Fajan's Rule
Mediumchemistry
Among LiCl, NaCl, KCl, and CsCl, which compound is expected to have the highest degree of covalent character according to Fajan's rules?
Select the correct option:
Solution
Incorrect! Answer:
LiCl
- 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.
- Compare Cation Sizes: Li⁺ (76 pm) < Na⁺ (102 pm) < K⁺ (138 pm) < Cs⁺ (167 pm). All cations have the same charge (+1).
- 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.
- 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.
- 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).
- Conclusion: LiCl has the highest covalent character among the given compounds due to the highest polarising power of Li⁺.
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About This Question
- Subject
- chemistry
- Chapter
- chemical bonding and molecular structure
- Topic
- fajan's rule
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
LiCl
- 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.
- Compare Cation Sizes: Li⁺ (76 pm) < Na⁺ (102 pm) < K⁺ (138 pm) < Cs⁺ (167 pm). All cations have the same charge (+1).
- 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.
- 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.
- 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).
- Conclusion: LiCl has the highest covalent character among the given compounds due to the highest polarising power of Li⁺.
This medium difficulty chemistry question is from the chapter chemical bonding and molecular structure, covering the topic of fajan's rule. It appeared in the 2025 exam.
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