Molecular Polarity and Electronegativity

Key Concepts

• Molecular Polarity: Determines how molecules interact at a molecular level.
• Electronegativity: Ability of an atom to draw electrons towards its nucleus.
  • More electronegative elements attract electrons more strongly.
  • Electronegativity values differ by element.

Electronegativity Trends

• Increases as you move up and to the right on the periodic table.
• Fluorine: The most electronegative element.
• Halogens (Group 7A): Highly electronegative.
• Noble Gases: Inert, do not engage in reactions, ignored in electronegativity trends.

Bond and Molecular Polarity

• Bond Polarity: Determined by differences in electronegativity.
  • Polar Bonds: Unequal sharing, presence of dipoles (partial charges).
    • More electronegative element gets a δ- (delta negative).
    • Less electronegative element gets a δ+ (delta positive).
  • Nonpolar Bonds: Equal sharing, no dipoles.

Dipoles

• Represented by an arrow pointing towards the more electronegative element.
• A plus sign is on the less electronegative element.
• Dipole moments indicate polar bonds.

Electronegativity Difference and Bond Type

• Nonpolar Bonds: Difference < 0.5.
• Polar Bonds: Difference between 0.5 and 2.0.
• Ionic Bonds: Difference > 2.0.

Molecule Polarity

• Nonpolar Molecules:
  • All nonpolar bonds or symmetrical polar bonds.
  • Symmetry in polar bonds leads to cancellation.
• Polar Molecules:
  • Contain asymmetrical polar bonds.
  • Different atoms on bonds prevent cancellation, resulting in polarity.

Identifying Polarity

• Comparison Examples:
  • Carbon (C) vs. Nitrogen (N): N is more electronegative.
  • Lithium (Li) vs. Chlorine (Cl): Cl is more electronegative.
  • Silicon (Si) vs. Nitrogen (N): N is more electronegative.
• Geometry and Polarity:
  • Linear (e.g., CO2): Can cancel if symmetrical.
  • Trigonal Planar: Can cancel if symmetrical.
  • Tetrahedral: Can cancel if symmetrical and identical.

Practical Examples of Molecule Polarity

• Diatomic Molecules (e.g., Cl2): Nonpolar if identical.
• Trigonal Planar Molecules: Could be polar if substituents differ.
  • Example: Different pulls from different atoms prevent cancellation.
• Water (H2O): Polar due to bent geometry, non-linear arrangement.

Conclusion

• Understand trends in electronegativity for predicting bond and molecular polarity.
• Recognize the importance of molecular geometry in determining overall molecule polarity.
• Utilize symmetry and identical atoms as key indicators of nonpolarity.