Four binary ionic solids are listed below together with the ionic radii of their constituent ions.
Ionic radii (pm):
• Cs⁺ 174, Na⁺ 102, Mg²⁺ 72, Al³⁺ 53
• F⁻ 133, Cl⁻ 181, O²⁻ 140, N³⁻ 146
Solids to be analyzed:
1. CsCl
2. NaCl
3. MgO
4. AlN
Assume each solid adopts its most common room-temperature crystal structure and that the numerical radii above are appropriate for estimating the closest cation–anion separation.
(a) Identify the solid among the four whose cation–anion separation is expected to be the smallest. Explain your reasoning using the numerical data provided.
(b) Rank the four solids from greatest-magnitude lattice energy (most exothermic) to least-magnitude lattice energy (least exothermic).
(c) Justify the ranking in part (b) by discussing how both the magnitude of ionic charge and the inter-ionic distance contribute to the relative lattice energies for these solids.