While it is taught that the chemical bonds are divided broadly into ionic and covalent types, however, in reality, most of the bonds are neither purely ionic nor purely covalent. These terms are used to indicate two extreme cases. The ionic bond refers to complete transfer of electrons from one atom to the other, whereas the pure covalent bond involves equal sharing of electrons. Nevertheless, the nature of chemical bonds in most of the compounds are somewhere in between above said two extremes.
In general, it is expected that the ionic compounds are soluble in polar solvents like water and show high melting and boiling points as well as exhibit electrical conductivity in the liquid state. As a rule, these compounds should be sparingly soluble in non polar solvents like benzene, carbon tetrachloride etc. However, it is observed that few of the ionic compounds are also fairly soluble in them and also exhibit somewhat less than anticipated melting points and electrical conductivity in the molten state .
It is also observed that some of the covalent compounds are soluble in water and exhibit electrical conductivity, though not very much, in the molten state.
This clearly shows more or less covalent nature is also possible for the ionic compounds and vice versa. These observations can be explained by the concept of polarization and thus led to the formulation of Fajan's rules which help in deciding the nature of chemical bonds.
The negative charge on an isolated anion is evenly distributed around the nucleus. 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.
Fajan's rules were developed based on the concept: "greater the polarization, greater is the covalent nature". There are two factors which are crucial in deciding the extent of polarization i.e.,
i) Polarizing power of cation: It is directly proportional to the charge density, which in turn is directly related to the charge and inversely related to the size; and
ii) The polarizability of an anion: It is the tendency to undergo polarization and is directly related to size as well as the negative charge. 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.
1) Smaller the cation, greater is the covalent nature. i.e., Larger the cation greater is the ionic nature.
2) Larger the anion, greater is the covalent nature. i.e., Smaller the anion greater is the ionic 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.e. pseudo inert gas configuration.
Author: Aditya vardhan Vutturi