The experimental lattice energies are −3217 kJ mol⁻¹ for SrO and −923 kJ mol⁻¹ for NaF. The ionic radii are Sr²⁺ = 132 pm, O²⁻ = 140 pm, Na⁺ = 102 pm, and F⁻ = 133 pm.
(a) Using the radii as center-to-center distances, calculate the theoretical ratio |E(SrO)| / |E(NaF)| based only on ionic charge and separation distance.
(b) Compare your theoretical ratio to the experimental ratio and state whether the simple electrostatic model overestimates or underestimates the relative lattice energies, briefly justifying your answer in one sentence.