Rudiments of Valence Bond Theory

What is valence bond theory?

Valence bond theory is used to explain chemical bonding with the help of quantum mechanics. As per the VB theory a covalent bond is formed due to the physical overlapping of half filled valence orbitals between two atoms.

Valence bond theory postulates

  • When the half filled valence orbitals of two atoms overlap on each other it results in the formation of covalent bond. It increases the electron density between the two atoms.
  • Only the unpaired electrons in valence orbitals can take part in bond formation.
  • The electron density around the atoms bonded is maximum.
  • The strength of the bond is determined based on the degree of overlapping. There are two types viz, pi bond and sigma bond.

Types of covalent bonds

  • Sigma bond – Covalent bond formed between two atoms by overlapping sidewise is called sigma bond.
  • Pi bond – Covalent bond formed between two atoms by overlapping of their inter nucleus axis is called sigma bond.

valence bond theory

Valence bond theory examples

Consider a water molecule. The electronic configuration of valence shell in each hydrogen atom is 1s1 whereas oxygen atom’s valence shell electron configuration is 2s2, 2px 2, 2py 1, 2pz 1. Therefore, two vacant electron positions are available in the oxygen atom which potentially form bonds with two hydrogen atoms considering that the electrons in the hydrogen and oxygen have opposite spins.

Applications of VB theory

  • The maximum overlap condition results in the formation of the strongest bonds. Therefore, the covalent bond formation in various molecules is explained well by this theory.
  • For instance, consider F2 molecule, the formation of F−F bond is made by the overlapping of pz orbitals of the two F atoms, where each has an unpaired electron. The nature of overlapped orbitals are not the same in F2 and H2 molecules, therefore, bond lengths and bond strengths differ between these molecules.
  • In an HF molecule, the formation of a covalent bond is obtained by the mutual sharing of electrons between F and H.

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