6.0 Fajan’s Rule

When two oppositely charged ions approach each other closely, the positively charged cation attracts the outermost electrons of the anion and repel its positively charged nucleus. This results in the distortion or polarization of the anion followed by some sharing of electrons between the two ions, i.e., the bond becomes partly covalent in character.

$1.$ Charge on Either of the ions:

As the charge on the cation increases, its tendency to polarize the anion increases. This brings more and more covalent nature in the electrovalent compound. Whereas with the increasing charge of anion, its ability to get polarized, by the cation, also increases.

For example, in the case of $NaCl$, $MgCl_2$ and $AlCl_3$ the polarization increases, thereby covalent character becomes more and more as the charge on the cation increases.

Similarly, lead forms two chlorides $PbCl_2$ and $PbCl_4$ having charges $+2$ and $+4$ respectively. $PbCl_4$ shows covalent nature.

Similarly among $NaCl$, $Na_2S$, $Na_3P$, the charge of the anions are increasing, therefore the increasing order of covalent character is $$NaCl < N{a_2}S < N{a_3}P$$

Polarisation of the anion increases as the size of the cation decreases i.e., the electrovalent compounds having smaller cations show more of the covalent nature.

For example, in the case of halides of alkaline earth metals, the covalent character decreases as we move down the group. Hence melting point increases in the order of $$BeC{l_2} < MgC{l_2} < CaC{l_2} < SrC{l_2} < BaC{l_2}$$

$3.$ Size of anion:

The larger the size of the anion, more easily it will be polarized by the cation i.e., as the size of the anion increases for a given cation, the covalent character increases. For example, in the case of halides of calcium, the covalent character increases from $F^–$ anion to $I^–$ anion i.e., $$\begin{equation} \begin{aligned} Ca{F_2} < CaC{l_2} < CaB{r_2} < Ca{I_2} \\ {\text{Increasing covalent character}} \\\end{aligned} \end{equation} $$ Similarly, in case of trihalides of aluminium, the covalent character increases with increase in size of halide anion i.e.,

$4.$ Nature of the cation:

Cations with $18$ electrons $(s^2p^6d^{10})$ in outermost shell polarize an anion more strongly than cations of $8$ electrons $(s^2p^6)$ type. The $d$ electrons of the $18$ electron shell screen the nuclear charge of the cation less effectively than the $s$ and $p$ electrons of the $18$-electron shell.

Hence, the $18$-electron cations behave as if they had a greater charge. Copper $(I)$ and Silver $(I)$ halides are more covalent in nature compared with the corresponding sodium and potassium halides although charge on the ions is the same and the sizes of the corresponding ions are similar.

This illustrates the effect of $18$-electron configuration of $Cu^+$ $(3s^2, p^6, d^10)$ and $Ag^+$ $(4s^2, p^6, d^{10})$ ions.

Example $1.$ The decomposition temperature of $Li_2CO_3$ is less than that of $Na_2CO_3$. Explain.

Solution: As $Li^+$ ion is smaller than $Na^+$ ion, thus small cation $(Li^+)$ will favour more covalent character in $Li_2CO_3$ and hence it has lower decomposition temperature than that of $Na_2CO_3$.