Salts of oxoacids

Since the alkali metals are highly electropositive, therefore their hydroxides are very strong bases and hence they form salts with all oxoacids ( H₂CO₃, H₃PO₄, H₂SO₄, HNO₃, HNO₂ etc) . They are generally soluble in water and stable towards heat. The carbonates (M₂CO₃)  of alkali metals are remarkably stable upto 1273 K, above which they first melt and then eventually decompose to form oxides. Li₂CO₃ , however is considerably less stable and decomposes readily.

This is presumably due to large size difference between Li⁺ and CO^2-₃  which makes the crystal lattice unstable.

Being strongly basic, alkali metals also form solid bicarbonates. No other metals forms solid bicarbonates though NH₄HCO₃ also exists as a solid. Lithium, however does not form solid bicarbonate though it does exist in solution. All the bicarbonate on gentle heating undergo decomposition to form carbonates with the evolution of CO₂. 

All the carbonates and bicarbonates are soluble in water and their solubilities increase rapidly on descending the group. This is due to the reason that lattice energies decrease more rapidly than their hydration energies on moving down the group.

Which of the following alkali metal halides has the lowest lattice energy?