JEE Main 2022 · 29 July · morning · Single correct · +4 / −1
Which of the following pair of molecules contain odd electron molecule and an expanded octet molecule?
- (a) BCl₃ and SF₆
- (b) NO and H₂ SO₄
- (c) SF₆ and H₂ SO₄
- (d) BCl₃ and NO
Answer and solution
Answer (b) NO and H₂ SO₄
Solution
Chemistry ma’am · 3 steps
Count twice: all valence electrons for odd-electron, only the central atom's for expanded octet.
- Odd-electron molecule. Add up every valence electron in the molecule. If the total is odd, one electron is left without a partner.
- Expanded octet. Now count only the electrons around the central atom. More than 8 is an expanded octet, and only atoms from period 3 onwards can manage that.
Molecule Total valence electrons Around the central atom Type BCl₃ 3 + 21 = 24, even 6 on B, fewer than 8 incomplete octet NO 5 + 6 = 11, odd 7 on N odd electron SF₆ 6 + 42 = 48, even 12 on S expanded octet H₂SO₄ 2 + 6 + 24 = 32, even 12 on S expanded octet - So. Only (b) has one of each: NO is the odd-electron molecule and H₂SO₄ has the expanded octet.
The idea to keep. Odd electron depends on the whole molecule's electron count. Expanded octet depends only on the central atom, and that atom must be from period 3 or below.
Answer from the NTA final answer key. Solution by PrepWiser.