Question

Why is polarizability of an anion directly proportional to its size?

Answer 1

Hint: Let us first understand what polarizability means. The polarizability of an electron cloud is a measure of how easily it can be distorted by an electric field. The electron cloud is usually associated with an atom, molecule, or ion. An electrode or a neighbouring cation or anion could be the source of the electric field. The species to which an electron cloud belongs is said to be polarizable if it is easy to distort.

Complete answer:
Polarizability of an anion is directly proportional to its size because larger anions have larger electron clouds, which are more easily distorted.
The size of an anion is dictated by how far its outermost shell is from the nucleus, as we all know. The atomic size grows as you proceed down the periodic table, because the outermost electrons are added further and further away from the nucleus.
This is also true in terms of ionic size. These outermost electrons are gradually better screened from the nucleus by the core electrons, in addition to being further away from the nucleus. This indicates that the attraction between these outermost electrons and the nucleus is weaker than the attraction between electrons on lower energy levels.
Polarizability refers to an anion's ability to become polarised. So, an anion's electron cloud must be distorted for it to become polarised.
This means that the more easily an anion's electron cloud can be distorted, the more polarizable it is. This is why anion polarizability is proportional to ionic size.

Note:
It must be noted that because the outermost electrons are located further away and are more screened from the nucleus, the larger the anions are, the more loosely held their outermost electrons are. This indicates that electron clouds are easily distorted, which explains why larger anions are more polarizable than smaller anions. When a positively charged cation approaches a larger anion, it may readily distort its electron cloud.