Question

why carbonates of group IA are more thermly stable then those of group II A

Answer 1

Carbonates are decomposed to carbon dioxide and oxide ion on heating. Whereas bicarbonates decomposed into carbonates water and carbon dioxide

The thermal stability of group 1 and 2 increases down the group as the polarizing power decreases

Due to same reason carbonates of group 1 is more stable than group 2

Small and highly charged metal ions possess more polarizing power and hence facilities the decomposition of carbonate into carbon dioxide and oxide ions