Difference between Valence Bond Theory and Molecular Orbital Theory
Valence Bond Theory (VBT) and Molecular Orbital Theory (MOT) are two fundamental concepts in chemistry that explain the formation of chemical bonds between atoms. Although both theories aim to describe the electronic structure of molecules, they differ in their approaches and assumptions. This article aims to highlight the key differences between Valence Bond Theory and Molecular Orbital Theory.
Valence Bond Theory
Valence Bond Theory, developed by Linus Pauling in the 1920s, is a simplified model that focuses on the overlapping of atomic orbitals to form covalent bonds. According to VBT, when two atoms come close together, their atomic orbitals overlap, and the electrons involved in the bond formation are localized between the nuclei of the bonded atoms. The overlapping orbitals can be sigma (σ) or pi (π) bonds, depending on the orientation of the orbitals.
The VBT assumes that the electrons in a bond are localized and that the bond is formed by the direct overlap of atomic orbitals. This theory also emphasizes the concept of hybridization, where atomic orbitals of an atom mix to form new hybrid orbitals with different shapes and energies, which are more suitable for bonding.
Molecular Orbital Theory
Molecular Orbital Theory, developed by Robert Mulliken and Friedrich Hund in the 1920s, is a more sophisticated model that considers the delocalization of electrons in a molecule. According to MOT, when two atoms come close together, their atomic orbitals combine to form molecular orbitals, which can be bonding, antibonding, or nonbonding.
In MOT, the electrons are not localized between the nuclei of the bonded atoms but are spread out over the entire molecule. This delocalization of electrons leads to the formation of molecular orbitals with different energies and shapes. The bonding molecular orbitals are lower in energy than the atomic orbitals, while the antibonding molecular orbitals are higher in energy.
Differences between VBT and MOT
1. Approach: Valence Bond Theory focuses on the overlap of atomic orbitals to form bonds, while Molecular Orbital Theory considers the combination of atomic orbitals to form molecular orbitals.
2. Electron Localization: In VBT, electrons are localized between the nuclei of the bonded atoms, whereas in MOT, electrons are delocalized over the entire molecule.
3. Bonding and Antibonding: VBT does not explicitly consider antibonding orbitals, while MOT includes both bonding and antibonding molecular orbitals.
4. Hybridization: VBT incorporates the concept of hybridization, which is not present in MOT.
5. Predictive Power: Molecular Orbital Theory is more accurate in predicting the properties of molecules, such as bond energies, bond lengths, and molecular geometries, compared to Valence Bond Theory.
In conclusion, while both Valence Bond Theory and Molecular Orbital Theory aim to explain the formation of chemical bonds, they differ in their approaches, assumptions, and predictive power. Molecular Orbital Theory provides a more comprehensive and accurate description of the electronic structure of molecules, making it the preferred model in modern chemistry.
