The Born-Haber Cycle

Formation of ionic bonds requires an electron transfer from a metal to a nonmetal atom - a process that is often endothermic. However, when elemental sodium and chlorine react to form a solid sodium chloride crystal, it's a highly exothermic process. But where is the energy coming from?

According to Coulomb’s law, cations and anions are attracted to each other by strong electrostatic forces into a solid array or lattice. The resulting lattice structure is stabilized by decreasing the potential energy, which is released as heat, an exothermic reaction. 

The total energy associated with the formation or breakdown of a crystalline lattice into its gaseous constituents is called lattice energy. 

In a solid ionic compound, a large number of charged particles interact with each other making it difficult to determine the exact value of the lattice energy experimentally. However, it can be calculated using Hess’s law in a hypothetical series of steps called the Born–Haber cycle, which represents the formation of an ionic compound from its constituent elements.

For example, the Born-Haber cycle for sodium chloride formation considers two alternate routes, one direct and another indirect. Th