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Which of the following alkali metal is most covalent:
1. LiCl
2. NaCl
3. KCl
4. CsCl

1 Answer

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Correct Answer - Option 1 : LiCl

Fajan's Rule:

  • Coulombic attraction between the cations and anions in certain cases leads to the deformation of the ions.
  • This deformation caused by one molecule to the other is called polarisation.
  • The extent to which the molecule is able to polarise the other is called its polarisation power.
  • The extent to which a molecule can get polarised is called its polarisability.
  • The rise in deformity of ions may give rise to increased electron density between the ions and this leads to a considerable amount of covalent bonding.
  • Polarisability decreases along with a period and increases along with a group.
  • Low positive charge and large size of cation and a small charge on anion and small size of anion favor the formation of ionic compounds.
  • A high positive charge and small size of cation and a high charge on anion and large size of anion favor the formation of covalent compounds.

Explanation:

  • The alkali metal chlorides LiCl, NaCl, KCl, and CsCl have the cations in oxidation state +1.
  • When we move down the group, the size of the cations increases whereas the size of the anions remains the same.
  • The size of Li+ is the least and thus the polarisability is highest.
  • As Li+ has high polarisation power, it polarises Cl- and forms the covalent compound LiCl.
  • On the other hand, the other chlorides like NaCl, CsCl, KCl are ionic in nature.
  • As we move down the group, the size of the cations increases and the polarisation power decreases. This resulted in a decrease of the covalent character down the group.
  • Hence, LiCl is the most covalent and CsCl is the least.

Hence, LiCl is the most covalent.

  • Li behaves differently than other alkali metals due to its high polarisation power which makes its compounds covalent.
  • Li is not able to form peroxides because of the same reason. 

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