Lecture on Drawing Lewis Structures

Introduction

• Focus on drawing Lewis structures.
• Importance of understanding the basics of drawing structures before moving on to geometries and resonance.
• Memorizing valence electrons for groups: 1 to 8 electrons.

Common Atoms in Lewis Structures

• Hydrogen
• Carbon
• Oxygen
• Nitrogen
• Fluorine
• Chlorine
• Sulfur
• Use of nonmetals for covalent Lewis structures (sharing electrons).

Steps in Drawing Lewis Structures

1. Calculate Total Valence Electrons:
   • Example: Carbon has 4 valence electrons, Hydrogen has 1.
2. Identify the Central Atom:
   • Usually carbon if present.
   • Least electronegative element if no carbon.
3. Drawing the Structure:
   • Use Lewis dot symbols.
   • Connect with single bonds.
   • Ensure total electrons add up to initial count.

The Octet Rule

• Central atom prefers 8 valence electrons.
• Example: Carbon should have 8 electrons around it.
• Applies to most Lewis structures.

Examples: Drawing Lewis Structures

• Phosphorus Trichloride (PCl3):

  • Central atom: Phosphorus
  • Steps: Calculate valence electrons, draw bonds, check octet rule.

• Carbon Dioxide (CO2):

  • Double bonds for satisfying octet rule.

• Ethanol (C2H5OH):

  • Multiple central atoms (carbon, oxygen).
  • Use given chemical formula to deduce structure.

Exceptions to the Octet Rule

• Incomplete Octet:

  • Less than 8 valence electrons.
  • Common in Boron and Beryllium.

• Expanded Octet:

  • More than 8 valence electrons.
  • Occurs in atoms from the third period and below (e.g., phosphorus, sulfur).

Lewis Structures with Charges (Ions)

• Cations:
  • Subtract electrons from total.
• Anions:
  • Add electrons to total.
• Enclose structures in brackets with charge indicated.

Practice and Encouragement

• Emphasis on practice and understanding of Lewis structures.
• Encouragement to believe in one's capability to master the content.