Concept: Fajan's Rule

Fajan's rule helps to predict the covalent character in an ionic compound. According to Fajan's rule, the covalent character of an ionic bond increases with:

• Smaller size of the cation
• Larger size of the anion
• Greater charge on the cation
• Greater polarizability of the anion
• Cations with pseudo noble gas configuration (e.g., $C{u}^{+}$, $A{g}^{+}$) have greater polarizing power than cations with noble gas configuration (e.g., $N{a}^{+}$, $K^{+}$) of similar size.

In the given compounds (LiCl, BeCl${}_2$, BCl${}_3$, CCl${}_4$), the anion (Cl$ext{}^{-}$) is the same. We need to compare the covalent character based on the cation's properties.

Why (C) is correct:

The cations are Li$ext{}^{+}$, Be$ext{}^2+$, B$ext{}^3+$, and C$ext{}^4+$. These elements belong to the same period (Period 2) in the periodic table. As we move from left to right across a period, the nuclear charge increases, and the atomic size decreases. Consequently, the ionic size of the cations decreases, and their charge increases.

• Cation charge: Li$ext{}^{+}$ (+1) < Be$ext{}^2+$ (+2) < B$ext{}^3+$ (+3) < C$ext{}^4+$ (+4)
• Cation size: Li$ext{}^{+}$ > Be$ext{}^2+$ > B$ext{}^3+$ > C$ext{}^4+$ (due to increasing effective nuclear charge)

According to Fajan's rule, a smaller cation size and a greater positive charge on the cation lead to higher polarizing power, which in turn increases the covalent character of the bond.

Therefore, the order of covalent character is:

LiCl < BeCl${}_2$ < BCl${}_3$ < CCl${}_4$

Option Analysis:

• A) LiCl < BeCl${}_2$ > BCl${}_3$ > CCl${}_4$: This order is incorrect as it implies a decrease in covalent character after BeCl${}_2$.
• B) LiCl > BeCl${}_2$ < BCl${}_3$ < CCl${}_4$: This order is incorrect as it suggests LiCl has higher covalent character than BeCl${}_2$, and also has an incorrect initial comparison.
• C) LiCl < BeCl${}_2$ < BCl${}_3$ < CCl${}_4$: This order correctly follows Fajan's rule, where covalent character increases with increasing charge and decreasing size of the cation.
• D) LiCl > BeCl${}_2$ > BCl${}_3$ > CCl${}_4$: This order is incorrect as it suggests a decreasing covalent character, which is opposite to what Fajan's rule predicts for these compounds.

Correct Answer: (C)