Lecture: Drawing Lewis Structures for Covalent Compounds

Key Concepts

• Lewis Structures (Electron Dot Diagrams): Representation of molecules showing how valence electrons are arranged among atoms in a molecule.
• Covalent Bonds: Bonds formed between non-metal atoms by sharing valence electrons.
• Types of Covalent Bonds: Single, double, and triple bonds.
• Valence Electrons: Electrons in the outermost energy level of an atom involved in bonding.
• Octet Rule: Atoms tend to form bonds to have 8 valence electrons (except H and He needing 2).

Steps to Draw Lewis Structures

1. Ensure Compound is Covalent:
   • Must involve only non-metal atoms (right side of the periodic table).
2. Count Total Valence Electrons:
   • Add the valence electrons for all atoms in the molecule.
3. Identify the Central Atom:
   • Usually the least electronegative atom or the atom that forms the most bonds.
4. Draw Single Bonds to the Central Atom:
   • Represent shared electron pairs with lines.
5. Distribute Remaining Electrons as Lone Pairs:
   • First complete the octet for terminal atoms.
6. Form Double or Triple Bonds if Necessary:
   • Ensure central and terminal atoms have a complete octet.

Examples

Example 1: H₂O (Water)

• Valence Electrons: H: 1 (x2) + O: 6 → Total = 8 electrons
• Central Atom: Oxygen (forms 2 bonds)
• Structure:
  • O in center with single bonds to H.
  • Distribute lone pairs on oxygen to fulfill octet.

Example 2: CF₄ (Carbon Tetrafluoride)

• Valence Electrons: C: 4 + F: 7 (x4) → Total = 32 electrons
• Central Atom: Carbon (forms 4 bonds)
• Structure:
  • C in center with single bonds to 4 F.
  • Distribute lone pairs on F to fulfill octet.

Example 3: PF₃ (Phosphorus Trifluoride)

• Valence Electrons: P: 5 + F: 7 (x3) → Total = 26 electrons
• Central Atom: Phosphorus (forms 3 bonds)
• Structure:
  • P in center with single bonds to 3 F.
  • Distribute lone pairs on F and remaining pair on P to fulfill octet.

Example 4: SO₂ (Sulfur Dioxide)

• Valence Electrons: S: 6 + O: 6 (x2) → Total = 18 electrons
• Central Atom: Sulfur
• Structure:
  • S in center with single bonds to 2 O.
  • Adjust to form one double bond to fulfill octet.

Example 5: CO₂ (Carbon Dioxide)

• Valence Electrons: C: 4 + O: 6 (x2) → Total = 16 electrons
• Central Atom: Carbon
• Structure:
  • C in center with double bonds to 2 O.
  • Distribute lone pairs on O to fulfill octet.

Important Tips

• Always verify the total count of valence electrons.
• Form double or triple bonds only if necessary to meet the octet rule.
• Practice with different molecules to become proficient.

Conclusion

• Follow the systematic steps to draw accurate Lewis structures.
• Focus on achieving the octet rule for stability.
• Active practice with varied examples is essential for mastery.