Lecture on Bond Polarity and Molecular Polarity

Introduction to Polarity

• Definition of Polarity:
  • A molecule is neutral if protons and electrons are equal.
  • If electrons are evenly distributed, the molecule is non-polar.
  • Uneven electron distribution results in a polar molecule.
• Example of Water:
  • Water is a polar molecule.
  • Oxygen is more electronegative than hydrogen, pulling electrons and creating a negative side on oxygen.
  • Water has an uneven electron distribution due to this electronegativity difference.

Determining Molecular Polarity

• Factors Influencing Polarity:
  • Molecular Shape: Bent shapes, like water, are non-symmetrical leading to polarity.
  • Bond Polarity: Difference in electronegativity between bonded atoms.
    • Electronegativity increases across the periodic table (left to right, bottom to top).
• Example of Water:
  • Hydrogen's electronegativity: 2.1
  • Oxygen's electronegativity: 3.5
  • Large difference indicates a strong bond polarity.

Examining Other Molecules

• Sulfur Difluoride (SF2):
  • Bent, non-symmetrical structure.
  • Sulfur is partially positive; fluorines are partially negative.
  • Creates a dipole due to large bond polarities.
• Carbon Dioxide (CO2):
  • Linear triatomic, symmetrical shape.
  • Even distribution of oxygens around carbon.
  • Bond polarities cancel out, resulting in no dipole.
• Mixed Atom Example (SCO):
  • Asymmetrical distribution, creating a dipole.
  • Oxygen side is more negative; sulfur side is more positive.

Symmetry and Polarity

• Boron Trifluoride (BF3):
  • Symmetrical, bond polarities cancel out, no dipole.
• Substitution Example (BF2Cl):
  • Substituting chlorine changes bond polarities.
  • Resultant dipole due to non-cancellation.

Flowchart for Determining Molecular Polarity

• Steps to Determine Polarity:
  1. Check for polar bonds (electronegativity difference > 0).
  2. Assess symmetry of outside atoms (shape of the molecule).
  3. Symmetrical distribution cancels bond polarities.
  4. Different outside atoms lead to non-canceling polarities.
• Special Case: Diatomic molecules, e.g., HCl (polar), Cl2 (non-polar).

Tetrahedral Molecule Polarity

• Example: Methane (CH4):
  • Symmetrical, non-polar due to canceling bond polarities.
• Substituting Fluorine in Methane:
  • CH3F: Substitution creates a polar molecule.
  • Further substitutions maintain polarity until all four hydrogens are replaced.
  • Carbon tetrafluoride (CF4) is non-polar due to symmetrical distribution.

Conclusion

• Understanding molecular shape, bond polarity, and symmetry is crucial in determining if a molecule is polar or non-polar.
• Flowcharts can help visualize and simplify the process of analyzing molecular polarity.