Question

In which of the following sets, molecules are correctly arranged in the decreasing order of covalent character? I. \( \text{AlCl}_{3} > \text{MgCl}_{2} > \text{NaCl} \) II. \( \text{BeCl}_{2} > \text{MgCl}_{2} > \text{CaCl}_{2} \) III. \( \text{CaI}_{2} > \text{CaBr}_{2} > \text{CaCl}_{2} \)

• A. \( I, II \) only
• B. \( I, II, III \)
• C. \( II, III \) only
• D. \( I, III \) only

Hint:

Fajan’s rule shortcut: “Small + highly charged cation + large anion ⇒ more covalent character.”

Answer 1

The Correct Option is B

Concept: According to Fajan’s rules, covalent character increases when:

• Cation size decreases.

• Cation charge increases.

• Anion size increases.
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Step 1: Check Set I.
\[ \text{Al}^{3+},\ \text{Mg}^{2+},\ \text{Na}^{+} \] Higher charge and smaller size of cation increases polarizing power: \[ \text{AlCl}_3 > \text{MgCl}_2 > \text{NaCl} \] So Set I is correct. ---

Step 2: Check Set II.
Cations have same charge \(+2\), so size dominates: \[ \text{Be}^{2+} < \text{Mg}^{2+} < \text{Ca}^{2+} \] Smaller cation ⇒ more covalent character: \[ \text{BeCl}_2 > \text{MgCl}_2 > \text{CaCl}_2 \] So Set II is correct. ---

Step 3: Check Set III.
Same cation \( \text{Ca}^{2+} \), so anion size dominates: \[ \text{I}^- > \text{Br}^- > \text{Cl}^- \] Larger anion ⇒ more polarizable ⇒ more covalent character: \[ \text{CaI}_2 > \text{CaBr}_2 > \text{CaCl}_2 \] So Set III is correct. ---

Step 4: Conclusion.
All three sets follow Fajan’s rules correctly: \[ \boxed{I,\ II,\ III} \] ---