Alkali chlorides, lattice energies

These trends are apparent In the values of lattice energy that appear in Table Notice, for example, that the lattice energies of the alkali metal chlorides decrease as the size of the cation increases, and the lattice energies of the sodium halides decrease as the size of the anion increases. Notice also that the lattice energy of MgO is almost four times the lattice energy of LiF. Finally, notice that the lattice energy of Fc2 O3, which contains five ions in its chemical formula, is four times as large as that of FeO, which contains only two ions in its chemical formula. 

He assumed that for salts made up of combinations of spherical or tetrahedral ions the lattice energy could be well approximated by assigning to the crystal structure (usually unknown) a Madelung constant equal to that of sodium chloride and estimating the repulsive contribution to the lattice energy by a Bom-Mayer expression similar to that which holds approximately for the alkali halides. These assumptions lead to the following expressions for the lattice energy U ... 

In the sulphides, selenides, tellurides and arsenides, all types of bond, ionic, covalent and metallic occur. The compounds of the alkali metals with sulphur, selenium and tellurium form an ionic lattice with an anti-fluorite structure and the sulphides of the alkaline earth metals form ionic lattices with a sodium chloride structure. If in MgS, GaS, SrS and BaS, the bond is assumed to be entirely ionic, the lattice energies may be calculated from equation 13.18 and from these values the affinity of sulphur for two electrons obtained by the Born-Haber cycle. The values obtained vary from —- 71 to — 80 kcals and if van der Waal s forces are considered, from 83 to -- 102 kcals. 

The radius ratio r+jr for each of the alkali halides is shown in table 3.03. Consideration of these values reveals that CsCl, CsBr and Csl would, indeed, be expected to have the caesium chloride structure, and that the majority of the remaining halides would be expected to show the sodium chloride arrangement. There are, to be sure, a number of halides with r+jr > 0 7 which, nevertheless, have the sodium chloride rather than the caesium chloride structure. Fig. 3.08, however, emphasizes that energetically there is little difference between these two structures when the radius ratio is large, and there are in any case other factors contributing to the lattice energy which we have so far ignored in our discussion. 

The lattice energy of francium chloride, FrCl, has been estimated to be —632 kj/mol. Calculate a value for the radius of the Fr cation. List any assumptions that you make in doing the calculation. Does your result make sense relative to the other alkali metals Briefly explain. 

Consider the lattice energies of the following alkali metal chlorides ... 

The binary alkali metal hydrides, MH (M = Li, Na, K, Rb, Cs) crystallise with the sodium chloride structure and are ionic [68]. Their INS spectra. Fig. 6.23, show peaks related to the density of transverse and longitudinal optic states due to the antiphase motions of the hydride ion and the M" cations in the lattice unit cell [69] ( 4.3). The first overtones were also seen. The acoustic bands, at lower energy transfer, were weaker than the optical bands since the acoustic bands arise from in-phase motions of the hydride ion and the cations the hydrogen mean square displacement is smaller in the acoustic than in the optical modes. 

---