The forces of attraction existing among the molecules of a substance (gaseous, liquid or solid) are called intermolecular forces. Attractive intermolecular forces are known as van der Waals forces. Greater the intermolecular forces, higher is the melting and boiling point. Attractive intermolecular forces are known as van der Waals' forces. Different types of intermolecular forces are given below.
London Forces or Dispersion Forces
This force of attraction was proposed by the German physicist F. London, and for this reason, force of attraction between two temporary dipoles is known as London forces and is also called dispersion forces.
We know that in non-polar molecules, the dipole moment is zero because of symmetrical distribution of their electronic charge cloud. But it may be possible at any point of time, the electron cloud of the molecule may be distorted so that an instantaneous dipole or momentary dipole is produced in which one part of the molecule is slightly more negative than the other part. This momentary dipole induces dipoles in the neighbouring molecules.
Thus, the force of attraction exists between them and are exactly same as between permanent dipoles. This force of attraction is known as London forces or Dispersion forces. These forces are always attractive and the interaction energy is inversely proportional to the sixth power of the distance between two interacting particles (i.e. 1/r6 where r is the distance between two particles).
These forces are important only at short distances (~500 pm) and their magnitude depends on the polarizability of the particle. The strength of these forces increases with the increase in molecular mass, molecular size, number of electrons and surface area of the molecule.
Dipole - Induced Dipole Forces
This type of attractive forces operate between the polar molecules and nonpolar molecules. Permanent dipole of the polar molecule induces dipole on the electrically neutral molecule (i.e. nonpolar molecules) by deforming its electronic cloud.
Thus, an induced dipole is developed in the other molecule. In this case also interaction energy is proportional to 1/r6 where r is the distance between two molecules. Induced dipole moment depends upon the dipole moment present in the permanent dipole and the polarisability of the electrically neutral molecule.
Dipole - Dipole Forces
Dipole-dipole forces act between molecules possessing the permanent dipole (i.e. polar molecules). The polar molecules interact with neighbouring molecules. This interaction is stronger than the London forces but is weaker than ion-ion interaction because only partial charges are involved. The attractive force decreases with the increase of distance between the dipoles.
The interaction energy is also inversely proportional to distance between polar molecules. Dipole-dipole interaction energy between stationary polar molecules (as in solids) is proportional to 1/r3 and that between rotating polar molecules is proportional to 1/r6, where r is the distance between polar molecules. Besides dipole-dipole interaction, polar molecules can interact by London forces also. Thus cumulative effect is that the total of intermolecular forces in polar molecules increase.
Comparison Chart
Below is a comparative infographic summarizing all three intermolecular forces side by side for quick revision.
