Question

Polarisation is the distortion of the shape of anion by an adjacently placed cation. Which of the following statements is correct?
A. Maximum polarisation is brought about by cation of high charge.
B. Maximum polarisation is brought about by cation of low charge.
C. A large cation is likely to bring about a large degree of polarisation.
D. A large anion is likely to undergo a large degree of polarisation.

Answer 1

Hint: This question can be answered with the help of Fajan rule. According to Fajan rule, polarisation of anion is influenced by the charge and size of cation. Fajan rule helps to determine the nature of chemical bonds.

Complete Step by Step Solution:
The term polarisation means the bond that is formed between two polar atoms. For example: When a hydrogen atom is associated with a chlorine atom the polar chlorine atom produces the non-polar hydrogen atom into a polar molecule under dipole induced forces. This combination is known as polarisation.

In an isolated anion, the negative charge is evenly distributed but in the presence of cation the negative charge density of anion is deformed. The ability of a cation to deform the electron density of anion towards itself or we can say that the ability of cation to pull the electron density of anion towards itself is known as Polarising power. And the tendency by which the anion undergoes polarisation is known as Polarizability.

Smaller is the size and higher is the charge on cation will have higher polarising power. This means cation can polarise the electron charge density of the nearby anion to a greater extent.For anions with larger size and high charge, polarizability will be high.
Thus, Option (A) is correct.

Note: Polarising power and polarizability determines the nature of bond. Smaller is the size of cation (large polarising power) or larger the size of anion (high polarizability) gives us the greater covalent nature in an ionic bond.