Chemistry I / Lewis Structures, Formal Charge and Resonance
Practice question · Match the pairs

Match each molecule to the way it departs from a simple octet structure.

  • BF3\mathrm{BF_3}
  • PCl5\mathrm{PCl_5}
  • NO
  • O3\mathrm{O_3}
  • Needs resonance rather than one structure
  • Odd total electron count
  • Expanded octet on the central atom
  • Incomplete octet on the central atom
Hints
  1. Separate too few, too many, unpairable, and unrepresentable.
  2. Ozone obeys the octet rule; its problem is that one drawing cannot capture it.
Show the answer
  • BF3\mathrm{BF_3} → Incomplete octet on the central atom
  • PCl5\mathrm{PCl_5} → Expanded octet on the central atom
  • NO → Odd total electron count
  • O3\mathrm{O_3} → Needs resonance rather than one structure
Why

Boron falls short at six, phosphorus exceeds at ten, NO has an unpairable odd electron, and ozone satisfies the octet rule but requires a resonance hybrid to describe honestly.

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