Group 2 Compounds

Group 2 compounds — particularly the oxides, hydroxides, carbonates, and nitrates — display systematic trends that are directly linked to the size and charge density of the metal cation. Understanding these trends is essential for Edexcel A-Level questions on thermal stability, acid-base behaviour, and real-world applications.

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Thermal Decomposition of Carbonates

Group 2 carbonates decompose on heating to form the metal oxide and carbon dioxide:

MCO₃(s) → MO(s) + CO₂(g)

The key trend is that thermal stability increases down the group. MgCO₃ decomposes at roughly 350 °C, while BaCO₃ requires temperatures above 1300 °C.

Why Does Stability Increase?

The explanation lies in the polarising power of the metal cation. Smaller cations (like Mg²⁺) have a high charge density, meaning their positive charge is concentrated in a small volume. This high charge density distorts (polarises) the electron cloud of the large carbonate ion (CO₃²⁻), weakening the C–O bonds within it and making decomposition easier.

As you descend the group, the cations become larger, their charge density decreases, and they polarise the carbonate ion less effectively. The C–O bonds remain stronger, and a higher temperature is needed to decompose the carbonate.