The Molecular orbital theory initially referred to emphasize the bonding in molecules as the valence band theory could not explain the bonding in molecules. These are the molecules that generally involve some attribute resonance.
Resonance indicates that a bond cannot be single or double but is a product of hybrid involving both.
Valence Band theory
The valence band theory highlights only the bonding present in single, double and triple bond but does not explain in detail about resonance bonding.
The Rules of Molecular Orbital theory:
First principle ofMolecular orbital theory:
The number of molecular orbitals produced will be always equal to number of atomic orbitals brought by the atoms that are in combination.
Higher energy is present in antibonding orbitals, however lower energy is present in Bonding molecular orbitals than their parent orbitals.
Electrons of the molecule are present at orbitals from lowest to successively higher energy
Atomic orbitals join together to form molecular orbitals most effectively when the atomic orbitals are of the same energy.
Principle 1: Example – Hydrogen (H2) Each hydrogen atom has a single valence bond orbital, this being the 1s orbital.
Two molecular orbitals can be formed by the constructing and destructing overlap of these two atomic orbitals.
With two 1s atomic orbitals only two molecular orbitals show formation from them. This is the First Principle.
According to MO Theory, the two molecular orbitals that emerge are s (sigma = bonding) and s* (sigma star = antibonding).
Principle 2 & 3: This interaction of atomic orbitals, which gives rise to the molecular orbitals,. In the given specific case of hydrogen each of the isolated atoms has one electron in its 1s orbital.
When the atoms combine to form H2 the two electrons may be joined (with opposite spins) in the bonding molecular orbital.
This principle explains why the molecular orbitals occupy the electrons initially, however stability is achievable by lowering the energy.
Therefore molecular orbitals provide a lower energy resulting in a more stable state for the electrons.
hence they fill the orbitals first. This could explain the third principle statement also.
Principle 4: In hydrogen molecule the 1s orbitals are combined to produce a single lower energy s molecular orbital.
The fourth principle states that the stability of molecular orbitals are easily achieved when they are built on atomic orbitals of same energy,
In other words the 1s orbital should bond with 1s and the 2p orbital should combine with the 2p orbital so as to achieve stable molecular orbitals.
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