Definition: Polarization is the distortion of the electron cloud of an anion by a cation. When distortion is large, the bond acquires covalent character even in compounds traditionally called ionic.
1. Fajan's rules
- Small cation → high polarizing power → more covalent character.
- Large cation → low polarizing power → ionic character.
- Large anion → easily distorted → covalent character.
- High charge on either ion → greater polarization.
- Cations with pseudo noble-gas configuration (e.g., Zn²⁺, Cd²⁺, Hg²⁺, 18-electron) polarize more than same-size noble-gas cations (e.g., Ca²⁺, Mg²⁺).
2. The series FeCl₃ → FeCl₂ → NaCl
Higher charge and smaller size make FeCl₃ largely covalent (low melting, volatile) while NaCl is ionic (high melting, conducts when molten). This progression is exactly Fajan's rules in action.
3. Consequences of polarization
- Lower melting points and volatility (e.g., AgCl, AlCl₃).
- Solubility in non-polar solvents.
- Decreased ionic conduction in molten state.
- Colour in some compounds (e.g., CdS yellow, AgI yellow).
- Distorted crystal structures (e.g., AgCl in rock-salt but with covalent character).
| Factor | Effect on bond |
|---|
| Small cation | Covalent |
| Large cation | Ionic |
| Large anion | Covalent |
| High charge | Covalent |
| Pseudo-noble-gas cation | Covalent |
Note: Exam point: always quote the comparison NaCl vs AlCl₃ or CaF₂ vs CaI₂ to show covalent character.