What is Valence bond theory and how to apply VBT

We will study valence bond theory in terms of energy consideration and orbital overlap concept. we will learn here how to apply VBT

What is Valence bond theory

  1. When 2 atoms approach each other, new forces of attraction and repulsion are there.
  2. Attractive forces are present in between electron-nuclei.
  3. Repulsive forces are present between electron-electron and nuclei-nuclei.
  4. If forces of attraction is more than repulsive forces, then molecule forms like in H2 molecule.
  5. If forces of repulsion is more than attractive forces, then molecule does not forms like in He2 molecule.
  6. The energy released in the formation of 1 mole of bonds is called bond energy.

VBT in terms of energy consideration

Formation of H2 molecule

  1. When the H atoms are far apart, the P.E. is zero.
  2. The atoms comes closer together from infinite distance.
  3. The system starts losing energy, when the force of attraction is more than repulsive forces.
  4. As a result, P.E. of the system keeps on decreasing.
  5. At the equilibrium distance, the forces of repulsion balances the force of attraction.
  6. At this stage, there is a formation of H2 molecule
  7. Bond energy is the amount of the energy required to break 1 mole of bonds of the same kind.

Non formation of He molecule

  1. When the 2 atoms of He comes together, attractive forces ( 4 in number ) < repulsive forces ( 5 in number )
  2. The energy of the system rises, hence there is no formation of chemical bond.

VBT in terms of orbital overlap

Covalent bond is the bond formation by partial overlap of 2 half-filled A.O. having electrons with the opposite spin. There are 3 types of orbital overlap .

  1. s-s overlap in case of H2 molecule
  2. s-p overlap in case of H-F molecule
  3. p-p overlap in case of F-F molecule

Check out the post on VSEPR theory here

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