Lecture on Lewis Dot Structures and Valence Electrons

Introduction

• Lewis Dot Structures: A tool for making structural predictions or understanding small molecules.
• Valence Electrons: Outer electrons available for bonding.
  • Indicated by the group number in the periodic table.

Periodic Table and Group Numbers

• Group number indicates the number of valence electrons:
  • Group 1: 1 valence electron (e.g., Hydrogen, Lithium)
  • Group 2: 2 valence electrons (e.g., Beryllium)
  • Group 3: 3 valence electrons (e.g., Boron)
  • Group 4: 4 valence electrons (e.g., Carbon)
  • Group 5: 5 valence electrons (e.g., Nitrogen)
• Different naming conventions exist (e.g., Group 1 = 1A or 1).

Covalent Bonds

• Formed by sharing valence electrons.
  • Covalent Electrons: Shared valence electrons.

Drawing Lewis Structures

Boron Atom

• Symbol: B
• Valence Electrons: 3 (Group 3)
• Lewis Structure: Place 3 dots around the symbol B.

Nitrogen Atom

• Symbol: N
• Valence Electrons: 5 (Group 5)
• Lewis Structure: Place 5 dots around the symbol N.
  • Can be drawn in any order around the atom (left, right, top, bottom).

Formaldehyde Molecule (CH2O)

• Atoms Involved:
  • Carbon: Tendency for 4 bonds.
  • Oxygen: Tendency for 2 bonds.
  • Hydrogen: Tendency for 1 bond.
• Steps to Draw:
  1. Draw carbon (C) as central atom due to higher bond capacity.
  2. Place hydrogen atoms (H) and oxygen (O) around carbon.
  3. Draw valence electrons:
     • Hydrogen: 1 electron (Group 1)
     • Oxygen: 6 electrons (Group 6)
     • Carbon: 4 electrons (Group 4)
  4. Form covalent bonds by pairing valence electrons between atoms.

Octet Rule

• Each atom should have a full valence shell:
  • Hydrogen: 2 electrons (1st period)
  • Carbon: 8 electrons
  • Oxygen: 8 electrons
• Validate by circling each atom and counting shared and lone pair electrons.

Conclusion

• The final Lewis Dot Structure for formaldehyde:
  • Ensures all atoms satisfy the octet rule.
  • Depicts correct bonding and lone pairs according to electron tendencies.

Summary

• Lewis Dot Structures provide a visual representation of the valence electrons in atoms and molecules, helping understand bonding and structure.
• The octet rule is a key principle in ensuring stable configurations in molecules like formaldehyde.