Van der Waal's forces
All molecules (polar and non-polar) exert a weak attraction upon the other, due to the electrostatic attraction of the electrons of one molecule to the nuclei of the other. These forces are known as van der Waals forces. They occur in all kinds of molecular solids including non-polar molecules such as H2, O2, Cl2, CH4, etc. and are present in both solid and liquid states. It has been observed that even non-polar substances such as hydrogen, chlorine, noble gases etc. can be liquefied due to these forces. The existence of weak attractive forces among non-polar molecules in their liquid and solid states was first proposed by J.D. Van der Waal's.
Origin of Van der Waal's forces
In order to understand the origin of these forces, consider two helium atoms very close to each other. The ground state electronic configuration of helium is 1s2 .Each helium atom is uncharged because its electron cloud is symmetrically distributed around the nucleus. In the most favoured arrangement in the helium atom, the two electrons tend to stay as far apart as possible. Thus, they are present diametrically opposite to each other and the centres of positive and negative charges coincide. However, due to the motion of the electrons, we can imagine that for a fraction of time, the electron distribution is not symmetrical i.e. the centres of positive and negative charges do not coincide. As a result of instantaneous asymmetrical distribution of electrons in the atom, there is a small temporary dipole known as instantaneous dipole. This instantaneous asymmetry influences the electron distribution in the neighbouring atom and induces a dipole known as instantaneous induced dipole
Fig: 6.17 - Instantaneous dipole and instantaneous induced dipole in an atom
The attractive forces between the instantaneous dipole and instantaneous induced dipoles are called Van der Waal's forces.
Fig: 6.18 - Dipole - dipole forces in molecular solids
The Van der Waal's forces are responsible for the condensation of noble gases and other gases such as H2, O2, Cl2, CH4, etc., which have no residual bonding capacity. In all such cases, solids and liquids are formed through the Van der Waals' type of interactions. However, these forces are very weak forces.The strength of Van der Waal's forces depends upon:
- Larger size of the interacting molecules. As large molecules have much diffused charged clouds there are great chances of its distortion and producing instantaneous dipoles.
- Increase in number of electrons present in the molecule, leads to greater chances of distortion of electron clouds. For example, the boiling points of monoatomic noble gases increase from helium to radon.
- Molecular structure.
- The lowering of temperature.
- Increase in pressure.