Exceptions to the Octet Rule

Introduction

• The octet rule states that atoms should have eight electrons around them. However, there are exceptions.

Example: BH3

• Drawing the Lewis Structure:
  • Count valence electrons: Boron (3) + Hydrogen (1 each) = 6 total.
  • Hydrogen atoms need at least one bond (2 electrons each).
  • Boron has only 6 electrons (incomplete octet).
• Molecular Geometry: Trigonal planar (120° bond angle).

General Rules

• Second-row elements (C, N, O, F) cannot have more than 8 electrons (no expanded octets).
• Elements in the third row or below (Si, P, S, Cl) can have expanded octets (more than 8 electrons).

Reasoning

• Second-row elements: Only 2s and 2p sublevels (maximum 8 electrons).
• Third-row elements: 3s, 3p, and 3d sublevels (up to 18 electrons).

Example: PCl5 (Expanded Octet)

• Phosphorus: 5 valence electrons, Chlorine: 7 valence electrons each.
  • Total: 40 valence electrons.
  • No lone pairs on phosphorus.
• Phosphorus can have an expanded octet (10 electrons).

Example: ICl5

• Iodine: 7 valence electrons, Chlorine: 7 each.
  • Total: 42 valence electrons.
  • One lone pair on iodine.
• Has an expanded octet (12 electrons).
• Molecular Geometry: Square pyramidal.

Odd Number of Electrons

• Molecules with an odd number of electrons violate the octet rule.

Example: Nitrogen Monoxide (NO)

• Nitrogen: 5 valence electrons, Oxygen: 6 valence electrons.
  • Total: 11 valence electrons.
• Lewis Structure:
  • One possible structure: Nitrogen has 7, Oxygen has 8 (more stable, oxygen is more electronegative).
  • Another structure: Oxygen has 7, Nitrogen has 8 (less stable).

Example: Nitrogen Dioxide (NO2)

• Nitrogen: 5 valence electrons, Oxygen: 6 each.
  • Total: 17 valence electrons.
• Lewis Structure:
  • Consider lone pairs and formal charges.
  • Nitrogen typically near 7 electrons (cannot exceed 8).
  • Two structures with incomplete octets (one for nitrogen, one for oxygen).

Conclusion

• The octet rule is not universal; exceptions include incomplete and expanded octets, and odd number of electrons.

Key Terms

• Octet Rule: Atoms tend to have eight electrons in their valence shell.
• Incomplete Octet: Less than eight electrons around an atom (e.g., BH3).
• Expanded Octet: More than eight electrons around an atom (e.g., PCl5, ICl5).
• Odd Number of Electrons: Molecules that can't have all atoms obey the octet rule (e.g., NO, NO2).