For an ionic compound the lattice enthalpy is the heat energy released when one mole of solid in its standard state is formed from its ions in the gaseous state. This value cannot be determined directly and so we make use of changes for which data are available and link them together with an enthalpy cycle. This enthalpy cycle is based on the formation of the compound from its elements in their standard states. From John Slater From John Slater :

BORN-HABER CYCLE FOR SODIUM CHLORIDE

Atomisation of sodium Na (s) + 1/2 Cl 2 (g) Na(g) + 1/2 Cl 2 (g) kJ H= +107kJmol -1

Atomisation of chlorine Na (s) + 1/2 Cl 2 (g) Na(g) + 1/2 Cl 2 (g) Na(g) + Cl(g) kJ H = +121kJmol -1

+ First Ionisation of sodium e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- e-e- Na (s) + 1/2 Cl 2 (g) Na(g) + 1/2 Cl 2 (g) Na(g) + Cl(g) kJ Na + (g) + Cl(g) H = +502kJmol -1

First electron affinity of chlorine e-e- - Na (s) + 1/2 Cl 2 (g) Na(g) + 1/2 Cl 2 (g) Na(g) + Cl(g) kJ Na (s) + 1/2 Cl 2 (g) Na(g) + 1/2 Cl 2 (g) Na(g) + Cl(g) kJ Na (s) + 1/2 Cl 2 (g) Na(g) + 1/2 Cl 2 (g) Na(g) + Cl(g) kJ Na + (g) + Cl(g) Na + (g) + Cl - (g ) H = -355kJmol -1

Formation of sodium chloride Na (s) + 1/2 Cl 2 (g) Na(g) + 1/2 Cl 2 (g) Na(g) + Cl(g) kJ Na (s) + 1/2 Cl 2 (g) Na(g) + 1/2 Cl 2 (g) Na(g) + Cl(g) kJ Na (s) + 1/2 Cl 2 (g) Na(g) + 1/2 Cl 2 (g) Na(g) + Cl(g) kJ Na + (g) + Cl(g) Na + (g) + Cl - (g) NaCl(s) H = -411kJmol -1

Na (s) + 1/2 Cl 2 (g) Na(g) + 1/2 Cl 2 (g) Na(g) + Cl(g) kJ Na (s) + 1/2 Cl 2 (g) Na(g) + 1/2 Cl 2 (g) Na(g) + Cl(g) kJ Na (s) + 1/2 Cl 2 (g) Na(g) + 1/2 Cl 2 (g) Na(g) + Cl(g) kJ Na + (g) + Cl(g) Na + (g) + Cl - (g) NaCl(s) Lattice enthalpy for sodium chloride H = -786 kJmol - 1