Definition:

Fajans’ Rule predicts whether a bond will be more ionic or covalent based on polarisation of ions.

More polarisation ⇒ more covalent character

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What is Polarisation?

• Cation distorts the electron cloud of the anion
• This distortion = polarisation

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Factors Affecting Polarisation

1. Size of Cation

• Smaller cation → more polarising power → more covalent

✔️ Example:

• Li⁺ > Na⁺ > K⁺ (in polarising power)

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2. Charge on Cation

• Higher charge → more polarisation

✔️ Example:

• Al³⁺ > Mg²⁺ > Na⁺

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3. Size of Anion

• Larger anion → more easily polarised → more covalent

✔️ Example:

• I⁻ > Br⁻ > Cl⁻ > F⁻

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4. Charge on Anion

• Higher negative charge → more polarisation

✔️ Example:

• N³⁻ > O²⁻ > F⁻

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5. Electronic Configuration (Important Exception Concept)

• Cations with pseudo noble gas configuration (d¹⁰) show more polarisation than noble gas config

✔️ Example:

• Cu⁺ (d¹⁰) > Na⁺ (noble gas)

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✔️ 1. Compare Covalent Character:

• LiCl > NaCl > KCl (LiCl most covalent)

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✔️ 2. Melting Point Trend:

• More covalent → lower melting point

✔️ Example:

• AlCl₃ (covalent, low MP)
• NaCl (ionic, high MP)

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✔️ 3. Solubility:

• Ionic → soluble in water
• Covalent → soluble in organic solvents

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