Na₂CO₃ survives heating but Li₂CO₃ decomposes. Explain via diagonal relationship. Step 1: Small, highly polarising Li⁺ distorts the carbonate ion (like Mg²⁺ does). Step 2: Distortion weakens C–O bonds → Li₂CO₃ → Li₂O + CO₂ on heating. Step 3: Large Na⁺ can't polarise CO₃²⁻, so Na₂CO₃ is stable to heat. Step 4: Li behaves like Mg (diagonal), not like Na (its own group)!