Molecular Orbital Theory of Simple Diatomic Molecules and Ions. Introduction
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Molecular Orbital Theory of Simple
Diatomic Molecules and Ions
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.
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.
Bond order or bond multiplicity
Bond 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 dissociation energy and hence would be more stable than the species having lower value of bond order. For
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