Lecture on Lewis Structures

Introduction

• Focus: How to draw Lewis structures correctly.
• Importance: Foundation for understanding geometries, resonance, etc.

Key Concepts

• Valence Electrons: Electrons on the outermost shell of an atom.
• Common Atoms in Lewis Structures:
  • Hydrogen, Carbon, Oxygen, Nitrogen, Fluorine, Chlorine, Sulfur.
  • Typically seen in covalent Lewis structures where they share electrons.

Steps to Draw Lewis Structures

1. Determine Valence Electrons:
   • Count the total valence electrons using the periodic table.
2. Identify Central Atom:
   • Typically the least electronegative atom (except hydrogen).
   • Carbon is almost always central if present.
3. Draw the Structure:
   • Connect atoms using bonds.
   • Ensure the total number of valence electrons is correct.

Example: Methane (CH₄)

• Valence Electrons:
  • Carbon has 4.
  • Hydrogen has 1 (multiplied by 4 due to 4 H atoms) = 4.
  • Total = 8 valence electrons.
• Central Atom: Carbon.
• Structure: Carbon surrounded by 4 hydrogens, each forming a single bond.

Octet Rule

• Central atom should have 8 valence electrons.
• Example: In CH₄, Carbon has 8 valence electrons satisfying the octet rule.

Lone Pairs

• Electrons not participating in bonding, shown as pairs around atoms.
• Common in halogens like Fluorine and Oxygen.

Types of Bonds

• Single Bond: 2 electrons.
• Double Bond: 4 electrons.
• Triple Bond: 6 electrons.

Exceptions to the Octet Rule

1. Incomplete Octet:
   • Common in Boron and Beryllium.
   • Example: BF₃ where Boron has only 6 valence electrons.
2. Expanded Octet:
   • Atoms in the 3rd row and below can have more than 8 valence electrons.
   • Example: XeF₄ where Xenon has 12 valence electrons.

Lewis Structures with Ions

• Cations: Lose an electron, subtract from count.
• Anions: Gain an electron, add to count.
• Place structure in brackets with charge outside.

Practice and Encouragement

• Review steps and practice problems.
• Importance of understanding concepts to avoid repeating content.