Skip to content

Intermolecular forces

What are Intermolecular forces and types of Intermolecular forces :

Ion – Dipole forces : Ion dipole forces are mainly important in aqueous solutions of ionic substances such as NaCl in which dipolar water molecules surrounded the ions.

Water molecules are polar and in them, hydrogen atoms possess partial positive charges and oxygen atoms possess partial negative charges due to electronegativity difference between hydrogen and oxygen atoms. When ionic compounds like NaCl dissolve in water, they dissociate into component ions like 

Na^{+} and cl^{-} . Now the water molecules orient in the presence of ions in such a way that the positive end of the dipole is near an anion and the negative end of the dipole is near a cation.

Dipole – Dipole forces : Neutral but polar molecules experience dipole – dipole forces. These are due to the electrical interactions among dipoles on neighboring molecules. These forces are again attractive between unlike poles and repulsive between like poles and depend on the orientation of the molecules. The net force in a large collection of molecules results from many individual interactions of both types. The forces are generally weak and are significant only when the molecules are in close contact.

London dispersion forces : These forces result from the motion of electrons around atoms. Ex : atoms of helium. The electron distribution around a helium atom is for averaged over time spherically symmetrical. However, at a given instant the electron distribution in an atom may be unsymmetrically giving the atom a short-lived dipole moment. This instantaneous dipole on one atom can affect the electron distribution is neighbouring atoms and induce temporary dipoles in those neighbours. As a result, weak attractive forces develop known as London forces or dispersion forces. London forces are generally small. Their energies are in the range 1 – 10 KJ mol^{-1}.

London disperson forces
Dipole – Induced Dipole forces :  These forces are between polar molecules with permanent dipole moments and the molecules with no permanent dipole moment. Permanent dipole of the polar molecule induces dipole on the electrically neutral molecule by deforming into the electronic cloud.

Magnitude of these forces depends on the magnitude of the dipole moment of permanent dipole and polarization of neutral molecule. This interaction is proportional to \left ( \frac{1}{r^{2}} \right ), where r = distance between molecules.