London Force

London Forces: The intermolecular interactions exist between no polar molecules also. Consider two non polar molecules A and B in which the centre of positive charge coincides with that of the negative charge. ++- A B When the molecules A and B approach each other, there is a distortion in the distribution of the charge resulting in a small and momentary dipole in one molecule. This small dipole can then create another dipole in the second molecule which is called induced dipole. Thus, if -+-+ A B the momentary dipole of molecule A is as shown below;-+ – + A B

Such a distribution of charge leads to mutual attraction between the molecules. These induced dipole – induced dipole interaction are also known as London forces. London forces are the only forces of attraction possible between nonpolar molecules. These interactions are weaker than the dipole-dipole interactions and are of the order of 4 kJ mol-1. These forces vary with the distance between the molecules. If ‘r’ is the distance between the two molecules, then the London forces are proportional to 1/r6.