Fajan's Rule

Question

Among LiCl, NaCl, KCl, and CsCl, which compound is expected to have the highest degree of covalent character according to Fajan's rules?

RankGuru · Accepted Answer

1. **Recall Fajan's Rules**: Covalent character in an ionic compound increases when:
   - The cation is small with high charge (high charge density → greater polarising power).
   - The anion is large and easily polarisable.
2. **Compare Cation Sizes**: Li⁺ (76 pm) < Na⁺ (102 pm) < K⁺ (138 pm) < Cs⁺ (167 pm). All cations have the same charge (+1).
3. **Polarising Power**: Li⁺ has the smallest radius and therefore the highest charge density among the group 1 cations. It polarises the electron cloud of Cl⁻ most effectively.
4. **Effect on Bonding**: Greater polarisation of the anion's electron cloud means more sharing of electron density between cation and anion, i.e., more covalent character.
5. **Evidence**: LiCl is soluble in organic solvents (a hallmark of covalent character), while CsCl is not. LiCl also has a lower melting point (605 °C) compared to NaCl (801 °C).
6. **Conclusion**: LiCl has the highest covalent character among the given compounds due to the highest polarising power of Li⁺.

Fajan's Rule

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