Question

Lithium fluoride has high lattice energy than other alkali metal fluoride then also its melting point is low. why?

Answer 1

The reason of this anomaly has to do with Fajan's rules which explain that an ionic bond can be polarizable and better described as polarized ionic (in other words more covalent) for compounds that have smaller cations and/or larger anions. In addition the greater the charge of either cation or anion leads to a more polarized ionic bond.

The key here is to recognize that covalent compounds tend to have lower melting and boiling points than ionic compounds. Therefore, polarized ionic compounds will also have lower mp's and bp's than more truely ionic compounds.

Based on these rules you will expect the very small lithium cation to be able to polarize the fluoride ion more easily than sodium and because of this its values are lower than the corresponding sodium salt.