JEE Main · 2021 · 27 Aug 2021 · Shift 2 · s-Block Elements

For the alkali metal fluorides the standard enthalpy of formation is most negative for $\mathrm{LiF}$ and becomes progressively less negative on descending the group, because the lattice enthalpy that dominates the formation energy is largest for the small $\mathrm{L}{\mathrm{i}}^{+}$ and ${\mathrm{F}}^{-}$ pair. That same high lattice enthalpy outweighs the hydration enthalpy of $\mathrm{LiF}$, so its dissolution is strongly disfavoured. Among all the alkali metal halides $\mathrm{LiF}$ is the least soluble in water.