What is Vander-Waal force?

 The nature of Vander-Waal's force:

The matter is formed of atoms and molecules and has the tendency to occupy minimum space at low temperatures through transforming into liquid and solid states. From this we may conclude that the atoms or molecules do not lose their individuality, inspite of this attractive force. We may further conclude that there must be repulsive forces at short intermolecular distances. The simplest model giving rise to such forces was assumed by Vander-Waal , according to which,"The molecules are hard and impenetrable surrounded by attractive field". This model does not explain the observed compressibility of solids and doesn't agree with the modern concept of electronic structure of atom suggested by Bohr and Ruthurford according to which,"The atom consists of a positively charged nucleus surrounded by negatively charged electrons in shells". Hence it may be assumed that forces between atoms and molecules are due to interaction between the electrons and the nuclei of the two systems. Thus we are led to the conclusion that intermolecular forces are essentially electrostatic. However, there are also magnetic forces of attraction which are ordinarily small and may be neglected. These forces of attraction are generally referred to Vander Waal's forces. Except for atoms having closed shells such as Helium(He), Argon(Ar),Neon(Ne) etc., almost all other atoms are electrically unsymmetrical. Each of the such atoms are electrostatically equivalent to a positive and equal negative charge separated by a small distance i.e ., an electric dipole possessing an electric moment equal to the product of the charge and distance between the two charges. The molecules formed of such atoms will also behave as dipoles. The dipole moment of an atom continuously varies in magnitude and direction. Therefore, the average dipole moment averaged over all phases of electronic motion of an atom is always zero. In the case of molecules, the average is not necessarily zero. In the case of molecules, the average dipole moment of a molecules in not necessarily zero. if the average dipole moment of a molecule does not vanish, it will possess a permanent dipole moment. Such molecules are called polar molecules. On the other hand if the average dipole moment of a molecule vanishes; the molecule is said to be non-polar molecule. if the non-polar molecules are placed in an electric field, they have an average dipole moment and the molecule is said to be polarised while the dipole so formed is termed as induced dipole. On the other hand if the polar molecules are placed in an electric field, they possess changed dipole moment. thus the Vander Waal's force between the molecules of a matter(or gas), more conveniently the potential energy of the system, may be due to:

i) Dipole-dipole interaction.

ii)Dipole-induced dipole interaction.

iii)Dispersion interaction.