Question 8.1: Without consulting Table 8.1, arrange the ionic compounds Na...

Analyze   From the formulas for three ionic compounds, we must determine their relative lattice energies.
Plan   We need to determine the charges and relative sizes of the ions in the compounds. We then use Equation 8.4 qualitatively to determine the relative energies, knowing that (a) the larger the ionic charges, the greater the energy and (b) the farther apart the ions are, the lower the energy.

E_{el}=\frac{\kappa Q_{1}Q_{2} }{d}                          [8.4]

Solve   NaF consists of Na^{+}   and   F^{-} ions, CsI of Cs^{+} and I^{-} ions, and
CaO of Ca^{2+}   and   O^{2-} ions. Because the product Q_{1}Q_{2} appears in the numerator of Equation 8.4, the lattice energy increases dramatically when the charges increase. Thus, we expect the lattice energy of CaO, which has 2+ and 2- ions, to be the greatest of the three. The ionic charges are the same in NaF and CsI. The difference in their lattice energies thus depends on the difference in the distance between ions in the lattice. Because ionic size increases as we go down a group in the periodic table, we know that Cs^{+} is larger than Na^{+} and I^{-} is larger than F^{-}. Therefore, the distance between Na^{+} and F^{-} ions in NaF is less than the distance between the Cs^{+} and I^{-} ions in CsI. As a result, the lattice energy of NaF should be greater than that of CsI. In order of increasing energy, therefore, we have CsI < NaF < CaO.

Check Table 8.1 confirms our predicted order is correct.