A lattice energy needs a reaction, a sign convention, and a stated amount. Using the lattice-dissociation enthalpy convention for sodium chloride,The reverse lattice-formation enthalpy is negative with the same magnitude when initial and final states match. Chemistry 2e gives for this separation, per mole of sodium chloride formula units. This is about electronvolts per formula unit, not per individual ion. It concerns separated gaseous ions, rather than neutral chemical atoms, molten salt, or hydrated ions in water.[ref]
Chemistry 2e. §7.5, “Ionic Bond Strength and Lattice Energy”, positive convention, reverse convention and NaCl value
Also distinguish an internal energy change from an enthalpy change: . A static point-charge calculation omits motion and thermal contributions. Reversing a reaction reverses its energy change; replacing internal energy with enthalpy additionally changes the defined quantity. The Born-Haber cycle infers a lattice-formation enthalpy by combining other enthalpies, ionization energies, and electron affinities. These imagined stages are a thermodynamic accounting path, not a claim about the actual sequence of a chemical reaction.[ref]
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Lattice energy refers to the amount of energy that is released when ions in a gas phase come together to form a solid ionic compound. It can also be viewed as the energy required to separate one mole of an ionic solid into its gaseous ions. Lattice energy is a measure of the strength of the forces between the ions in an ionic solid.