Lecture Notes: Section 13.11 - Formal Charge and Resonance

Introduction

• Topic: Formal charge calculation and resonance in chemistry.
• Purpose: Understanding how to calculate formal charge and its significance.

Formal Charge

• Definition: Formal charge is a method to determine the charge distribution in a molecule.
• Formula:
  • Formal charge = Valence electrons - Lone pair electrons - (1/2 * Bonding electrons)
  • Note: (1/2 * Bonding electrons) is equivalent to the number of bonds.
• Calculation: Formal charge is calculated for each atom in a molecule.

Example: Ozone (O3)

• Atoms: Oxygen A, Oxygen B (middle), Oxygen C
• Calculations:
  • Oxygen A: Valence = 6, Lone pairs = 4, Bonds = 2 → Formal charge = 0
  • Oxygen B: Valence = 6, Lone pairs = 2, Bonds = 3 → Formal charge = +1
  • Oxygen C: Valence = 6, Lone pairs = 6, Bonds = 1 → Formal charge = -1
• Conclusion: Sum of formal charges = 0, indicating a neutral molecule.

Importance of Formal Charge

• Neutral Molecules: Sum of formal charges should equal zero.
• Charged Molecules: Sum of formal charges equals the molecule's actual charge.

Resonance

• Concept: Resonance describes equivalent structures of a molecule.
• True Structure: The real structure is an average of resonance structures, known as resonance hybrid.
• Example with Ozone:
  • Structure Options: Double bond either between left & middle or right & middle oxygens.
  • Resonance Hybrid: Average bond order is 1.5 (one and a half bonds).
• Notation: Use a double-headed resonance arrow to indicate equivalent structures.

Experimental Observations

• Bond Observation: Only one type of bond observed experimentally (not two types).
• Averaging Method: Calculate average bond order from resonance structures.

Further Example: Nitrite Ion

• Exercise: Calculate average bond for nitrite ion (NO2−).
• Result: Average bond order is 1.33 for each oxygen.

Minimizing Formal Charge

• Example: Thiocyanate (SCN−)
• Structures: Different valid Lewis structures with varying bond arrangements.
• Objective: Choose structures minimizing formal charge deviation from zero.
• Conclusion: Ignore structures with higher deviation from zero in formal charges.

Key Takeaways

• Minimization: Aim to have formal charges as close to zero as possible.
• Resonance Importance: Recognize equivalent structures and their average.

Conclusion

• Stay aware of the significance of formal charge and resonance in determining molecular structure.
• Safety Reminder: Stay safe when conducting chemistry experiments.