Consider two hypothetical solid ionic compounds AB and CD . The radii are r A + = r C + and r B − = r D − . Cation A + has an ns 2 np 6 configuration, while C + has an ns 2 np 6 ( n − 1 ) d 10

Question

Consider two hypothetical solid ionic compounds AB and CD . The radii are r A + ​ = r C + ​ and r B − ​ = r D − ​ . Cation A + has an ns 2 np 6 configuration, while C + has an ns 2 np 6 ( n − 1 ) d 10 configuration. Which of the following is most likely true?

TestHub · Accepted Answer

Concept: Pseudo-noble gas configuration vs. noble gas configuration, polarization, lattice energy.

Solution:

1. Lattice Energy:

$U \propto \frac{∣ Z _{+} Z _{-} ∣}{r _{+} + r _{-}}$

Since radii and charges are equal, their theoretical ionic lattice energies are similar.

2. Polarizing Power: Cation C⁺ (pseudo-noble gas) has poor shielding by d-electrons, resulting in a higher effective nuclear charge at the surface compared to A⁺.

3. Melting Point: The increased polarizing power of C⁺ leads to greater distortion of D⁻, increasing the covalency of CD. Covalent character reduces the melting point of predominantly ionic solids.

Therefore, AB (more ionic) will have a higher melting point.

Final Answer: Option (D)

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