AN DER WAALS-LONDON FORCES

Another explanation must therefore be found. Now we know that besides forces of an electrical character there are others which act between atoms. Even the noble gases attract one another, although they are non-polar and have spherically symmetrical electronic structures. These so-called van der Waals forces cannot be explained on the basis of classical mechanics and London was the first to find an explanation of them with the help of wave mechanics. He reached the conclusion that two particles at a distance r have a potential energy which is inversely proportional to the sixth power of the distance, and directly proportional to the square of the polarizability, and to a quantity p which is a function of the ionization and excitation energies of the atom, so that 

This formula explains why the boiling points of shielded halides increase from the fluorides to the iodides see Section 41). As the polarizability increases in this order more strongly than the third power of the distance, the energy required to separate the molecules in the liquid increases from the fluorides to the iodides. The increase 

Can the van der Waals forces lead to the formation of chemical compounds The two Cl atoms in a molecule Cl2 will certainly be attracted to one another by a van der Waals-London force but it can be very simply shown that these forces alone can never lead to the formation of a chemical compound. The peculiar property of these forces is that they always strive to bring like atoms together. Suppose we have a dilute gas mixture containing N atoms of A and N atoms of B further let us imagine that the van der Waals-London forces lead to the formation of molecules AB from these atoms the energy of the system is then 

If we were to form 0-5N molecules of A% and 0 5N molecules B2 then the energy would be 

The energy of the combination AB is thus higher than that of 0-5A2 + 0 5jB2. This result can be proved equally well for other combinations. Van der Waals-London forces, therefore, although very important in the mutual cohesion of atoms and molecules, can never lead to an actual chemical combination.  

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