The lattice energy is the energy released when one mole of the ionic crystal is formed from the gaseous ions.
OR It is also defined as the energy required to break one mole of the solid into isolated ions in the gas phase.
Explanation with examples:
Solids are composed of atoms, ions or molecules. However, many solids of daily importance are ionic in nature. As mentioned earlier these ions exist in a three dimensional array which is called as lattice.
When the appositely charged ions are brought close to each other, energy is released. There happens an energetic stability to the crystal lattice. It is expressed in kJ/mole.
Na+(g) + CI–(g) →NaCl(s) ΔH =- 792 kJ/mole
Or NaCl(s) → Na+(g) + CI–(g) ΔH = + 792 kJ/mole