DEAR Ravi ,
In case of ions , we associate polarising power of cation and polarisability of anion , which are the power of cation to distort the anions electronic cloud and power of anion to get distorted . The whole process is like that of electrical induction. the cation pulls the electronic cloud of anion to itself so that the elctron pair on anion is shared between two as in the covalent bond. Now greater the charge density on cation greater is its polarising power and correspondingly for anion. so bond become more and more covalent with smaller size of action and larger size of anion.
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