Polarity in Molecules Study Guide

Polarity in Molecules

Definition of Polarity

  • Polarity in a molecule determines whether valence electrons in that molecule are shared equally.

Key Determinants of Polarity

  • To determine the polarity of a molecule, consider the following factors:
    • Bond Polarity: The difference in electronegativity between the bonded atoms.
    • Geometry of the Molecule: The three-dimensional arrangement of the atoms in the molecule influences overall polarity.

Characteristics of Polar and Non-Polar Molecules

  • Asymmetric Molecules tend to be polar because of an uneven distribution of charge.
  • Symmetric Molecules without lone electrons on the central atom are always non-polar due to equal distribution of charge.

Steps to Determine the Polarity of a Molecule

  1. Draw the Electron Dot Structure of the molecule.
  2. Use the Electronegativity Chart to determine the difference in electronegativity for each bond:
    • Differences of:
      • 0—0.4 = Non-polar
      • 0.5—1.7 = Polar

Conditions for Non-Polar Molecules

  • A molecule is non-polar if:
    • Condition 1: Each bond in the molecule is non-polar and there are no lone electron pairs.
    • Condition 2: Every bond in the molecule has the same dipole, and there are no unbonded electron pairs on the central atom.
    • Condition 3: There are no net dipoles (the molecule exhibits symmetry).

Conditions for Polar Molecules

  • A molecule is polar if:
    • Condition 1: There is a net dipole moment present.
    • Condition 2: Each bond in the molecule is non-polar, but there are lone electron pairs on the central atom, resulting in asymmetry.
    • Condition 3: The bonds in the molecule have different polarities and/or there are unbonded electron pairs on the central atom.

Additional Considerations

  • Determining Molecular Polarity Depends on:
    • The bond polarity and lone electrons on the center atom.
    • The overall geometry of the molecule.

Polar Bonds vs. Polar Molecules

  • The effect of polar bonds on the overall polarity of the entire molecule heavily relies on the geometry of the molecule.
  • Example: Carbon Dioxide (CO2)
    • Contains two polar bonds but is linear and symmetric, therefore classified as Nonpolar due to the canceling of dipoles.