Lecture on Drawing Lewis Structures

Introduction

• Purpose: Learn how to draw Lewis structures correctly.
• Importance: Understanding the basics of Lewis structures is crucial for grasping molecular geometries and resonance.

Key Concepts

• Valence Electrons: Electrons in the outermost shell of an atom.
  • Memorize valence electrons for common atoms (e.g., H, C, O, N, F, Cl, S).
  • Groups: 1 valence electron, 2 valence electrons, etc., up to 8.
• Covalent vs. Ionic Compounds:
  • Covalent: Share electrons, consist of nonmetals.
  • Ionic: Transfer electrons, consist of metals and nonmetals.

Steps to Draw Lewis Structures

1. Calculate Total Valence Electrons:

   • Example: CH4
     • Carbon = 4 valence electrons
     • Hydrogen = 1 valence electron each (x4)
     • Total = 8 valence electrons

2. Identify Central Atom:

   • Generally, carbon is the central atom.
   • If no carbon, choose the least electronegative element.
   • Hydrogen can never be the central atom.

3. Draw the Structure:

   • Use single bonds to connect atoms.
   • Check octet rule: central atom should have 8 valence electrons.
   • Example: Methane (CH4) has 4 single bonds.

4. Check Structure:

   • Ensure total valence electrons match calculated amount.

Octet Rule

• Central atoms prefer to have 8 valence electrons.
• Apply the rule to confirm structure correctness.

Lone Pairs

• Electrons not involved in bonds but staying in pairs on an atom.
• Typically present on halogens, oxygen, sulfur, etc.

Types of Bonds

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

Examples & Practice

1. Phosphorus Trichloride (PCl3):

   • Calculate total valence electrons: P = 5, Cl = 7 (x3), Total = 26.
   • Phosphorus is the central atom, surrounded by Cl.
   • Check octet rule and electron count.

2. Expanded Octet:

   • Occurs in atoms from the third row and below (e.g., S, P).
   • Xenon Tetrafluoride (XeF4) as an example with 36 valence electrons, Xe as central atom.

Exceptions to Octet Rule

1. Incomplete Octet:

   • Less than 8 valence electrons on central atom.
   • Common in elements like B, Be.
   • Example: Boron Trifluoride (BF3).

2. Expanded Octet:

   • More than 8 valence electrons.
   • Involves elements in the third period and beyond.

Lewis Structures of Ions

• Consider charges: subtract electrons for cations and add for anions.
• Example: Ammonium Ion (NH4⁺).
  • Adjust total valence electron count due to charge.
  • Enclose the structure in brackets with charge notation.

Conclusion

• Practice drawing and identifying different Lewis structures.
• Understand the rules and exceptions to become proficient.

Encouragement: Continue practicing and believe in your capability to understand these concepts.