Enthalpy of lattice formation of NaCl is -788 kJ/mole This data is supported by

Lattice energy is the amount of energy required to separate constituent gaseous ions from one mole of an ionic crystal.

Lattice energy is the energy released when one mole of ionic crystal is formed from its constituent gaseous ions.

$$\begin{gathered} \mathrm{NaCl}\left(\mathrm{s}\right)\to {\mathrm{Na}}^{+}\left(\mathrm{g}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{g}\right), ∆{\mathrm{H}}_{\mathrm{U}}=+788 \mathrm{kJ}; \\ {\mathrm{Na}}^{+}\left(\mathrm{g}\right)+{\mathrm{Cl}}^{-}\left(\mathrm{g}\right)\to \mathrm{NaCl}\left(\mathrm{s}\right), ∆{\mathrm{H}}_{\mathrm{U}}=-788 \mathrm{kJ}; \end{gathered}$$