Fajans’ Rule
Table of Contents
Compounds described as ionic are not always completely ionic. Many show some covalent character. It is because a positive ion can distort, or polarize, the electron cloud of a neighboring negative ion. Fajans’ rule helps predict this covalent character. It does so by considering the cation’s polarizing power and the anion’s polarizability. [1–4]
Postulates of Fajans’ Rule
When two oppositely charged ions, such as X+ and Y–, come close to each other, the cation attracts the outer electrons of the anion. The cation pulls the anion’s electron cloud toward itself more strongly than it affects the anion’s nucleus. As a result, the electron cloud becomes distorted. This uneven attraction and repulsion distorts the anion’s electron cloud, causing polarization. [1,2]
The ability of a cation to distort the electron cloud of an anion is called its polarizing power. The ease with which an anion’s electron cloud can be distorted is called its polarizability.
According to Fajans’ rule, a bond shows more covalent character when the cation has high polarizing power, and the anion has high polarizability. If the polarization is small, the bond remains mostly ionic. If the polarization is large, electron density is shared to a greater extent, and the bond gains more covalent character.
The following summary shows how ion size, charge, and electronic configuration affect the ionic or covalent character of a compound.
| Factor | Favors More Ionic Character | Favors More Covalent Character |
|---|---|---|
| Size of cation | Large | Small |
| Charge on cation | Low | High |
| Size of anion | Small | Large |
| Charge on anion | Low | High |
| Cation electronic configuration | Noble gas | Cations with poorly shielding d-electrons, such as many d-block cations |
Factors Affecting Covalent Character
1. Effect of Cation Size and Charge [1,2]
A cation with a small size and a high positive charge has high charge density and therefore greater polarizing power. It can attract and distort the electron cloud of the anion more strongly, so the bond develops greater covalent character.
Example: BeCl2 is more covalent than BaCl2. Both compounds contain Cl– ions, but Be2+ is much smaller than Ba2+. Because of its smaller size, Be2+ has a higher charge density and greater polarizing power. It polarizes the chloride ion more strongly, making BeCl2 more covalent.
2. Effect of Anion Size and Charge
A larger anion is more easily polarized because its valence electrons are farther from the nucleus and are held less tightly. Anions with higher negative charges are also generally more polarizable. Therefore, compounds containing larger or more highly charged anions usually show greater covalent character.
For halide ions, polarizability increases in the following order:
F– < Cl– < Br– < I⁻
Example: AlI3 is more covalent than AlF3. Both compounds contain Al3+ ions, but I– is much larger than F–. Since the electron cloud of I– is easier to distort, AlI3 has greater covalent character than AlF3.
3. Effect of Cation Electronic Configuration
The electronic configuration of the cation can also affect its polarizing power. Cations with a noble-gas configuration usually provide better shielding and have lower polarizing power. In contrast, cations with d-electrons often shield the nuclear charge less effectively, so they can polarize anions more strongly than expected.
Example: HgCl2 is more covalent than CaCl2. Although both compounds contain 2+ cations and chloride ions, Hg2+ has d-electrons, whereas Ca2+ has a noble-gas configuration. As a result, Hg2+ has greater polarizing power than Ca2+, making HgCl2 more covalent.
Limitations
Although Fajans’ rule is helpful for comparing the covalent character of ionic compounds, it has some limitations. It is qualitative and does not give the exact percentage of ionic or covalent character. The rule works best for simple ionic compounds and may not fully explain bonding in complex compounds, coordination compounds, or compounds where factors such as lattice energy, hydration energy, crystal structure, molecular geometry, and electronegativity difference are important. Therefore, Fajans’ rule should be used mainly as a guide for comparing relative covalent character rather than as a complete explanation of bonding. [1]
