The lattice energy is defined as "a measure of the energy contained in the crystal lattice of a compound, equal to the energy that would be released if the component ions were brought together from infinity", therefore it will be larger in compounds composed by ions with high charge and ions with small radius
.
Thus, the largest is BeO (2+ 2- and both of them are small), next MgS (2+ 2- as the BeO, nonetheless it has bigger ions), afterwards KBr (1+ 1- considering Br- to be smaller than I-)
, then KI [1+ 1 considering I- larger than Br- and K+ smaller than Cs+] and finally CsI.
