Fajans’ Rule

In 1923, Kazimierz Fajans, with the help of X-ray crystallography, outlined a set of rules to predict the characteristics to determine if a bond is ionic or covalent in nature, known as Fajan’s rule. No bond is either completely ionic or completely covalent, indicating that every ionic bond has partial covalent characteristics and vice-versa, depending on the charge of cation and the relative size of cation and anion.

Polarisation

Polarisation is defined as the phenomenon of distorting negatively charged ion or anion electron clouds by the positively charged ion or cation.

Ionic bonds are formed by the transfer of electrons, due to which cations and anions are formed. The cations, which are positively charged, attract anion electron clouds which are negatively charged. Therefore, the cations distort the shape of anion electron clouds.

Due to this distortion, the negatively charged ion becomes less negative as some part of the electron cloud migrates towards the positive ion. Similarly, the positively charged ion will become less positive because it will acquire more electrons.

As the charge on each ion decreases, the ionic character of the bond decreases and the covalent character increases. Hence, if polarisation is more, then the covalent nature of the bond is more. Fajan’s rule gives the extent of polarisation.

Fajan’s Rule

The Postulates of Fajan’s rules are as follows:

1. Small size of the cation:

The smaller the cation’s size, the higher its electron density and greater is its tendency to distort the electron cloud of anions, resulting in greater polarisation. Thus, smaller is the size of the cation, greater is its polarising power. Hence, compounds with smaller cations have a more covalent character.

2. Large size of the anion:

The bigger the size of the anion, the electron cloud on it will be held less firmly by its nucleus, easily distorting itself towards the cation, resulting in greater polarisation. Thus, a larger anion will have greater polarisability. Hence, the compounds having larger anions are more covalent.

3. Large charge on cation or anion:

The larger the charge on the cation, the greater is its polarising power. Similarly, the larger the charge on the anion, the greater is its tendency to get polarised.

4. Electronic Configuration of cation:

Cations with 18 electrons in their outermost shell or having electronic configuration (n – 1) dn ns0 np0 can cause more distortion than cations with 8 electrons in their outermost shell or having electronic configuration ns2 np6. Because electrons in the d-orbital have a poor shielding effect and do not restrict the positive nucleus from polarised electron clouds, causing more distortion.

Hence, cations with an electronic configuration of (n – 1) dn ns0 np0 are more covalent.

Examples of Fajan’s rule

1. Lithium iodide (LiI) is the most covalent metal halide:

The compounds have a smaller cation, and larger anions have more covalence.

Lithium-ion is the smallest metal cation, and the Iodide ion is the most significant halide ion; hence Lithium iodide is the most covalent metal halide.

2. Cesium fluoride (CsF) is the most iconic metal halide:

The compounds with a larger cation and a smaller anion have more ionic character.

Cesium ion is the largest metal cation, Fluoride ion is the smallest halide ion, Cesium fluoride is the most ionic metal halide.

3. Lithium chloride (LiCl) is more covalent than Sodium chloride (NaCl):

The compounds with a smaller cation and a larger anion have more covalent character.

Since the size of Lithium-ion (Li) is much smaller than the size of Sodium ion (Na), LiCl is more covalent than NaCl.

4. The decreasing order of covalent character of Lithium compounds is as follows: LiI > LiBr > LiCl > LiF:

The compounds with a smaller cation and a larger anion are more covalent.

Since the decreasing order of size of anions is as follows: I- > Br- > Cl- > F-, Lithium halide is more covalent.

5. The size of Calcium ion and Sodium-ion is almost the same, but CaCl2 has a higher covalent character than NaCl:

The larger the charge on cation, the greater is its polarising power, and more will be its covalent character.

Since the charge on Calcium ion (+2) is greater than the charge on Sodium ion (+1), CaCl2 has a higher covalent character than NaCl. 

Application of Fajan’s Rule

1. Solubility of compounds in non-polar solvents.

Generally, ionic compounds are more soluble in polar solvents. But due to polarisation, ionic compounds develop some covalent characteristics. As a result, ionic compounds with significant covalent characteristics get dissolved in non-polar solvents.

Example: The order of solubility of Group 1 chlorides in non-polar solvents are as follows:

LiCl > NaCl > KCl > RbCl > CsCl

2. Melting points of ionic compounds decrease with an increase in polarisation.

Generally, ionic compounds possess higher melting points. But due to polarisation, ionic compounds develop some covalent character. As a result, ionic compounds with significant covalent character have lower melting points.

Conclusion 

Fajan’s Rule is used to predict the characteristics of a bond, whether it is covalent or ionic. For a more covalent nature, the size of the cations should be small, and size of the anions should be large, and the charge on both cations and anions should be greater. For more ionic nature, the size of the cations should be large and size of the anions should be small, and the charge on both cations and anions should be smaller.