Number of molecules from the following which are exceptions to octet rule is _________.
CO₂, NO₂, H₂ SO₄, BF₃, CH₄, SiF₄, ClO₂, PCl₅, BeF₂, C₂ H₆, CHCl₃, CBr₄

Solution

Chemistry ma’am · 3 steps

Check each central atom: short of 8, odd total, or more than 8 means it breaks the octet rule.

1. Three ways to break. A molecule is an exception if the central atom has fewer than 8 electrons (incomplete octet), more than 8 (expanded octet), or if the whole molecule has an odd number of valence electrons.

   Type	Molecules	Why	Count
   Incomplete octet	BF₃, BeF₂	6 electrons on B, 4 on Be	2
   Odd electron	NO₂, ClO₂	5 + 12 = 17 and 7 + 12 = 19, both odd	2
   Expanded octet	H₂SO₄, PCl₅	12 electrons on S, 10 on P	2
   Obeys octet	CO₂, CH₄, SiF₄, C₂H₆, CHCl₃, CBr₄	every C or Si forms 4 bonds, exactly 8 electrons	0

2. Watch SiF4. Silicon is in period 3 and can expand its octet, but in SiF₄ it forms only 4 bonds. It has exactly 8 electrons, so it follows the rule.
3. So. The exceptions are NO₂, H₂SO₄, BF₃, ClO₂, PCl₅ and BeF₂, giving 6.

The common slip: 7. Counting SiF₄ as an expanded octet just because Si is in period 3 gives 7. Si has only 4 bonds there, so exactly 8 electrons.

The idea to keep. Being able to expand is not the same as expanding. Count the electrons actually around the central atom, and separately check whether the whole molecule's total is odd.

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