What is the lattice energy?

The lattice energy of a crystalline solid is usually defined as the energy of formation of the crystal from infinitely-separated ions, molecules, or atoms, and as such is invariably negative.

When ##NaCl## dissolves in water, the crystals disappear on the macroscopic scale. On the atomic scale, the ##Na^+## and ##Cl^-## ions in the crystal are released into solution. The lattice energy of a salt therefore gives a rough indication of the solubility of the salt in water because it reflects the energy needed to separate the positive and negative ions in a salt.

Opposite of this picture is this example: The lattice energy of ##NaCl## is the energy given off when ##Na^+## and ##Cl^-## ions in the gas phase come together to form the lattice of alternating ##Na^+## and ##Cl^-## ions in the ##NaCl## crystal.