Consider the ionic compounds KF, NaCl, NaBr, and LiCl. (a) Use ionic radii (Figure 7.8) to estimate the cation–anion distance for each compound. (b) Based on your answer to part (a), arrange these four compounds in order of decreasing lattice energy. (c) Check your predictions in part (b) with the experimental values of lattice energy from Table 8.2. Are the predictions from ionic radii correct?

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Elin HesselLv2

23 May 2020

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NaCl and KF have the same crystal structure. The only difference between the two is the distance that separates cations and anions. (a) The lattice energies of NaCl and KF are given in Table 8.2. Based on the lattice energies, would you expect the Na – Cl or the K – F distance to be longer? (b) Use the ionic radii given in Figure 7.8 to estimate the Na – Cl and K – F distances.

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In the ionic compounds LiF, NaCl, KBr, and RbI, the measured cation–anion distances are 2.01 Å (Li–F), 2.82 Å (Na–Cl), 3.30 Å (K–Br), and 3.67 Å (Rb–I), respectively. (a) Predict the cation–anion distance using the values of ionic radii given in Figure 7.8. (b) Calculate the difference between the experimentally measured ion–ion distances and the ones predicted from Figure 7.8. Assuming we have an accuracy of 0.04 Å in the measurement, would you say that the two sets of ion–ion distances are the same or not? (c) What estimates of the cation–anion distance would you obtain for these four compounds using bonding atomic radii? Are these estimates as accurate as the estimates using ionic radii?