The oxides CaO and SrO each contain doubly charged ions and adopt the same crystal…
CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
The oxides CaO and SrO each contain doubly charged ions and adopt the same crystal structure. The relevant ionic radii (pm) for gaseous ions are listed.
Ion
Radius / pm
Ca²⁺
100
Sr²⁺
118
O²⁻
140
Treat the nearest-neighbour cation–anion separation as the sum of the listed radii and assume the magnitude of the electrostatic attraction between a single cation–anion pair is proportional to rqcqa, where qc and qa are the ionic charges.
(a) Calculate the ratio of the electrostatic attraction magnitude for a CaO ion pair to that for a SrO ion pair, reporting your answer to two significant figures.
(b) Identify which oxide is expected to possess the larger lattice energy and state the reasoning behind your selection.
(c) Relate your conclusion in part (b) to the observed difference in melting points between CaO (melting point ≈ 2570 °C) and SrO (melting point ≈ 2430 °C).