Question

What is the basic difference between the Valence bond theory and Molecular orbital theory?

Answer 1

Both theories tell the formation of molecules or compounds from atoms. Valence bond theory tells the formation by the participation of the electrons between the atoms and in Molecular orbital theory, the molecule formation is explained by the participation of atomic orbitals.

Complete answer:
We know that compounds and molecules are formed by the combination of atoms. Valence bond theory and Molecular orbital theory are two theories that explain the formation of a molecule.
Valence bond theory explains the formation of the molecule by the participation of electrons as this theory assumes that the electrons in the atom are in the original atomic orbitals and these electrons are used in the bonding. By drawing the Lewis dot structure of the atoms and the formation of a bond between the atoms are explained in this theory. This is due to the fact electrons of the atom are assumed localized.
The molecular orbital theory explains the formation of molecules by the participation of atomic orbitals. The atomic orbitals of two atoms combine to form molecular orbitals. But the atomic orbitals of both the atoms should have the same symmetry and the same energy. This theory explains the formation of the molecule by orbitals because it assumes the electrons are delocalized. The number of atomic orbitals combined is equal to the number of molecular orbitals formed.

Note:
Earlier the ${{O}_{2}}$ was assumed that it has all paired electrons according to the Valence bond theory and it is a diamagnetic molecule but the Molecular orbital theory showed that ${{O}_{2}}$ is paramagnetic and it has unpaired electrons.