Question

$\text{ZnC}{{\text{O}}_{3}}$ is thermally more stable than $\text{MgC}{{\text{O}}_{3}}$, because
A) $\text{Mg}{{\left( \text{OH} \right)}_{2}}$ exhibits only basic properties while $\text{Zn}{{\left( \text{OH} \right)}_{2}}$ is amphoteric
B) Polarising action of $\text{Z}{{\text{n}}^{+2}}$ with 18-electronic configuration on the anion is larger than that of $\text{M}{{\text{g}}^{+2}}$ with a noble gas-electronic configuration and of same size and charge.
C) Both (a) and (b) are correct
D) None of the above are correct.

Answer 1

Hint:
Both $\text{ZnC}{{\text{O}}_{3}}$ and $\text{MgC}{{\text{O}}_{3}}$ are ionic compounds, in which the ions are bonded to each other in a lattice. The more the ions are equal in size to each other, the higher will be the strength of the lattice and hence the more stable it will be.

Complete step by step solution:
The polarising activity of the cation is dependent upon the size and the charge upon the cation and also on its electronic configuration. Due to the stable electronic configuration and the half-filled d-subshell and p-subshell configuration, the radius of the zinc ions is very small and hence it has high polarising power as compared to the magnesium ion, whose size is quite small in comparison to the carbonate anion.
The magnesium ions have only four electrons in its 1s and 2s orbitals and hence the structure is more compact in case of the zinc, which has the full-filled 3d-subshell which has higher penetrating power than the s-orbitals and hence more compact structure. So, polarising action of $\text{Z}{{\text{n}}^{+2}}$with 18-electronic configuration on the anion is larger than that of $\text{M}{{\text{g}}^{+2}}$with a noble gas-electronic configuration and of same size and charge.

So, the correct answer is option B.

Note:
The polarising power of the cation is equal to the ratio of the charge on the cation to the radius of the cation. So, when the charge of two cations are similar, the radius of the cation becomes the deciding factor. Higher the polarising power of the cation, higher will be the strength of attraction between the cation and the anion and hence stronger will be the bond.