Chapter 1:- Structure and Reactivity 1st year Book
Organic Chemistry
(Page 5)

Van der Waals Forces 

van der Waal forces are a type of weak intermolecular interaction that exists in neutral molecules. The magnitude of these forces is maximum in the case of solids and decreases on passing from solid to liquid and from liquid to gaseous states. The attractive forces which keep the molecules in position in a molecular crystal are called Van der Waal forces. van der Waal forces are arisen by the following molecular interactions :

(i) Dipole-Dipole interactions
(ii) Ion-Dipole interactions
(iii) London forces

(i) Dipole-Dipole interactions :

Polar molecules which have permanent dipoles are attracted mainly due to electrical interactions, known as ‘Dipole-Dipole Interaction‘.


Hydrogen bonding in water, HF, ammonia, lower alcohols, carboxylic acids, biomolecules, etc. due to dipole-dipole interactions. The magnitude of this type of interaction depends upon the dipole moment of the molecule concerned. The greater the dipole moment, the greater the dipole-dipole interaction. Because of these attractive interactions, gaseous dipoles are easily liquefied.

Dipole-dipole interaction in hydrofluoric acid.

(ii) Ion-Dipole interactions:

If an ionic solid is to dissolve in polar solvents like water, ammonia, alcohols, etc. some energy is required to overcome the force of inner-ionic attraction. If water is the solvent, this energy is supplied by the interaction of water molecules with the ions. This is known as ‘Ion-Dipole Interaction‘. In this case, the positive pole of the water molecules migrates towards the anion and the negative pole towards the cation and interacts with them. The ion-dipole interaction results in the release of a considerable amount of energy which becomes available for the separation of the opposite ions of the ionic solids, and thus, the ionic solid may dissolve.

Ion-dipole interaction
(iii) London forces:

In 1930, F. London provided an explanation for the existence of forces of attraction between non-polar molecules, based on quantum mechanics. According to his view, electrons of a neutral molecule keep on oscillating with respect to the nuclei of the atoms. As a result of this, at a given instant, the positive charge may be concentrated in one region and a negative charge in another region of the same molecule. Thus, a non-polar molecule may become self-polarised for very short time intervals. This self-polarised molecule may induce a dipole moment in a neighboring molecule as shown in figure 1.12

original molecule original dipole due to induced dipole electron oscillation, Origin of cohesive forces in non-polar molecules.
Figure 1.12: Origin of cohesive forces in non-polar molecules.
The electrostatic forces of attraction between induced dipoles and the original dipoles due to electron oscillation are known as ‘London forces‘. London forces vary inversely as the seventh power of the distance between them.
van der Waal forces exist in nonpolar molecules such as N2, O2, Cl2, Br2, etc., and even in non-polar monoatomic molecules such as He, Ne, Ar, etc.
Factors influencing the magnitude of van der Waals Forces:
Van der Waal forces increase with the increase in the number of electrons in the molecule. Thus, boiling points of halogens increase on moving down a group, i.e. from F to I.
Molecule: CH4 SiH4 GeH4
No. of e.p.’s:  10 18 36
B.P.(K) : 112 161 183
Similarly, boiling points of noble gases increase in the order: He<Ne<Ar<Kr<Xe<Rn.
Noble gas He Ne Ar Kr Xe Rn
B.P.(K) 4.1 27.0 67.2 121.0 165.0 211.2

The other factor which affects the magnitude of van der Waal forces is molecular size. Larger the molecular size, the stronger the van der Waals forces. e.g., the Higher boiling point of ethane(185 K) from methane(112 K) is due to the bigger size of ethane than that of methane.

  PAGE NO. 5

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