The Energetics of Ionic Bond Formation

This chapter consists of two sections, one being a general discussion of the stable forms of the elements, whether they are metals or non-metals, and the reasons for the differences. The theory of the metallic bond is introduced, and related to the electrical conduction properties of the elements. The second section is devoted to a detailed description of the energetics of ionic bond formation. A discussion of the transition from ionic to covalent bonding in solids is also included. 

The energetics of ionic bond formation helps explain why many ions tend to have noble-gas electron configurations. For example, sodium readily loses one electron to form Na, which has the same electron configuration as Ne ... 

When a metal salt dissolves in water, the cation and anion are hydrated we discuss the energetics of this process in Section 6.9, but for now, we consider the interactions between the individual ions (freed from their ionic lattice on dissolution) and the solvent molecules. Consider the dissolution of NaCl. Figure 6.5a shows a schematic representation of the formation of the inner hydration shell around Na. The O Na interaction can be described in terms of an ion-dipole interaction, while the solvation of the anion can be described in terms of the formation of hydrogen bonds between Cl and H atoms of surrounding H2O molecules. 

The trend In metallic character is caused by the decrease in ionisation energy as the radius of the atom increases, making it energetically favourable for the formation of ionic bonds. 

As mentioned in Chapter 2.1, the formation of radicals requires the homolytic cleavage of a covalent bond. Energetically such homolytic cleavage is particularly favorable in gas-phase reactions and for liquid-phase reactions in nonpolar solvents. In polar solvents, however, the energy contribution from the solvation of ionic species formed in heterolytic cleavage reverses the picture and heterolytic cleavage becomes more favorable. 

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