AL Chemistry

Each of Mg2+ ion and Ca2+ ion carries two positive charges. Since Mg2+ ion is smaller than Ca2+ ion, Mg2+ has a greater charge density (greater charge : volume ratio). Mg2+ ion thus has a greater polarizing power, and Mg2+ ion polarizes CO32- ion to a greater extent. In MgCO3, the more polarized CO32- ion is more unstable which will decompose more readily to less polarizable O2- ion. As a result, MgCO3 is less stable than CaCO3.