Question

Which of the following cannot be classified as an ionic compound based on basic bonding concept?
A.\[{\text{AlC}}{{\text{l}}_3}\]
B.\[{\text{CuC}}{{\text{l}}_3}\]
C.\[{\text{PC}}{{\text{l}}_3}\]
D.\[{\text{KCl}}\]

Answer 1

Hint:On moving down the group, atomic size increases as new shells are added to the atom. The greater the size of the atom lesser will be its electronegativity. The greater the difference in electronegativity, the more ionic the bond is.

Complete step by step answer:
Ionic bond is just a very strong electrostatic force of attraction between cation and anion. Generally a cation is formed by metal because cation is a positively charged species and metals are electropositive in nature, which tends to donate electrons. The anion is formed generally by non metals which have a tendency of gaining electrons are electronegative elements. The ionic bonds are non directional in nature. The favourable condition for the formation of ionic bond is:
Low ionization energy of metal so that it will easily lose electrons to form cation.
High electron affinity of non-metal so as it could accept the electron and form an anion.
Large size cation so that it has less polarizing power.
Small-sized anion so that polarizing could be minimum high lattice energy so as to minimise the energy of crystal for better stability.
Combining cation and anion must be different by equal to or more than \[1.7\] electronegativity.
In \[{\text{KCl}}\] the cation is larger in size and anion is smaller in size, it makes the atoms very suitable for the formation of ionic bonds. Hence \[{\text{KCl}}\] is ionic in nature.
The charge on cation is same in case of \[{\text{AlC}}{{\text{l}}_3}\] , \[{\text{CuC}}{{\text{l}}_3}\] and \[{\text{PC}}{{\text{l}}_3}\] . But the electronegativity difference in \[{\text{AlC}}{{\text{l}}_3}\] and \[{\text{PC}}{{\text{l}}_3}\] is not up to the requirement. Hence they both are not considered as ionic. Electronegativity of aluminium, phosphorus and chlorine is \[1.61,{\text{ }}2.19{\text{ and 3}}{\text{.16}}\]

Thus, the correct options are A and C.

Note:
Fajan’s rule tells us about the percentage covalent character in ionic bonds. It states that the cation having the smaller size and anion will larger size favours the formation of covalent bond. The smaller the cation is the more will be it polarising power due to high charge density. The bigger the anion is the more is its polarizability and it increases the covalent character in an ionic bond.