Question

Write a short note on Fajan's Rules.

Answer 1

In inorganic chemistry, Fajans' rules, formulated by Kazimierz Fajans in 1923,[1][2][3] are used to predict whether a chemical bond will be covalent or ionic, and depend on the charge on the cation and the relative sizes of the cation and anion. They can be summarized in the following table:

Chart illustrating the relationship between atomic and ionic radius

Ionic Covalent

Low positive charge High positive charge

Large cation Small cation

Small anion Large anion

Thus sodium chloride (with a low positive charge (+1), a fairly large cation (~1 Å) and relatively small anion (0.2 Å) is ionic; but aluminium iodide (AlI3) (with a high positive charge (+3) and a large anion) is covalent.

Polarization will be increased by:

high charge and small size of the cation

Ionic potential Å Z+/r+ (= polarizing power)

High charge and large size of the anion

The polarizability of an anion is related to the deformability of its electron cloud (i.e. its "softness")

An incomplete valence shell electron configuration

Noble gas configuration of the cation produces better shielding and less polarizing power

e.g. Hg2+ (r+ = 102 pm) is more polarizing than Ca2+ (r+ = 100 pm)

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Answer 2

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Here is your answer

» For a given cation, covalent character Increases with increase in size of the anion

» For a given anion, covalent character increases with a decrease in the size of the cation

» Covalent character Increseas with increasing charge either on cation or an anion

» Covalent character is higher for compounds with cations with pseudo noble gas electronic configuration