Lattice energy alkali metal chlorides

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. 

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 ... 

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 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. 

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