The lattice energy of an ionic compound is the energy required to separate one mole of the compound into its constituent ions in the gas phase.
Lattice energy∝(r++r−)1
The lattice energy is inversely proportional to the sum of the ionic radii (r++r−) of the ions involved. When the size of the ions increases, the distance between them also increases, leading to a decrease in the attractive forces between the ions and a decrease in the lattice energy. Therefore, if the size of the ions increases, the lattice energy generally decreases.
So Order : NaF>NaCl>NaBr>NaI