Intermolecular forces

Forces of attraction and repulsion between interacting atoms/molecules other than electrostatic or covalent bonds are called inter molecular forces.

Attractive inter molecular forces: Known as van der Waal’s forces


1. Dispersion forces/ London forces:

  • Force of attraction between temporary dipoles.
  • Always attractive.
  • Significant only at short distances between interacting particles
  • Magnitude depends on polarizability of the particle.
    E α 1/r⁶
    E = interacting energy.
    r = distance between two particles.

2. Dipole – dipole forces:

  • Force of attraction between permanent dipoles.
  • Stronger than London forces, weaker than ion-ion interaction.
    E α 1/r³ —- For stationary polar molecules.
    E α 1/r⁶ —- For rotating polar molecules.

3. Dipole induced dipole forces:

  • Acts between a permanent dipole and a non-polar molecule.
  • The Permanent dipole of the polar molecule induces dipole on the electrically neutral molecule by deforming its electronic cloud. The non-polar molecule acquires charge.
  • Force depends on strength of permanent dipole and polarizability of non-polar molecule.
    E α 1/r⁶

4. Hydrogen bonding:

  • It is a type of dipole-dipole force.
  • Occurs in molecules containing N – H, O – H and H – F bonds (also found in H – Cl bond).
  • Strength of the hydrogen bond is depends on the coulombic interaction between the lone-pair electrons of the electro-negative atom of one molecule and the hydrogen atom of another molecule.

Repulsive inter molecular forces:

  • Repulsion between the electron clouds and that between the nuclei of two molecules comes into play when particles come in close vicinity of each other.
  • Magnitude of the repulsion rises as the distance separating the molecules decreases.
  • In liquids and solids the atoms and molecules are very close to each other. Hence the repulsive forces start acting on them. So they are hard to compress.