Fajan's Rule
Covalent Character in Ionic Bonds | Chemical Bonding
1. What is Polarization?
No ionic bond is 100% ionic. When a cation approaches an anion, the electron cloud of the anion gets attracted towards the cation and distorted. This deformation of the anion's electron cloud is called Polarization.
- Polarizing Power: The ability of a Cation to distort the anion.
- Polarizability: The tendency of an Anion to get distorted.
2. Fajan's Rules
Kazimierz Fajans formulated rules to predict the extent of covalent character. Greater Polarization $\Rightarrow$ Greater Covalent Character.
Rule 1: Size of Cation (Small is better)
Smaller cations have higher charge density, thus higher polarizing power.
Example: $LiCl > NaCl > KCl$ (Covalent character decreases).
Rule 2: Size of Anion (Large is better)
Larger anions hold their valence electrons loosely (high polarizability).
Example: $LiF < LiCl < LiBr < LiI$ (Covalent character increases).
Rule 3: Charge on Ions (High is better)
Higher charge on cation increases polarizing power; higher charge on anion increases polarizability.
Example: $NaCl < MgCl_2 < AlCl_3$ (Covalent character increases).
Rule 4: Electronic Configuration
Cations with Pseudo-noble gas configuration ($ns^2np^6nd^{10}$, 18 valence $e^-$) have higher polarizing power than those with Noble gas configuration (8 valence $e^-$) due to poor shielding by d-electrons.
Example: $CuCl$ (Covalent) vs $NaCl$ (Ionic). ($Cu^+$ has 18 $e^-$, $Na^+$ has 8 $e^-$).
3. Applications & Consequences
A. Melting Point
Ionic compounds have high MP. As covalent character increases, Melting Point decreases.
Example: $CaF_2$ (High MP) vs $CaI_2$ (Lower MP).
B. Solubility
Ionic compounds are soluble in polar solvents (water). Covalent compounds are soluble in non-polar solvents (benzene).
Example: $AgF$ is soluble in water; $AgI$ is insoluble (highly covalent).
C. Color
Polarization causes distortion of electron clouds, which can lead to color (transition of electrons).
Example: $AgCl$ (White) $\rightarrow$ $AgBr$ (Pale Yellow) $\rightarrow$ $AgI$ (Dark Yellow). (Polarization increases).
Practice Quiz
Test your understanding of Fajan's Rules.
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