Lattice Energy

Lattice energy, also called lattice enthalpy, of an ionic solid/crystal is a measure of the strength of bonds in that ionic compound. The lattice energy is the change in energy that occurs when an ionic solid is separated into isolated ions in the gas phase.

Lattice energy is denoted by the symbol U.
Lattice energy depends on two factors:

1. Charges on the ions
2. The distance between the centers of neighboring positive and negative ions. The greater the magnitude of ion charge, the greater is the lattice energy.

Lattice energy works against the solution process, so an ionic solid with relatively large lattice energy is usually insoluble.
Lattice energies cannot be measured directly, but experimental values are obtained from thermodynamic data using the Born-Haber cycle. Theoretical values for lattice energy may be calculated. The ions are treated as point charges, and the electrostatic (columbic) energy E between two ions of opposite charge is calculated: 

Where,
Z⁺ and Z^- are the charges on the positive and negative ions
e is the charge on an electron
r  is the inter-ionic distance
No is the Avogadro constant
A is the Medelung constant

In the case of NaCl, the lattice energy is the energy released by the reaction

This would amount to -787 kJ/mol.