Use the Born-Haber cycle
Write the formation reaction for the ionic compound
Break the reaction into steps: atomization, ionization, electron affinity, and bond dissociation if needed
Apply Hess’s law to relate all steps
Solve for the lattice energy from the energy balance
Use the formula: lattice energy = sum of energies of ion formation and atomization steps minus enthalpy of formation, with sign convention adjusted as needed
Alternatively, use the Born-Landé equation if ionic charges, ionic radii, and crystal structure are known
Use the formula: (U = dfrac{N_A M z^+ z^- e^2}{4pi varepsilon_0 r_0}left(1 – dfrac{1}{n}right))
Identify the correct Madelung constant, ionic charges, internuclear distance, and Born exponent
Substitute the values and calculate the lattice energy
Check the sign convention used in your source, since lattice energy may be reported as positive or negative