Lecture Notes: Lewis Structures and Exceptions to the Octet Rule

Introduction to the Octet Rule

• Octet Rule: Atoms typically strive to have 8 electrons in their valence shell.
• Exceptions: Some molecules do not follow the octet rule.

Example of Incomplete Octet: BH3

• Lewis Structure of BH3:
  • Valence Electrons: Boron (3) + 3 Hydrogens (1 each) = 6 total.
  • Bonding: Each H forms a bond, using 6 electrons (3 bonds).
  • Result: Boron has less than 8 electrons (incomplete octet).
• Molecular Geometry: Trigonal planar with bond angle of 120°.

Elements with Incomplete and Expanded Octets

• Second Row Elements: Carbon, Nitrogen, Oxygen, Fluorine
  • Cannot have more than 8 electrons (no expanded octet).
  • Can have an incomplete octet.
• Third Row Elements and Below: Silicon, Phosphorus, Sulfur, Chlorine
  • Can have an expanded octet (more than 8 electrons).
  • Reason: Third energy level includes 3s, 3p, and 3d sublevels, allowing for up to 18 electrons.

Example of Expanded Octet: PCl5

• Lewis Structure of PCl5:
  • Valence Electrons: Phosphorus (5) + 5 Chlorines (7 each) = 40 total.
  • Calculation: No lone pairs on phosphorus (40 - 40 = 0).
  • Result: Phosphorus has 10 electrons (5 bonds).

Another Example of Expanded Octet: ICl5

• Lewis Structure of ICl5:
  • Valence Electrons: Iodine (7) + 5 Chlorines (7 each) = 42 total.
  • Calculation of Lone Pairs: 42 - 40 = 2 (1 lone pair on iodine).
  • Molecular Geometry: Square pyramidal; Iodine has 12 electrons around it.

Odd-Electron Molecules

• Example: Nitric Oxide (NO)
  • Valence Electrons: Nitrogen (5) + Oxygen (6) = 11 total.
  • Lewis Structure Considerations:
    • Nitrogen can form 3 bonds; Oxygen can form 2 bonds.
    • Structural options lead to one atom having an incomplete octet.
  • Electronegativity Consideration: Oxygen (more electronegative) will take the extra electron.

Example of Another Odd-Electron Molecule: NO2

• Valence Electrons: Nitrogen (5) + 2 Oxygens (6 each) = 17 total.
• Calculation of Bonds and Lone Pairs:
  • Nitrogen typically forms 3 bonds; Oxygen forms 2 bonds.
  • Lone pairs calculated: 17 - 16 = 1 lone pair.
• Resulting Structure:
  • Nitrogen has 7 electrons; best configuration has one double bond and one single bond with appropriate lone pairs on oxygen.

Conclusion

• General Rule: Odd number of electrons violates the octet rule.
• Key Takeaway: Understand the exceptions to the octet rule for better comprehension of Lewis structures.