Valence Bond Theory (VBT) Vs. Molecular Orbital Theory (MOT): 10 Differences With Examples

SHARE

Valence bond theory is a molecular theory that is used to define the chemical bonding of atoms in a molecule. Valence bond theory is based on localized bond approach, in which it assumes that the electrons in a molecule occupy atomic orbitals for the individual atoms. As a chemical bonding theory, it explains the bonding between two atoms is caused by the overlap of half-filled atomic orbitals. The two atoms share each other’s unpaired electron to form a filled orbital to form a hybrid orbital and bond together. Sigma and pi bonds are part of valence bond theory. Valence bond theory was first proposed by W. Heitler and F. London in 1927

 In contrast, molecular orbital theory is a basic theory that is used to define the chemical bonding of a molecule by use of hypothetical molecular orbitals. The molecular orbital theory is a way of looking at the structure of a molecule by using molecular orbitals that belong to the molecule as whole rather than to the individual atoms.

When simple bonding occurs between two atoms, the pair of electrons forming the bond occupies a molecular orbital that is a mathematical combination of the wave functions of the atomic orbitals of the two atoms involved. Molecular orbital theory was first proposed by F. Hund and R.S. Mulliken in 1932.

The Difference

  1. Valence bond theory was first proposed by W.Heitler and F. London in 1927 whereas molecular orbital theory was first proposed by F. Hund and R.S. Mulliken in 1932.
  2. The valence bond theory defines the hybridization of molecular orbitals whereas the molecular theory does not define anything about hybridization of orbitals.
  3. Valence bond theory can only be applied for diatomic molecules whereas molecular orbital theory can be applied on polyatomic molecules.
  4. In valence bond theory, bonds are localized to two atoms and not molecules while in molecular orbital theory, bonds are localized to both two atoms and molecules.
  5. Valence theory explains about molecules occupying atomic orbitals whereas molecular orbitals theory explains about the mixing of atomic orbitals when forming molecules.
  6. In valence bond theory, resonance plays an important role whereas in molecular orbital theory, resonance does not play any role.
  7. In valence bond theory, atoms which are involved in the bond formation, maintain their individual characteristic nature. In contrast, in molecular orbital theory, atomic orbitals which form molecular orbitals, do not retain their individual characteristic nature.
  8. In valence bond theory, there is no explanation of paramagnetic character of oxygen whereas in molecular orbital theory there is an elaborate explanation of paramagnetic character of oxygen.
  9. In valence bond theory, the resulting molecular orbital is obtained by the combination of two wave functions of two unpaired electrons. On the other hand, in molecular orbitals theory, formation of the molecular orbitals is based on the LCAO approximation method, whereby atomic orbitals corresponding of the valence shell of two, only takes part in the formation of molecular orbitals.
  10. In valence bond theory, some of the valence electrons are represented as not shared and not involved in the formation of the molecule. On the contrary, in molecular orbital theory, all the electrons of the valence shell are represented as having taken part in the bonding.
  11. Valence bond theory has simplicity and convenience especially in terms of calculations whereas molecular orbital theory is somehow complex and tedious especially in terms of calculations.

ALSO READ: Bonding Vs. Antibonding Molecular Orbitals

Valence Bond Theory (VBT)  Vs. Molecular Orbital Theory (MOT) In Tabular Form

BASIS OF COMPARISON VALENCE BOND THEORY (VBT)  MOLECULAR ORBITAL THEORY (MOT)
Description Valence bond theory is a molecular theory that is used to define the chemical bonding of atoms in a molecule. Valence bond theory is based on localized bond approach, in which it assumes that the electrons in a molecule occupy atomic orbitals for the individual atoms. Molecular orbital theory is a basic theory that is used to define the chemical bonding of a molecule by use of hypothetical molecular orbitals. The molecular orbital theory is a way of looking at the structure of a molecule by using molecular orbitals that belong to the molecule as whole rather than to the individual atoms.  
Proposed By Valence bond theory was first proposed by W.Heitler and F. London in 1927. Molecular orbital theory was first proposed by F. Hund and R.S. Mulliken in 1932.  
Application It can only be applied for diatomic molecules.   It can be applied on polyatomic molecules.  
Bond Localization In valence bond theory, bonds are localized to two atoms and not molecules.   In molecular orbital theory, bonds are localized to both two atoms and molecules.  
What The Theory Explains The theory explains about molecules occupying atomic orbitals.   The theory explains about the mixing of atomic orbitals when forming molecules.  
Resonance In valence bond theory, resonance plays an important role.     In molecular orbital theory, resonance does not play any role.
Retention of Individual Characteristics Atoms which are involved in the bond formation, maintain their individual characteristic nature.   Atomic orbitals which form molecular orbitals, do not retain their individual characteristic nature.
Explanation Of Paramagnetic Character Of Oxygen There is no explanation of paramagnetic character of oxygen.   There is an elaborate explanation of paramagnetic character of oxygen.  
Formation The resulting molecular orbital is obtained by the combination of two wave functions of two unpaired electrons. Formation of the molecular orbitals is based on the LCAO approximation method, whereby atomic orbitals corresponding of the valence shell of two, only takes part in the formation of molecular orbitals.  
Representation Of Valence Electrons Some of the valence electrons are represented as not shared and not involved in the formation of the molecule.   All the electrons of the valence shell are represented as having taken part in the bonding.
Complexity Valence bond theory has simplicity and convenience especially in terms of calculations.   Molecular orbital theory is somehow complex and tedious especially in terms of calculations.