So, lattice enthalpy of formation is defined as the 'standard enthalpy change when one mole of an ionic crystal lattice is formed from its constituent ions in gaseous form'.The strength of this lattice depends on two factors:1) Sizes of the ions: Larger ions lead to a more positive lattice enthalpy of formation value...forming a weaker lattice. This is because, in larger ions charges become further apart so there are weaker attractive forces between ions.2)Charges on the ions: Higher charge = greater electrostatic attraction between oppositely charged ions leading to a more negative lattice enthalpy value...forming a stronger lattice.