Question

The correct order of the increasing covalent character of the following is:
(A) $SiCl_4\;<\;AlCl_3\;<\;CaCl_2\;<\;KCl$
(B) $KCl\;<\;CaCl_2\;<\;AlCl_3\;<\;SiCl_4$
(C) $AlCl_3\;<\;CaCl_2\;<\;KCl\;<\;SiCl_4$
(D) none of these

Answer 1

The covalent character in the molecule increases with increase in the charge of the cation.
To arrange the given molecules in the increasing order of the covalent character, arrange these molecules in the increasing order of charge on the cations.

Complete answer:
Fajans rule can be used for the determination of the covalent character of ionic compounds.
The ionic compound has maximum covalent character, if its cation has maximum charge.
The ionic compound has minimum covalent character, if its cation has minimum charge.
In the ionic compounds ${\text{KCl, CaC}}{{\text{l}}_2}{\text{, AlC}}{{\text{l}}_3}{\text{ and SiC}}{{\text{l}}_4}$ the cations are ${{\text{K}}^ + }{\text{, C}}{{\text{a}}^{2 + }}{\text{, A}}{{\text{l}}^{3 + }}{\text{ and S}}{{\text{i}}^{4 + }}$ respectively. The increasing order of the charge on the cations is ${{\text{K}}^ + }{\text{ < C}}{{\text{a}}^{2 + }}{\text{ < A}}{{\text{l}}^{3 + }}{\text{ < S}}{{\text{i}}^{4 + }}$. Hence, the increasing order of the covalent character of the compounds is $KCl\;<\;CaCl_2\;<\;AlCl_3\;<\;SiCl_4$.
Silicon tetrachloride has maximum covalent character,as silicon cation has maximum charge.
Potassium chloride has minimum covalent character, as potassium cation has minimum charge.

Hence, the correct option is the option (B).

Note: According to Fajan’s rule, if an ion has stable electronic structure, and small ionic charge, and the cation has larger size then the anion, then the ionic compound is easily formed.
On the other hand, if a cation has 18-electron structures, and large ionic charge, and the cation has smaller size then the anion, then the covalent compound is easily formed.