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The difference in thermal stability of $\ce{CaCO3}$ and $\ce{MgCO3}$ can be explained using Fajans' rules, but why not using the lattice energy method?

Why is the thermal stability of $\ce{CaCO3}$ higher even though the lattice energy decreases down a group (as the size of cations increases)?

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  • $\begingroup$ Smaller magnesium ion destabilises the carbonate anion more. Electrostatic and chemical stabilities go against each other here, with the latter having the upper hand. $\endgroup$
    – Poutnik
    Aug 25 at 8:03
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The so called "lattice energy" method doesn't work for all compounds, same is the case with Fajan's rule. These are theories which have been developed to explain the characteristics and make it somewhat believable to the general audience but we cannot compare all compounds using one theory or the other because each theory has its own drawbacks.

Lattice energy is directly proportional to the mod of product of charges on individual ions, which is same in this case, further it is inversely proportional to the radius of the resulting compound, and you're right that $\ce{CaCO_3}$ has lower lattice energy implying that it has lower ionic character, which further implies that it should have lesser thermal stability but here's the drawback. Lattice energy isn't the dominating factor when it comes to ionic character.

Diamond has very high lattice energy, you can't say it is ionic, can you?

Fajan's rule along with size compatibility and polarization decides the ionic character of a compound. By Fajan's rule we have: covalent character directly proportional to the extent of polarization, implying that the smaller cation will cause greater distortion of electron cloud as compared to the larger cation. This means that $\ce{CaCO_3}$ would have higher extent of polarization, i.e. higher covalent character. This would cause slight thermal instability, and $\ce{MgCO_3}$ would be relatively more thermally stable.

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    $\begingroup$ Welcome to Chemistry StackExchange and thanks for posting an answer! You might want to use \ce{} to format chemical formulae. $\endgroup$
    – harry
    Aug 25 at 12:25

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