Lecture Notes: Bond Polarity and Dipole Moments

Introduction

• The focus of this tutorial is on bond polarity and dipole moments.
• We'll evaluate bond polarity and dipole moments in various bonds and molecules.

Determining Bond Polarity

• Covalent Bonds: Formed between two nonmetals.
• Electronegativity Values: Key to determining polarity.
  • Nonpolar Bond: Electronegativity difference (ΔEN) < 0.5
  • Polar Covalent Bond: ΔEN > 0.5

Examples

Carbon-Oxygen Bond

• Electronegativity Values:
  • Carbon: 2.5
  • Oxygen: 3.5
• Electronegativity Difference: 3.5 - 2.5 = 1.0 (Polar bond)
• Bond Polarity:
  • Oxygen: Partial negative charge
  • Carbon: Partial positive charge
• Dipole Moment: Arrow points from carbon to oxygen.
• A net dipole moment indicates the molecule is polar.

Oxygen-Fluorine Bond

• Electronegativity Values:
  • Oxygen: 3.5
  • Fluorine: 4.0
• Electronegativity Difference: 0.5 (Polar bond)
• Bond Polarity:
  • Fluorine: Partial negative charge
  • Oxygen: Partial positive charge
• Dipole Moment: Arrow points from oxygen to fluorine.

Sulfur-Hydrogen Bond

• Electronegativity Values:
  • Hydrogen: 2.1
  • Sulfur: 2.5
• Electronegativity Difference: 0.4 (Nonpolar bond)
• Bond Polarity:
  • Sulfur: Partial negative charge
  • Hydrogen: Partial positive charge
• Dipole Moment: Arrow points from hydrogen to sulfur.

Molecular Polarity

Water (H2O)

• Structure: Bent or V-shaped.
• Oxygen-Hydrogen Bonds:
  • ΔEN: 1.4 (Polar bond)
  • Oxygen: Partial negative charge
  • Hydrogen: Partial positive charge
• Dipole Moments: Point towards oxygen.
• Polarity: Net dipole moment in positive y-direction, overall polar molecule.

Methane (CH4)

• Structure: Tetrahedral.
• Carbon-Hydrogen Bonds:
  • ΔEN: 0.4 (Nonpolar bond)
  • Carbon: Partial negative charge
  • Hydrogen: Partial positive charge
• Dipole Moments: Cancel out due to symmetry, resulting in a nonpolar molecule.

Carbon Dioxide (CO2)

• Structure: Linear.
• Carbon-Oxygen Bonds:
  • ΔEN: 1.0 (Polar bond)
  • Oxygen: Partial negative charge
  • Carbon: Partial positive charge
• Dipole Moments: Arrows point towards oxygen, opposite and cancel out, resulting in a nonpolar molecule.

Conclusion

• Molecules can have polar bonds but be overall nonpolar due to symmetry and cancellation of dipole moments.
• Understanding bond polarity and molecular polarity is essential for predicting molecular behavior.