Li2CO3 decomposes on heating because the Li+ ion exerts a strong polarising action and distorts the electron cloud of the nearby oxygen atom of the large CO32– ion. This results in the weakening of the C—O bond and strengthening of the Li—O bond. This ultimately facilitates the decomposition of Li2CO3 into Li2O and CO2. The lattice energy of Li2O is higher than the lattice energy of Li2CO3. This also favours decomposition of Li2CO3. Na+ ion being bigger in size, the lattice energy of Na2O is less stable than that of Na2CO3. Therefore, Na2CO3 does not decompose on heating.