Calcium chloride (CaCl₂) and strontium chloride (SrCl₂) are both hygroscopic salts composed of doubly charged cations and singly charged anions. Their measured melting points are 772 °C for CaCl₂ and 875 °C for SrCl₂. Relevant ionic radii (pm) are given below.
Ion
Radius / pm
Ca²⁺
100
Sr²⁺
118
Cl⁻
181
Both compounds adopt closely related lattice geometries.
(a) Using the radius data, predict which chloride has the higher lattice energy. Provide a clear explanation that references Coulombic considerations.
(b) The prediction from part (a) appears inconsistent with the experimental melting-point data. Offer a rational explanation that reconciles the apparent discrepancy, considering at least one structural or entropic factor beyond simple lattice-energy magnitude.
(c) Describe, at the particulate level, how the arrangement of ions in an alkaline-earth chloride crystal leads to its characteristic brittleness when the crystal is struck.