Types of Secondary Valence Lattices

True dispersion forces are present to all appearances in the lattices of inert gases. These substances crystallize in very highly symmetrical arrangement—cubic face-centered— both the lattice constants and the heats of sublimation are easily obtainable experimentally. Eisenschitz and London, by employing the calculation mentioned on page 93 of the quantum mechanical component of the van der Waals forces for the lattice energy of a cubic crystal, which coheres only through such force effects, have derived an equation of the form 

3 Energy of ionization. a = Polarizability of the lattice units, ro = Distance of the lattice units. 

by its aid, we calculate the heats of sublimation of lattices which cohere by virtue of true dispersion forces, we obtain the data given in Table 62, which agree well with the experimental values cited in column 4. Mar-genau has improved this agreement still further by taking into account the interaction of dipole and quadrupole terms of the zero vibration. 

It is interesting that in HCl and HBr, which already possess quite considerable dipoles, there is an appreciable deviation in the sense that the lattice energy calculated on the basis of dispersion force alone is too small, because interactions of a dipole nature intervene. The fact, however, that in these lattices the quantum mechanical forces play a very important part is clear from the data in Table 62 and from the calculations of Born and Kornfeld, who for a long time have tried to adopt a true 

Substance a X 102 in cm Lattice energy calculated in Cal/mol Lattice energy observed in Cal/mol extrapolated to 0 abs. 

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