CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
The group-2 oxides MgO, CaO, SrO, and BaO are important refractory materials. Selected crystal radii are listed below.
Ion | Crystal radius / pm
Mg²⁺ | 72
Ca²⁺ | 100
Sr²⁺ | 118
Ba²⁺ | 135
O²⁻ | 140
The decomposition temperature of MgO (conversion to Mg(g) + O(g)) is approximately 3500 K.
Answer the following questions about the series.
(a) Rank the four oxides in order of decreasing magnitude of lattice energy. Clearly indicate ties if any.
(b) Provide a detailed justification for your ranking in part (a), explicitly addressing how both ionic charge (constant across the series) and ionic size influence the trend.
(c) Predict the qualitative trend in decomposition temperatures for CaO, SrO, and BaO relative to the value given for MgO. Explain your reasoning.
(d) Suppose the anion in each lattice were changed from O²⁻ (radius 140 pm) to S²⁻ (radius 184 pm) while keeping the same cations. Discuss how this substitution would modify the lattice-energy trend you derived in part (a) and defend your answer.