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Answer: The correct sequence of increasing covalent character is NaCl < LiCl < BeCl₂. According to Fajan’s Rules, covalent character increases with smaller cation size and higher positive charge. Na⁺ is large and +1, Li⁺ is smaller and +1, while Be²⁺ is very small and +2, making BeCl₂ the most covalent. Also, covalent character increases as the electronegativity difference between atoms decreases. Be–Cl has the lowest electronegativity difference, so it is the most covalent. Thus, considering both polarization and electronegativity, NaCl is the most ionic and BeCl₂ the most covalent. Hence, option (D) is correct.
Complete Answer:
Let’s understand why only one option is correct and the others are incorrect:
Option (A): BeCl₂ < LiCl < NaCl — This option is completely incorrect because it wrongly places BeCl₂ as the least covalent compound. According to Fajan’s Rules, Be²⁺ has the highest charge and the smallest size, which gives it the highest polarizing power, and thus BeCl₂ should be the most covalent, not the least. Na⁺ is the largest cation with the lowest polarizing ability, so NaCl is the most ionic, not the most covalent as this option implies. Therefore, this sequence violates both the size/charge logic and electronegativity difference rule, making it entirely incorrect.
Option (B): LiCl < NaCl < BeCl₂ — This option is partially correct because it correctly places BeCl₂ as the most covalent compound. However, it incorrectly suggests that NaCl is more covalent than LiCl, which is false. Lithium ion (Li⁺) is smaller than sodium ion (Na⁺) and therefore has a higher polarizing power, making LiCl more covalent than NaCl. By placing NaCl in the middle, this option violates Fajan’s Rules, which clearly indicate that smaller cations lead to greater covalent character. Hence, although BeCl₂ is correctly positioned, the rest of the sequence is flawed.
Option (C): BeCl₂ < NaCl < LiCl — This option also places BeCl₂ at the beginning, wrongly implying it has the least covalent character, which is scientifically incorrect. As explained earlier, Be²⁺, due to its small size and high charge, leads to maximum polarization and hence forms a highly covalent compound. Also, it incorrectly ranks NaCl as more covalent than LiCl, which again goes against Fajan’s Rules because Na⁺ is larger and less polarizing than Li⁺. Therefore, both the placement of BeCl₂ and the ordering between NaCl and LiCl are wrong in this option.
Option (D): NaCl < LiCl < BeCl₂ — This option correctly follows Fajan’s Rules and the concept of electronegativity difference. Sodium ion (Na⁺) is the largest and has a +1 charge, so it causes very little distortion of the chloride ion (Cl⁻) electron cloud, making NaCl the most ionic and least covalent. Lithium ion (Li⁺) is smaller than Na⁺, so it polarizes Cl⁻ more, giving LiCl more covalent character than NaCl. Beryllium ion (Be²⁺) is very small and has a +2 charge, which causes strong polarization of Cl⁻, resulting in maximum covalent character in BeCl₂. Also, Be–Cl has the smallest electronegativity difference, reinforcing its covalent nature. Hence, this is the correct order of increasing covalent character.
Hence, only option (D) correctly follows the trend based on cationic size, charge, and electronegativity differences, giving the true order of increasing covalent character as NaCl < LiCl < BeCl₂. All other options either misplace BeCl₂ or incorrectly order NaCl and LiCl.
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