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FAJAN'S RULE

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

These factors are --

(i)- Small size of cation: Smaller size of cation greater is its polarizing power i.e. greater will be 

                                        the covalent nature of bond.

(ii)-Larger size of anion: Larger the size of anion greater is its polarizing power i.e. greater will be 

                                          the covalent nature of bond.

(iii)-Large charge on either of 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.

(iv)-Electronic configuration of the cation: For the two ions of the same size and charge, one with

                                                                         a pseudo noble configuration (i.e.18 electrons in                                                                                     outermost shell) will be more polarizing than a cation 

                                                                         with noble gas configuration (i.e., 8 electrons in                                                                                       outer most shell).