The enthalpy of formation is more negative for

L.E: →LiF>NaF>KF>RbF>CsFAs the size of cation increases attractive forces on the F- ion decreasestherefore the ionic bond strength decreases, the energy released decreases i.e., lattice energy [L.E] decreasesNOTE : The amount of energy released when one mole of ionic compound formed from its oppositely charged gaseous ions is called L.E, here energy released therefore it is denoted with negative signNag++Clg-→NaCls+L.E