Question 1

What decides the coordination number of an ionic solid?

Accepted Answer

The radius ratio r⁺/r⁻ (cation radius divided by anion radius). As the cation gets larger relative to the anion, more anions can pack around it. Standard bands: 0.225–0.414 gives coordination number 4 (tetrahedral, ZnS-type), 0.414–0.732 gives 6 (octahedral, NaCl-type), and 0.732–1.000 gives 8 (cubic, CsCl-type). Size sets the shape.

Question 2

Why is the lattice energy of MgO much higher than NaCl?

Accepted Answer

Lattice energy follows U ∝ (Z⁺·Z⁻)/(r⁺+r⁻). MgO has 2⁺ and 2⁻ ions, so the charge product Z⁺·Z⁻ is 4 versus 1 for NaCl, and Mg²⁺/O²⁻ are also smaller ions. Both factors raise the lattice energy — NaCl is ≈ 788 kJ/mol while MgO is ≈ 3795 kJ/mol, about five times stronger. That is why MgO melts at 2852 °C versus 801 °C for NaCl.

Question 3

How many formula units are in a NaCl unit cell?

Accepted Answer

Four. In the rock-salt structure the Cl⁻ ions form a face-centred cubic arrangement (4 per cell) and Na⁺ ions occupy all the octahedral holes (also 4 per cell), giving Z = 4 formula units of NaCl per unit cell. This is a frequently tested value.

Question 4

What are the coordination numbers of NaCl, CsCl and ZnS?

Accepted Answer

NaCl is 6:6 (octahedral, radius ratio ≈ 0.52), CsCl is 8:8 (cubic, ratio ≈ 0.93), and ZnS (zinc blende) is 4:4 (tetrahedral, ratio ≈ 0.40). The coordination number of the cation equals that of the anion in each because the formula ratio is 1:1.

Question 5

Why are ionic solids hard, brittle and high-melting?

Accepted Answer

Ions are locked in a rigid three-dimensional lattice held by strong electrostatic forces (the lattice energy), so a lot of energy is needed to break them apart — hence high melting points and hardness. They are brittle because forcing one layer to slide brings like charges next to each other; the sudden repulsion splits the crystal along a clean plane.

Ionic Lattice — Charge Sets Strength, Size Sets Shape

Practice

Learned it? Now prove it under the clock.