This rule was formulated by Kazimierz Fajan in 1923 and it is used in predicting whether a chemical bond is expected to be predominantly ionic or covalent. This depends on the relative charges and sizes of the cations and anions.

If two oppositely charged ion are brought together,the nature of bond between them depends upon the effect of one ion on the cation. Fajan’s rule is based on covalent nature of an ionic compound;for any ionic compound, there’s always a degree of covalency.

Fajans rule for predicting whether a bond is predominantly covalent or ionic goes thus;

Covalent                                       Ionic

Small cation(<=1.0n°)             Large cation(>1.0n°)

Large anion                             small anion

High charges                         Low charges

The bond in a compound A+B- may be considered to be 100% ionic. It will always have some degree of covalent character.

Note: when two oppositely charged ion A+ and B- approach each other the cation attract electrons in  the outermost shell of the anion but repel the positively charged nucleus. This results in a distortion,deformation or polarization of the anions. The tendency of the anion to become polarized by the cation is known as its polarizability.

The polarizing power and polarizability that enhances the formation of covalent bond is favored by the following;

  1. Small cation: the high polarizing power stems from the greater concentration of positive charge on a small area. This explains why LiBr is more covalent than KBr.
  2. High charges: as the charges on an ion increases, the electrostatic att of the cation for the outer electron of the anion increases resulting in the degree of covalent bond formation increasing.
  3. Large onions: high polarizability stems from the larger side where the outer electrons can be easily distorted by the cation. This explains why for common halides, iodides

For the period, cation charges increases the size of the atom decreases .

For the group, anion size increases

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