Alkaline earth metals lattice energy

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

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 lattice energies of the alkaline-earth metal oxides have been calculated from equations based on the Born model and containing terms accounting for van der Waals interactions. The results, in kcal mol"1, are MgO, —905 CaO, -815 SrO, -767 and BaO, -736. These lattice energies, when combined with the appropriate thermochemical data, give 149 8kcal... 

TABLE 9.1 Lattice Energies and Melting Points of Some Alkali Metal and Alkaline Earth Metal Halides and Oxides... 

The alkaline earth metals are not as soluble as the alkali metals because of higher lattice energies associated with the cation. For example, potassium sulfate (K2SO,) is soluble in water, but calcium sulfate (CaSO,) is not. 

TABLE 4.5 Lattice Energies of Some Alkali and Alkaline Earth Metal Halides at OK (kJ/mol) ... 

Explain the trend in the lattice energies of the alkaline earth metal oxides. 

As the size of the alkaline earth metal ions increases, so does the distance between the metal cations and oxygen anions. Therefore, the magnitude of the lattice energy decreases accordingly because the potential energy decreases as the distance increases. 

Compounds of the first of the above-mentioned classes are conveniently called "metal-like refractory compounds, in view of their external and, particularly, internal resemblance to metals and intermetallic compounds The chemical bond in the lattices of these compounds, in addition to the s and p electrons of the metallic and nonmetallic components, respectively, is also formed by the electrons of the deeper, incomplete d and / levels of the transition metals, to which belong almost all metallic components of the metal-like refractory compounds. Isolated atoms of metals of the odd subgroup of group II, the alkaline earth metals, do not have any electrons in the d and / shells, but in compounds with nonmetals, energy states corresponding to these shells may occur. 

The general applicability of these ideas was perhaps first pointed out in the classic articles of Flood and Forland (1947) where the stability properties of a wide variety of oxyanions were correlated with the nature of the cation. Since the decomposition reaction in such cases can be viewed as the production of a smaller or more basic fragment ion plus a more molecular species, the reaction obviously will be enhanced by a more acidic or polarizing cation. For example, the stability of the COs " ion with respect to decomposition to oxide and CO2 decreases with a decrease in atomic number and hence size in either the alkali metal or the alkaline earth metal series, whereas the stability progressively decreases even more with the smaller ions Mg +, Mn +, Cd +, Pb +, Ag+, Zn +, and Fe +. Of course, the relative stabilities of the product oxides must also be considered in a quantitative comparison. Electrostatically the process can be viewed as a competition between the formation of the more stable oxide lattice (plus CO2) and the lower lattice energy of the carbonate plus the bond energy associated with CO2 + CO,3. The elimination of oxide plus SO3... 

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