Question

Explain alkali metal carbonates do not decompose on heating except ${Li}_2{CO}_3$.

Answer 1

Group 1 alkali metal carbonates are thermally stable except $L{i}_{2}C{O}_3$. $L{i}_{2}C{O}_3$ decomposes on heating and gives $L{i}_{2}O+C{O}_2$. The easy explanation for this behavior may be stated as, the size of alkali metals increase down the group so $Li$ is the smallest and $Cs$ is the biggest in group 1 . So, now $L{i}^{+}$ ion is small and charge density is high (charge/volume) due to this small size . So it has a strong polarising power and strong polarizing effect of small $L{i}^{+}$ can not stabilize large polarizable $C{O}_3{}^{2-}$ ion. That's why it is unstable. Going down the group ,the cation size increase and carbonates are stabilized .