U = −(NAMz⁺z⁻e²)/(4πε₀r₀) × (1 − 1/n)
Lattice energy is the energy released when gaseous ions form a crystalline ionic solid. It is a direct measure of the strength of ionic bonds. The Born-Landé equation: U = −(N_A × A × z⁺ × z⁻ × e²)/(4πε₀r₀) × (1 − 1/n), where A = Madelung constant (geometry), z = ion charges, r₀ = nearest-neighbor distance, n = Born exponent (electron repulsion, typically 5-12). Higher charges and smaller ions give larger lattice energies: LiF (1037 kJ/mol) > NaCl (787) > KBr (682). MgO (3850 kJ/mol) has very high lattice energy due to 2+ and 2− charges. Lattice energy determines melting points, solubility (via the Born-Haber cycle), and hardness of ionic compounds. It cannot be measured directly but is calculated from thermodynamic cycles.