That positive charge then exerts an attractive force on the electron cloud of the other ion, which has accepted the electrons from the aluminium or other positive ion. Two contrasting examples can illustrate the variation in effects. In the case of aluminium iodide an ionic bond with much covalent character is present. The large charge pulls on the electron cloud of the iodines. Now, if we consider the iodine atom, we see that it is relatively large and thus the outer shell electrons are relatively well shielded from the nuclear charge.
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But in presence of a cation, the electron density is distorted towards it. Thus the negative charge is unevenly distributed over the anion. One end of the anion gets relatively more negative charge than the other end. This condition is referred to as polarization. Note that the cation is also slightly polarized in presence of anion. Due to polarization, the electron density is now spread out in between the anion and cation. This condition is more or less similar to the covalent bonding. There are two factors which are crucial in deciding the extent of polarization i.
Larger anions are distorted easily. We can easily conclude that greater the polarizing power of cation and greater the polarizability of anion, greater is the polarization and hence greater will be the covalent nature. Thus, 1 Smaller the cation, greater is the covalent nature.
It is also observed that the nuclear attraction is not properly shielded in transition metal ions which are also smaller in size. The cumulative effect is higher polarizing power and more covalent nature of compounds containing these ions, most of which have 18 electrons in their outer shell i.
Fajans' Rules for Chemical Bonds