Intermolecular forces

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

1. Attractive inter molecular forces:

Known as van der Waal’s forces

Types:

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.

2. 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.