Lecture on Drawing Dot Structures and Resonance Structures

Introduction to Dot Structures

• Application of rules to draw dot structures for the nitrate anion.
• Introduction to resonance structures.

Calculating Valence Electrons

• Nitrogen:
  • Group 5 on the periodic table.
  • 5 valence electrons.
• Oxygen:
  • Group 6 on the periodic table.
  • 6 valence electrons each.
  • 3 oxygen atoms: 6 x 3 = 18 valence electrons.
• Total Calculation:
  • 18 (Oxygen) + 5 (Nitrogen) = 23 valence electrons.
  • Add 1 electron for negative charge: 24 valence electrons.

Dot Structure of Nitrate Anion

• Central Atom: Nitrogen (less electronegative than Oxygen)
• Bonds:
  • Nitrogen bonded to 3 Oxygens.
  • Initial valence electrons used in bonds: 6.
  • Remaining valence electrons: 24 - 6 = 18.
• Distributing Electrons:
  • Complete octet for oxygens.
  • 6 additional electrons needed per oxygen.
  • Total 18 electrons used (6x3).

Nitrogen Octet Issue

• Nitrogen only has 6 electrons, needs 8 for octet.
• Solutions:
  • Convert lone pairs from oxygens to form double bonds with nitrogen.

Possible Resonance Structures

1. First Structure:
   • Double bond with top oxygen.
   • Other oxygens single bonded, all electrons accounted for.
2. Second Structure:
   • Double bond with bottom left oxygen.
3. Third Structure:
   • Double bond with bottom right oxygen.

Understanding Resonance

• Resonance structures are attempts at representing the ion's structure.
• Resonance Hybrid:
  • Structures should be thought of as a hybrid with electrons delocalized over all oxygens.
  • Bond characteristics:
    • Stronger than single bond.
    • Weaker than double bond.
  • Evidence: Bond lengths between nitrogen and oxygen are equal.

Conclusion

• The idea of resonance implies delocalization rather than switching between structures.
• The ion is best represented by a resonance hybrid rather than individual resonance structures.