A student investigates the melting behavior of three crystalline salts – sodium…
CHEMISTRY · Multi-part · AP · Chemistry · Compound Structure and Properties · Compound Structure and Properties
Problem
A student investigates the melting behavior of three crystalline salts – sodium chloride (NaCl), magnesium oxide (MgO), and potassium iodide (KI). Measured ionic radii (in picometres, pm) for the gaseous ions are given.
Ion
Radius / pm
Na⁺
102
Mg²⁺
72
K⁺
138
Cl⁻
181
O²⁻
140
I⁻
220
Assume that, to a first approximation, the centre-to-centre distance between neighbouring cations and anions in each crystal equals the sum of their tabulated radii.
(a) Rank the three salts from highest to lowest melting point.
(b) Justify your ranking by discussing both the magnitude of ionic charge and the relative cation–anion separation in each lattice.
(c) Based on the same reasoning, predict which salt will display the greatest brittleness when a mechanical force is applied. Explain your answer in terms of how the lattice responds to the displacement of ionic layers.