Section 13.12: Exceptions to the Octet Rule

Introduction

• Discussion of exceptions to the octet rule.
• Focus on compounds that are electron deficient (less than 8 electrons around an atom).

Electron Deficient Compounds

• Example: Boron trifluoride (BF₃)

  • 24 valence electrons distributed among 4 atoms.
  • Two Lewis structures:
    • Obeying the octet rule: Double bond between Boron and Fluorine.
    • Breaking the octet rule: Boron with 6 electrons.

• Quiz Question: Calculate the formal charge of fluorine.

  • Formula: Formal charge = Valence - Lone pair electrons - 1/2 bonding electrons
  • Fluorine:
    • Valence: 7
    • Lone pair electrons: 4
    • Bonding electrons: 4
    • Calculation results in a +1 formal charge.

Implications on Formal Charges

• Discrepancy with Fluorine having a +1 formal charge, despite being the most electronegative element.
• When formal charges are calculated:
  • Boron: -1
  • Fluorines: 0
• More stable structure: Electron deficient boron with lower formal charges.

Atoms Allowed to be Electron Deficient

• Beryllium, Boron, and sometimes Aluminum
  • These can be electron deficient (6 or 4 electrons around them) if it results in lower formal charges.
• Hydrogen as an already known electron deficient (wants only 2 electrons).
• Default choice is to complete the octet unless minimizing formal charge requires deficiency.

Hypervalent Compounds

• Question: Can elements have more than 8 electrons?
• Applicable to elements past period 2 (rows 3, 4, etc.)
  • These can be hypervalent (more than 8 electrons).
  • Examples:
    • Phosphorus pentachloride (PCl₅): Phosphorus has 10 electrons.
    • Sulfur hexafluoride (SF₆): Sulfur has 12 electrons.
  • These structures match experimental observations.

Rule of Thumb

• Obey the octet rule unless breaking it minimizes formal charge.
• Hypervalency or electron deficiency should only be invoked to achieve zero or minimized formal charges.

Conclusion

• Always aim to minimize formal charges with the available electrons and atoms, which may require breaking the octet rule.
• Lecture ends with a reminder to "stay safe."