10.1.3—Thermal decomposition trends

Heating Group 2 carbonates gives a metal oxide and carbon dioxide; heating nitrates gives a metal oxide, nitrogen dioxide and oxygen. Thermal stability generally increases down the group.

Larger M²⁺ ions have lower charge density and polarise the anion less, so the carbonate or nitrate ion is less distorted and decomposes less readily.

MgCO₃ decomposes more readily than BaCO₃. The products follow MgCO₃ → MgO + CO₂ and 2M(NO₃)₂ → 2MO + 4NO₂ + O₂.

Do not explain the trend only by “stronger bonds down the group”. The key comparison is cation polarising power and anion stability.