Molecular Orbital Theory: Atomic Structure

Molecular Orbital Theory

To describe the covalent bond formation and nature of electron sharing, two theories have been proposed: Valence Bond Theory (VBT) and Molecular Orbital Theory (MOT). In Valence Bond Theory, only the half filled orbitals of valence shell take part in bond formation and the remaining orbitals retain their identity. But Molecular Orbital Theory (MOT) suggests the combination of all atomic orbitals having comparable energy and proper symmetry. Molecular Orbital Theory (MOT) was developed by F. Hund and R.S Mulliken in 1932. Main postulates of this theory are :

1. Atomic orbitals of comparable energy and proper symmetry combine together to form molecular orbitals.

2. The movement of electrons in a molecular orbital is influenced by all the nuclei of combining atoms. (Molecular orbital is poly centric in nature)

3. The number of molecular orbitals formed is equal to the number of combining atomic orbitals. When two atomic orbitals (AO's) combine together two molecular orbitals (MO's) are formed. One molecular orbital possess higher energy than corresponding atomic orbitals and is called anti bonding molecular orbital (ABMO) and the other has lower energy and is called bonding molecular orbitals (BMO).

4. In molecules electrons are present in molecular orbitals. The electron filling is in accordance with Pauli's exclusion principle, Aufbau principle and Hund's rule.

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