Alkali metals lattice energy

Figure 4,4 Standard enthalpies of formation (A// and lattice energies (plotted as —t/O for alkali metal halides and hydrides.

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. 

For simple monovalent metals, the pseudopotential interaction between ion cores and electrons is weak, leading to a uniform density for the conduction electrons in the interior, as would obtain if there were no point ions, but rather a uniform positive background. The arrangement of ions is determined by the ion-electron and interionic forces, but the former have no effect if the electrons are uniformly distributed. As the interionic forces are mainly coulombic, it is not surprising that the alkali metals crystallize in a body-centered cubic lattice, which is the lattice with the smallest Madelung energy for a given density.46 Diffraction measurements... 

Incorporation of heavier alkali metals into ate complexes is usually disfavored because of the large lattice energies of the simple salt elimination products MX. However, Stephan et al. have reported that reaction of Cp2ZrHCl with KPH(mes ) in the presence of KH gives the... 

Whether or not enthalpies of sublimation of the alkali metals are approximately the same, lattice energies of a series such as LiCl(s), NaCl(s), KCl(s), RbCl(s), and CsCl(s) will vary approximately with the size of the cation. A smaller cation will produce a more exothermic lattice energy. Thus, the lattice energy for LiCl(s) should be the most exothermic and CsCl(s) the least in this series, with NaCl(s) falling in the middle of the series. 

Fig. 4.4. Surface energy predicted by the jellium model. A test case of the accuracy of the jellium model, conducted by Lang and Kohn (1970). Only for four alkali metals, Na, K, Rb, and Cs, are the predictions of surface energy by the jellium model fair. For most metals, with r, < 2.5 bohr, the surface energy predicted by the jellium model is negative, contradicting seriously with experimental facts. On the other hand, the calculated values of surface energies with crystal lattices agree much better with the values measured experimentally. (After Lang and Kohn, 1970).

TABLE 1.16 Lattice energies of some alkali and alkaline earth metal halides at 0 K... 

Cesium hydroxide. (CAS 21351-79-i (. CsOH, is the strongest of the Use alkali metal hydroxides, as would be expected front its position in the periodic table (francium hydroxide, when prepared, would he expected to be stronger). For the same reason, it has the lowesl lattice energy of the live (135.6 kcal per mole). 

TABLE 3.7. Heats of Formation and Lattice Energies of Alkali Metal Halides ... 

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