Lecture Notes: Lewis Dot Structures

Introduction

• The focus of this lecture is on Lewis dot structures.
• This method is slow but reliable.
• Primarily used to represent covalent bonding.

Purpose of Lewis Dot Structures

• Visual representation of covalent bonds.
• Shows:
  • Connectivity between atoms.
  • Number of electrons shared (bond type: single, double, triple).
  • Number of lone pair electrons.
• Does not show molecular shape or angles.

Electron Sharing in Covalent Bonds

• Single Bond: Pair of electrons shared (e.g., H-H).
• Double Bond: Four electrons shared (e.g., O=O).
• Triple Bond: Six electrons shared (e.g., N≡N).
• Only certain elements (C, N, O, P, S) form multiple bonds.

Exceptions and Considerations

• Octet Rule: Aim for 8 electrons around each atom, with exceptions.
• Exceptions:
  • Hydrogen requires only 2 electrons.
• Lone Pairs: Non-bonding electrons affecting molecular properties.

Steps to Drawing Lewis Dot Structures

1. Determine Valence Electrons: Use only valence electrons to form bonds.
2. Central Atom: Usually the least electronegative.
3. Terminal Atoms: Typically hydrogen or other atoms bonded to the central atom.
4. Distribute Electrons: Ensure each atom satisfies the octet rule.
5. Adjust for Multiple Bonds: If necessary, double or triple bond elements like C, N, O.

Example Structures

Cl2 (Chlorine Molecule)

• Cl-Cl with single bond.
• Each Cl has a complete octet.

H2O (Water)

• O is in the center, H atoms at the ends.
• Total 8 electrons; Oxygen completes its octet.

O2 (Oxygen Molecule)

• Initial proposal with single bond uses too many electrons.
• Correct structure: O=O with a double bond.

NH4⁺ (Ammonium Ion)

• Positive charge reduces valence count by one.
• Nitrogen in center with four hydrogens.
• Full octet for nitrogen, hydrogens satisfied with two electrons.
• Structure enclosed in brackets with charge indicated.

O3 (Ozone)

• Initial proposal uses too many electrons.
• Correct structure involves double bonding one O to the central O.
• Central oxygen has one lone pair; total 18 electrons.

Conclusion

• Practice by constructing structures for different molecules.
• Key focus on electron count and octet rule.
• Remember the limitations and purpose of Lewis dot structures.

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• Homework: Create a Lewis dot structure for O3 and identify the lone pairs on the central oxygen atom.
• Stay safe, Chem 1A!