Chemical Bonding and Molecular Structure Part - 14 | Class 11th

ü  Molecular Orbital Theory:
 

    This theory was developed by F.Hund and R.S Mulliken. The important postulates of this theory are:

1. When two atomic orbitals combine or overlap, they lose their identity and form new orbitals. The new orbitals formed are called molecular orbitals.

2. Only those atomic orbitals can combine to form molecular orbitals which have comparable energies and proper orientations.

3. The number of molecular orbitals formed is equal to the number of combining atomic orbitals.

4. When two atomic orbitals combine, they form two new orbitals called ‘bonding molecular orbital’ and ‘antibonding molecular orbital’.

5. The bonding molecular orbital (BMO) has lower energy and hence greater stability than the corresponding antibonding molecular orbital (ABMO).

6. The bonding molecular orbitals are represented by σ (sigma), π (Pi), δ (delta) and antibonding are represented by σ*, π*, δ*.

7. The shapes of the molecular orbitals formed depend upon the type of the combining atomic orbitals.

8. The molecular orbitals are filled according to 3 rules – Aufbau principle, Pauli’s exclusion principle and Hund’s rule.

Ø  Formation of molecular orbitals – Linear Combination of Atomic Orbitals (LCAO) method:

Molecular orbitals are formed by the combination of atomic orbitals by an approximate method known as Linear Combination of Atomic Orbitals (LCAO). According to this theory, the combinations of atomic orbitals take place by addition and subtraction of wave functions of atomic orbitals. The M.O formed by the addition of A.Os is called the bonding molecular orbital (BMO) and by the subtraction of A.Os is called the anti-bonding molecular orbitals (ABMO).

Bonding Molecular Orbitals                        Anti Bonding Molecular Orbitals

1. Formed by additive effect of                      1. Formed by subtractive effect of

atomic orbitals           atomic orbitals 

2. lower energy, greater stability                    2. higher energy, lower stability

3. represented by σ and π                                3. represented by σ* and π*.

 Ø  Conditions for combination to atomic orbitals to form molecular orbitals:

1. The combining atomic orbitals should have comparable energy.

2. The combining atomic orbitals must have proper orientation.

3. The extent of over lapping should be large.

Ø  Types of Molecular Orbitals:

M.Os of diatomic molecules are designated as σ (sigma), π (Pi), δ (delta) etc. the σ (sigma) M.Os are symmetrical about the bond axis, while the pi M.Os are not symmetrical.

Linear combination of two 1s atomic orbitals produces two M.Os – a BMO σ1s and an ABMO σ*1s.

Similarly linear combination of two 2s atomic orbitals produces two M.Os – σ2s and σ*2s.

If z-axis is taken as the inter nuclear axis, linear combination of two 2p_z orbitals produces two sigma M.Os σ2p_z and σ*2p_z.

                While the combination of 2p_x and 2p_y orbitals produce M.Os which are not symmetrical                          about the bond axis. So two 2p_x orbitals produces 2 sigma M.Os – π2p_x and π*2p_x and                    two 2p_y orbitals produce 2 M.Os – π2p_y and π*2p_y.