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Example 7.10: Lattice Energy Comparisons

The precious gem ruby is aluminum oxide, \(\ce{Al2O3}\), containing traces of \(\ce{Cr^3+}\). The compound \(\ce{Al2Se3}\) is used in the fabrication of some semiconductor devices. Which has the larger lattice energy, \(\ce{Al2O3}\) or \(\ce{Al2Se3}\)?

Solution

In these two ionic compounds, the charges Z⁺ and Z^- are the same, so the difference in lattice energy will depend upon \(R_{\mathrm{0}}\). The \(\ce{O^2–}\) ion is smaller than the \(\ce{Se^2–}\) ion. Thus, \(\ce{Al2O3}\) would have a shorter interionic distance than \(\ce{Al2Se3}\), and \(\ce{Al2O3}\) would have the larger lattice energy.