Fajans' rules

In inorganic chemistry, Fajans' Rules, formulated by Kazimierz Fajans in 1923, 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:

Ionic	Covalent
Low positive charge	High positive charge
Large cation	Small cation
Small anion	Large anion

The magnitude of polarization or increased covalent character depends upon a number of factors.  These factors are,

(1) Small size of cation: 

Smaller size of cation greater is its polarizing power i.e. greater will be the covalent nature of the bond.

(2) Large size of anion: 

Larger the size of anion greater is its polarizing power i.e. greater will be the covalent nature of the bond.

(3) Large charge on either of the two ions: 

As the charge on the ion increases, the electrostatic attraction of the cation for the outer electrons of the anion also increases with the result its ability for forming the covalent bond increases.

(4) Electronic configuration of the cation: 

 For the two ions of the same size and charge, one with a pseudo noble gas configuration (i.e. 18 electrons in the outermost shell) will be more polarizing than a cation with noble gas configuration (i.e., 8 electron in outer most shell).

Thus sodium chloride (with a low positive charge (+1), a fairly large cation (~1 Å) and relatively small anion (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)

What are the effects of Polarization?

1. Due to higher lattice energies of Bromides and iodides they have higher stability than expected from theoretical calculations. This results in formation of a polar covalent Bond. This covalent bond is more stable than ionic bond.

2. With increase in degree of polarization, the solubility of ionic compounds in polar solvents decreases.

3. With increase in degree of polarization, the hardness of ionic compounds decreases.

Fajan's rule states that covalent character is more likley when:

 the  positive ion has an outer electron configuration that is not a noble-gas  configuration.

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