Why is `Li_(2)CO_(3)` decomposed at a lower temperature whereas `Na_(2)CO_(3)` at higher temperature ?

Explanation

`Li_2CO_3` is less stable to heat as compared to `Na_2CO_3`. This is due to the fact that `Li^(+)` ions being very small in size polarise large cations leading to the formation of more stable `Li_2O and CO_2`. This is why `Li_2CO_3` decomposes at a lower temperature. On the other hand, `Na^(+)` ions being larger are unable to polarise `CO_3^(2-)` ions. Hence, `Na_2CO_3` is stable at higher temperatures.