Molecular Orbital Theory
Molecular orbital theory is a method developed in the beginning of twentieth century by F. Hund and R. S. Mulliken to describe the structure and properties of different molecules. Before molecular orbital theory, there was no clear and distinctive theory about the bonding and non-bonding electrons and how their arrangement in a molecule affects the structure of the molecule. In this method the electrons are not assigned to a particular bond between atoms, rather they are treated as moving under the influence of the nuclei of the molecule. The space in a molecule in which the probability of finding an electron is maximum is known as molecular orbital. The atomic orbitals combine together to form molecular orbitals. This combination is also known as linear combination of atomic orbitals (LCAO). The molecular orbitals are of three types:-
- BONDING MOLECULAR ORBITALS
If the electrons present in the molecular orbital have higher probability of being around the nucleus of bonding atoms than anywhere else, then the molecular orbital is known as bonding molecular orbital. This kind of molecular orbital strengthens the bond between two atoms.
- ANTI-BONDING MOLECULAR ORBITALS
If the electrons present in the molecular orbital have higher probability of not being around the nucleus of bonding atoms than anywhere else, then the molecular orbital is known as anti-bonding molecular orbital. This kind of molecular orbital weakens the bond between two atoms.
- NON-BONDING MOLECULAR ORBITALS
If the electrons present in the molecular orbital belong to atomic orbitals that don’t interact positively or negatively with another atomic orbitals, then that molecular orbital is known as non-bonding molecular orbital. This kind of molecular orbital doesn’t affect the bond between two atoms.
Few salient features of MOT:
- The electrons in atomic orbitals are influenced by one nucleus but in case of molecular orbitals the electrons are affected by one or more nucleus depending upon the number of atoms present in the nucleus. Thus an atomic orbital is monocentric and a molecular orbital is polycentric.
- The molecular orbitals like atomic orbitals, are filled in accordance with the aufbau principle, Pauli’s exclusion principle and the Hund’s rule.
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