Alkali Halogenide Gas Molecules

It might be thought that this treatment would provide a poor approximation because of the neglect of polarization of each of the two ions in the electric field of the other.36 However, there is reason to think that the neglect of polarization does not introduce great error. First, the effect of multipole polarization as well as of the partial covalent character of the bonds is taken into account in the treatment of the crystals by the evaluation of the Bom exponent n from the observed compressibility and of the repulsion factor from the observed interionic distance. Second, in the gas molecule, in which there is dipole polarization mainly of the anion, its effect in causing increased attraction of the ions may be largely neutralized by the increased repulsion caused 

34 These values should be decreased by a few percent to take account of the structures involving a bond formed with a pair of r electrons of the h-1 genide ion and a w orbital of the alkali ion. 

During the last few years the application of the techniques of microwave spectroscopy to these gas molecules has provided much precise information about their properties.28 It has been found87 that the observed values of the internuclear distances can be closely approximated by use of an equation analogous to Equation 13-12. The potential energy of the molecule, analogous to 13-10 for the crystal, is 

By differentiating V with respect to r and equating the derivative to zero, the equilibrium internuclear distance rfl is found to be 

These values of n+ and n may be used to predict values of the equilibrium interionic distance for the five molecules for which experimental values have not yet been obtained. The predicted values are given in parentheses. 

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The alkali halogenide gas molecules MX present a still more extreme case, the bonds being essentially ionic with only a small amount of covalent character. For cesium chloride, involving the most electropositive of the metals and one of the most electronegative of the nonmetals, the electron affinity of the nonmetal (86 kcal/mole) is about as great as the ionization potential of the metal (89 kcal/mole), so that at large intemuclear distances the ionic structure Cs+Cl is about as... 

The values of the electric dipole moments of the alkali halogenide gas molecules are found to be about 80 percent of cr0 for example, for KC1 the value of r0 is 2.671 A and that of n is 10.48 D which is 82 percent of er0. The deviation of from er0 is about the magnitude expected to result from the polarization of each ion in the electric field of the other, and because of the uncertainty in the theoretical calculation of the polarization correction it is not possible to say more than that the observed moments agree roughly with those expected for completely ionic structures. 

Work on the direct determination of the electron affinities of halogen atoms was begun by Mayer,7 who, with his students, measured directly the equilibrium constant for dissociation of alkali halogenide gas molecules into ions or of gas halogenide ions into atoms and electrons. Other methods have also been used, especially some involving mass... 

Table 13-11.—Intebionic Distances in Alkali Halogenide Gas Molecules...

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