Which one of the following decomposes easily on heating ?

$$\begin{gathered} \mathrm{Stability} \mathrm{of} {\mathrm{I}}_{\mathrm{A}} \mathrm{group} \mathrm{carbonates}: \\ {\mathrm{Li}}_2{\mathrm{CO}}_3<{\mathrm{Na}}_2{\mathrm{CO}}_3<{\mathrm{K}}_2{\mathrm{CO}}_3<{\mathrm{Rb}}_2{\mathrm{CO}}_3<{\mathrm{Cs}}_2{\mathrm{CO}}_3 \end{gathered}$$

As the size of cation increases the stability of carbonate increases,
As the larger size cation will stabilise the larger size anion and hence Cs₂CO₃ is more stable where as Li₂CO₃ is highly unstable and it easily undergoes decomposition and gives CO₂ and more stable Li₂O
 

Lithium being very small polarises the large size ${\mathrm{CO}}_3^{-2}$  ion leading to formation of more stable Li₂O and CO₂