Question

Molecular Orbital Theory (MOT) is an advanced theory of chemical bonding. 

1. Write the salient features of MOT. 

2. What is meant by LCAO? Illustrate using hydrogen molecule. 

3. What are the conditions for the combination of atomic orbitals?

Answer 1

1. 

• The electrons in a molecule are present in the various molecular orbitals.
• The atomic orbitals of comparable energies and proper symmetry combine to form molecular orbitals. 
• The electron in a molecular orbital is influenced by two or more nuclei depending upon the number of atoms in the molecule. 
• The number of molecular orbitals formed is equal to the number of combining atomic orbitals. When two atomic orbitals combine, two molecular orbitals are formed. One is known as bonding molecular orbital while the other is called antibonding molecular orbital. 
• The bonding molecular orbital has lower energy and hence greater stability than the corresponding antibonding molecular orbital. 
• The electron probability distribution around a group of nuclei in a molecule is given by a molecular orbital.
• The molecular orbitals are filled in accordance with the Aufbau principle obeying the Pauli’s exclusion principle and the Hund’s rule.

2. LCAO refers to the linear combination of atomic orbitals. It is an approximate method used to explain the formation of molecular obritals. Consider hydrogen molecule consisting of two atoms A and B. Each hydrogen atom has one electron in the 1s orbital. The atomic orbitals of these atoms can be represented by the wave functions ψ_A and ψ_B . Mathematically, the formation of molecular orbitals can take place by addition and by subtraction of wave functions of individual atomic orbitals. 

ψ_MO = ψ_A ± ψ_B 

Therefore, the two molecular orbitals σ and σ* are formed as:

 σ* = ψ_A – ψ_B

The molecular orbital σ formed by the addition of atomic orbitals is called the bonding molecular orbital while the molecular orbital σ* formed by the subtraction of atomic orbital is called antibonding molecular orbital. 

The energy level diagram is shown below:

3. 

• The combining atomic orbitals must have the same or nearly same energy. 
• The combining atomic orbitals must have the same symmetry about the molecular axis. 
• The combining atomic orbitals must overlap to the maximum extent.