Lattice energy group 1 halides

From Fig. 6 it is seen that in the group of the alkali halides the heat of formation always increases from iodine to fluorine and also from lithium to caesium, this latter with the exception, however, of the fluorides. In this group the sequence is just reversed in this case, in view of the small radius of the negative fluorine ion, the decrease of the lattice energy predominates over that of the ionization energy, which decreases much more slowly than proportional to i /r+. 

Figure 9.7 Trends in lattice energy. The lattice energies for many of the alkali-metal halides are shown. Each series of four points represents a given Group 1A(1) cation left side) combining with each of the Group 7A(17) anions (bottom). As ionic radii increase, the electrostatic attractions decrease, so the lattice energies of the compounds decrease as well. Thus, LiF (smallest ions shown) has the largest lattice energy, and Rbl (largest ions) has the smallest.

Effect of ionic size. As we move down a group, ionic radii increase, so the electrostatic energy between cations and anions decreases thus, lattice energies should decrease as well. Figure 9.7 shows that, for the alkali-metal halides, lattice energy decreases down the group whether we hold the cation constant (LiF to Lil) or the anion constant (LiF to RbF). 

Figure 21.2.1. Showing the relationship between lattice energy and melting poinF for the Group 1 halide salts.

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