understand.

The simple Lewis and VSEPR models are based in the one-center orbitals of individual atoms. The molecular orbital theory involves sophisticated hybridization models, according to which the orbital can be modified by the interaction with other atoms.

However, the valence bond models, explained by the molecular orbital theory, are normally very limited in their applicability and predictive power. This is because the valence electrons are governed by positive centers.

Molecular Orbital Theory Notes

When the force of attraction between an electron and two nuclei exceeds the electrostatic repulsion between the two nuclei, chemical bonding occurs. The electron must be in the binding region in order to do so.

If the electron is located on another side in an anti-binding region, it in fact, enhances the repulsion between two nuclei and pushes them away.

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If you want to visualize a molecular orbital, you can do it easily by picturing two separate isolated atoms having their own electron orbitals. These are just orbitals of those atoms. The next thing is to predict the manner in which these atomic orbitals interact with each other and bring the atoms close to each other.

When the inter-nuclear distance complies with the molecule under study, the corresponding orbitals will be the molecular orbitals of the new molecule.

To see the working nature of this, let us consider a simple molecule, i.e. a hydrogen molecule. The molecule consists of two nuclei having charge +1 and shares a single electron between them.

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When two hydrogen nuclei move towards each other, the 1s orbitals of the isolated atoms merge to form a new molecular orbital. The greatest electron density of the new molecular orbital falls between the two nuclei.

At this position, the force exerted by electrons on the nuclei is maximum. Thus this arrangement gives rise to a bonding molecular orbital.

Since the phenomena occur in a three-dimensional region of space, it is symmetrical about the imaginary line that connects the centers of the two nuclei. Based on the usual nomenclature, this is called σ (sigma) orbital.

Types of Molecular Orbitals

According to the molecular orbital theory, there are two types of primary molecular orbitals which are formed due to the linear combination of atomic orbitals. These orbitals are mentioned below.

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1. Anti Bonding Molecular Orbitals

In anti-bonding molecular orbital, the electron density of the two bonding atoms is concentrated behind the nuclei. As a result, the distance between the nuclei of the two atoms increases. 

The anti-bonding molecular orbital weakens the bonds between two atoms.

2. Non-Bonding Molecular Orbitals

There is no symmetry between two bonding atomic orbitals in the case of the non-bonding molecular orbital. The molecular orbital thus formed doesn’t have any positive or negative interaction between each other.

 The bond between the atoms is not affected by these types of orbitals.

Molecular Orbital Theory Diagram

The bonding and anti-bonding orbitals are usually depicted by the molecular orbital diagram. Below mentioned is the molecular orbital diagram of the dihydrogen ion H2+. 

The atomic valence electrons (which are represented by the left and right boxes) at first fills the lower-energy molecular orbitals, and then it fills the higher ones. This is same as that of the atomic orbitals. 

Therefore, the single electron of the molecule goes into the bonding orbital and leaves the anti-bonding orbital empty.

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Orbitals have space for a maximum of two electrons, and therefore the bonding orbital in H2+ is half-filled. This single electron doesn't have the required energy the lower the potential energy of one mole of hydrogen nuclei pairs by 270 kJ, thereby making them stick together and behave like distinct molecular species.

H2+ is highly stable in an energetic sense and is an extremely reactive molecule. It even reacts with itself, and hence these ions are not commonly found.

Salient Features of the Molecular Orbital Theory

  1. The molecular orbitals are created because of the overlapping of the atomic orbitals. The atomic orbitals merge with each other to form the molecular orbital.

  2. The electrons of the molecules fill the new energy states of the molecular orbitals, similar to filling up of the energy states of the atomic orbitals.

  3. The probability of finding the distribution of electrons around the nuclei of a molecule is given by the molecular orbital.

  4. The two atomic orbitals which combine to form the molecular orbital should possess energy values of similar orientation. For example, 1s can combine with only 1s and both with 2s or 2p.

  5. The number of molecular orbitals creates equal to the number of combinations of the atomic orbitals.

  6. The shape of the molecular orbitals depends on the shape of the atomic orbitals.

FAQ (Frequently Asked Questions)

Q1. Why is the Molecular Orbital Theory so crucial?

Ans: The molecular orbital theory is more powerful than the valence bond theory and describes the geometry of the molecule in a better way. 

However, this power holds a huge value when viewed with respect to the model that represents its core importance.

Q2. What are the cons of Molecular Orbital Theory (MOT)?

Ans: Some of the demerits of the molecular orbital theory are:

i. It is based on the principles of quantum mechanics and is hard to understand for beginners.

ii. It does not give any idea about the shape of the molecule.

iii. Some of the properties of the molecules are not described by it; hence the valence bond theory is required.

iv. The explanation of polyatomic molecules is very difficult.

Q3. Give the specific difference between valence bond theory & Molecular Orbital Theory.

Ans: Valence bond theory deals with the electrons filling up atomic orbitals, whereas molecular orbital theory deals with electron filling molecular orbitals. 

Q4. What is the difference between Sigma and Pi Bond?

Ans: Sigma bond is formed due to the linear overlapping of the atomic orbitals of two atoms. 

In a pi bond, the overlap of electrons occurs both above and below the axis connecting the nuclei of two atoms.