Lecture Notes: Drawing Lewis Structures

Introduction

• Focus on how to draw Lewis structures.
• Importance of knowing how to draw correctly for understanding geometries, resonance, etc.
• Foundation in valence electrons is crucial.

Valence Electrons

• Definition: Electrons on the outermost shell of an atom.
• Memorize number of valence electrons per group:
  • Group 1: 1 valence electron
  • Group 2: 2 valence electrons
  • Group 13: 3 valence electrons
  • Group 14: 4 valence electrons
  • Group 15: 5 valence electrons
  • Group 16: 6 valence electrons
  • Group 17: 7 valence electrons
  • Group 18: 8 valence electrons
• Focus on non-metals like Hydrogen, Carbon, Oxygen, Nitrogen, Fluorine, Chlorine, Sulfur.

Covalent vs Ionic Compounds

• Ionic: Metal and non-metal, transfer electrons.
• Covalent: Non-metals, share electrons.

Steps to Draw Lewis Structures

1. Calculate Total Valence Electrons

   • Example: CH₄ (Methane)
   • Carbon: 4 electrons; Hydrogen: 1 electron x 4 = 4 electrons.
   • Total = 8 valence electrons.

2. Identify the Central Atom

   • Typically Carbon if present.
   • If Carbon isn't present, use the least electronegative element.
   • Hydrogen is never the central atom.

3. Draw the Structure

   • Place the central atom and surround it with other atoms.
   • Create bonds using pairs of electrons.
   • Ensure total electron count matches calculated total.
   • Apply the octet rule (8 valence electrons for stability).

Octet Rule

• Central atom prefers 8 valence electrons for stability.
• Example: For Carbon in CH₄, counts to 8 valence electrons.

Lone Pairs and Multiple Bonds

• Lone pairs: Electrons not involved in bonding.
• Single bond: 2 electrons.
• Double bond: 4 electrons.
• Triple bond: 6 electrons.
• Example: CO₂ (Carbon Dioxide): Double bond between Carbon and Oxygen.

Exceptions to Octet Rule

1. Incomplete Octet

   • Applies to atoms with less than 4 valence electrons (e.g., Boron, Beryllium).
   • Example: BF₃ (Boron Trifluoride)

2. Expanded Octet

   • Possible in elements from the third row and below.
   • Example: PF₅ (Phosphorus Pentafluoride)
   • Elements like Phosphorus and Sulfur can expand octets.

Lewis Structures of Ions

• Cations (Positive charge): Subtract electrons.
• Anions (Negative charge): Add electrons.
• Example: NH₄⁺ (Ammonium) and ClO₄⁻ (Perchlorate).
• Structure enclosed in brackets with charge indicated.

Practice

• Emphasize frequent practice.
• Use steps for drawing and reviewing structures.
• Understand the exceptions and rules.
• Keep motivated and believe in your capability to master these concepts.