27.1.1—Trends: the trend in the thermal stability

Heating Group 2 nitrates and carbonates gives an oxide plus gaseous products. Thermal stability generally increases down the group as larger M²⁺ ions polarise the large anions less strongly.

A small, high-charge-density cation distorts carbonate or nitrate electron density, weakening the anion and making decomposition easier. Compare ionic radius and charge rather than memorising isolated temperatures.

MgCO₃ decomposes more readily than BaCO₃. The trend applies to both anions but the actual decomposition temperatures differ.

Do not explain stability by saying “all bonds get stronger down the group”; the key is cation polarisation of the anion.