State $Fajans$ rules for the partial covalent character of ionic bonds.

(N/A) The smaller the size of the cation and the larger the size of the anion,the greater the covalent character of an ionic bond.
The greater the charge on the cation,the greater the covalent character of the ionic bond.
For cations of the same size and charge,the one with electronic configuration $(n-1)d^n s^0$,typical of transition metals,is more polarising than the one with a noble gas configuration,$ns^2 np^6$,typical of alkali and alkaline earth metal cations.
The cation polarises the anion,pulling the electronic charge toward itself and thereby increasing the electronic charge density between the two nuclei. This is precisely what happens in a covalent bond,i.e.,the build-up of electron charge density between the nuclei.
The polarising power of the cation,the polarizability of the anion,and the extent of distortion (polarisation) of the anion are the factors that determine the percent covalent character of the ionic bond.