Factors Affecting Lattice Energy

Once you can calculate lattice energies using Born-Haber cycles, the next step is to understand why lattice energies vary between different ionic compounds and what these variations can tell us about the nature of bonding.

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Factors That Determine Lattice Energy

Lattice energy depends on the electrostatic attraction between the oppositely charged ions in the lattice. Two main factors control the strength of this attraction:

1. Ionic Charge

The greater the charge on the ions, the stronger the electrostatic attraction and the more exothermic (more negative) the lattice energy.

This follows from Coulomb's law. The force of attraction between two charged particles is proportional to the product of their charges:

F ∝ (q⁺ × q⁻) / r²

For example:

• NaCl (Na⁺ and Cl⁻, charges 1+ and 1−): ΔH°latt = −787 kJ mol⁻¹
• MgO (Mg²⁺ and O²⁻, charges 2+ and 2−): ΔH°latt = −3850 kJ mol⁻¹

The product of charges in MgO is 2 × 2 = 4, compared to 1 × 1 = 1 for NaCl. The lattice energy of MgO is roughly 4–5 times larger, consistent with the charge effect (and helped by smaller ionic radii).

2. Ionic Radius