Dear Student,
Fajans rule states that the polarising power is directly proportional to the radius of anion.
Hence bigger the anion, the more covalent it is.
Size of anion in increasing order is: F<Cl < Br < I.
Hence, LiF is the most ionic whereas LiI is most covalent as Iodine is larger than Chlorine and Bromine.
Therefore increasing order of Covalent character is:
LiF<LiCl < LiBr < LiI
Regards
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