MOLECULAR ORBITAL THEORY
The molecular orbital theory was proposed.by E. Hund and R.S. Muliken in 1932 and the basic features of this theory are as follows.
>Electrons of a molecule are present in various molecular orbitals just like the electrons of atoms are present in atomic orbitals.
>Molecular orbitals are formed by the mixing of atomic orbitals of comparable energies and proper symmetry.
>An electron in an atomic orbital is infuenced by one nudeus and thus is monocentnc while in a molecular orbital it is influenced by two or more nudei of the molecule and thus is polycentric.
>The total number of molecular orbitals formed are equal to the number of combining atomic orbitals.
>Bonding molecular orbital has lower energy and greater stability than the corresponding antibonding molecular orbital.
>The molecular orbitals are also filled by electrons in accordance with Aufbau principle obeying the Pauli's exclusion principle and the Hund's multiplicity rule.
The linear combination of atonic orbitals to form molecular orbitals takes place if the following conditions are satisfied.
> The combining atomic orbitals must have the same or the nearly same energy.
>The extent of overlapping between the atomic orbitals of two atoms should be maximum.
>The combining atomic orbitals must have the same symmetry about the internuclear axis.
>Molecular orbital energy level diagram grves many valuable intormation regarding the behaviour of molecules as followings.
A. Stability of a Molecule
If Na,is the number of electrons present in bonding orbitals and Nb, is the number of electrons present in anti-bondng electrons, then the molecule is stable when Na>Nb
B. Bond Order
Bond order may be defined as one half the diliference between the number of electrons present in
the bonding and the bonding orbitals i.e.
Bond Order= Nb - Na
2
The positive value of bond order indicates a stable molecule. Higher the bond order greater would be the bond dissociation energy. More over higher the bond order value smaller would be the bond length.
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