Theory of the van der Waals bond

THEORY OF THE VAN DER WAALS BOND van der Waals lattice energy 

The origin of the van der Waals forces has been the subject of much work which we shall discuss only briefly here. In 1912 Keesom considered the case of uncharged molecules possessing a permanant dipole / , and attributed the forces to the attraction between these dipoles. As a result of this so-called orientation effect he derived an expression for the interaction energy between atoms or molecules at a distance r apart of the form 

While these two effects are undoubtedly relevant in solids in which molecules are polar, neither can play any part in the numerous structures, such as those of the inert gases, methane, hydrogen, benzene and many others, in which the atoms or molecules possess no permanent dipole moment. Moreover, even when a dipole moment does exist, the forces predicted are far smaller than those experimentally observed. Some other component of interatomic force must therefore exist, and it was London who first ascribed this to a dispersion effect associated with the dynamic polarization of an atom or molecule arising from its zero-point motion. Even if we consider a completely symmetrical system, with no permanent dipole moment, it will still be possible for such a system to possess an instantaneous moment, since at any given instant the electrons will not necessarily be distributed with the high symmetry 8 

The van der Waals lattice energy of a crystal is the energy required to disperse the structure into an assemblage of widely separated molecules and may therefore be directly compared with its heat of sublimation. Some values of this latter quantity are given in table 6.02 and it will be seen that on the whole agreement is satisfactory, especially when it is remembered that we have taken no account of the effect of the repulsive forces which must operate to confer on molecules their characteristic sizes. 

A comparison of the lattice energies given in table 6.02 with those of ionic structures (tables 3.04 and 3.05) reveals that the van der Waals bond has a strength one or more orders of magnitude smaller than that of the ionic link. Its weakness compared with the covalent 

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