Order of Covalent bond;
A. KF > KI ; LiF > KF
B. KF < KI ; LiF > KF
C. SnCl₄ > SnCl₂ ; CuCl > NaCl
D. LiF > KF ; CuCl < NaCl
E. KF < KI ; CuCl > NaCl
Choose the correct answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Use Fajans' rules on each half: a small, highly charged cation and a big, soft anion mean more covalent.

1. Fajans' rules. Covalent character grows when the cation polarises the anion more. That happens with a smaller cation, a higher cation charge, a larger anion, or a cation with an 18-electron (pseudo noble gas) outer shell such as Cu⁺.

   Statement	First comparison	Second comparison	Verdict
   A	KF > KI: wrong, I⁻ is larger and more polarisable	LiF > KF: right, Li⁺ is smaller	false
   B	KF < KI: right	LiF > KF: right	true
   C	SnCl₄ > SnCl₂: right, Sn⁴⁺ has the higher charge	CuCl > NaCl: right, Cu⁺ has an 18-electron shell	true
   D	LiF > KF: right	CuCl < NaCl: wrong, it is the other way round	false
   E	KF < KI: right	CuCl > NaCl: right	true

2. Why Cu+ wins. Cu⁺ and Na⁺ are close in size, but the 18 outer electrons of Cu⁺ shield its nucleus poorly. So Cu⁺ distorts Cl⁻ much more, and CuCl is more covalent.
3. So. A statement is correct only if both of its comparisons are right. That leaves B, C and E, which is option (d).

The idea to keep. For covalent character, ask how strongly the cation can distort the anion: small size, high charge or an 18-electron shell on the cation, and a large anion, all push it up.

Answer from the NTA final answer key. Solution by PrepWiser.