Molecular Orbital Theory of Simple

Diatomic Molecules and Ions

Introduction

Molecular orbital theory (MOT) is another approach to explain the formation of

covalent bonds in molecules or ions. This theory was mainly developed by Hund

and Mulliken in 1932 and later by Lennerd Jones and Coulson. This theory is also

sometimes called Hund-Mulliken theory. This theory is based on the Linear

Combination of Atomic Orbitals (LCAO) of the atoms constituting the molecule or

ion and hence is also called LCAO—MO theory.

According to this theory, all the electrons in a given molecule or ion are

considered to be present in molecular orbitals. Thus these electrons move under

the influence of all the nuclei of all the constituent atoms making the molecule or

ion.

Necessary conditions to be satisfied by atomic orbitals to combine

together to form molecular orbitals

1.The combining atomic orbitals must have the same or almost the same

energy.

2.The combining atomic orbitals must be able to overlap to the maximum

extent to form molecular orbitals, since greater is the extent of overlap between

the combining atomic orbitals, greater will be the electron density in the region

between the nuclei of the combining atoms and hence stronger will be the bond

between them.

3.The combining atomic orbitals must have the same symmetry about the

molecular axis.

Disallowed overlaps of atomic orbitals on the basis of their symmetry. The

shaded portions indicate the overlap regions.

Comparison between sigma and pi molecular orbitals.

Comparison between atomic orbitals and molecular orbitals

Similarities.

1. The molecular orbitals in a molecule or an ion have different energy and different shape and size as atomic orbitals in an atom have.

2. The filling of atomic orbitals with electrons takes place in accordance with Aufbou principle, Pauli's exclusion principle and Hund's rule of maximum multiplicity. Molecular orbitals are also filled with electrons according to these principles.

3. Like atomic orbitals, a molecular orbital can be represented by a wave function of an atomic orbital is called atomic orbital wave function and that for a molecular orbital is called molecular orbital wave function.

4. Each atomic orbital wave function in an atom is associated with a set of quantum numbers which determine the energy and shape of the atomic orbital. Similarly each molecular orbital in a molecule is also associated with a set of quantum numbers which determine the energy, shape and size of molecular orbital.

Bond order or bond multiplicityBond order of a molecule or an ion is the measure of the strength or stability of the bond. Numerically bond order (B.O.) is equal to half of the difference between the total number of electrons present in bonding molecular orbitals (nb) and the total number of electrons present in the antibonding molecular orbitals (na). Thus

B.O. = (nb — nu)/2

(ii) Bond dissociation energy , stability and bond length. Greater is the value of bond order, greater is the value of bond dissociation energy and hence greater is the stability of the bond or the species, i.e., the species with higher value of bond order will have higher value of bond dissocia tion energy and hence would be more stable than the species having lower value of bond order. For

Comparison between valence bond theory (VBT) and molecular

orbital theory (MGT)

Similarities.

1. Both the theories explain the formation of covalent bond.

2. According to both the theories, a covalent bond (VBT) or a molecular orbital

(MOT) is formed when two atomic orbitals having the same symmetry and same

energy overlap with each other.

3. According to both the theories, a covalent bond (VBT) or a molecular orbital

(MOT) has directional characteristics.

4. Both the theories predict that, when covalent bond is formed, an increase in

the electron density between the nuclei takes place. This increased electron

density holds the two atoms to- gether and thus a bond is established.

5.VBT does not explain about magnatc character & bond order but MOT

explains.

LALTA PRASADPGT (CHEMISTRY)

KV BANAR JODHPURRAJASTHAN

THANKS