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Molecular Polarity Analysis

Sep 8, 2025

Overview

This lecture discusses how to determine the polarity of molecules by analyzing their Lewis structures, shapes, and bond dipoles, using several molecular examples.

Determining Molecular Polarity

  • The polarity of a molecule depends on both its shape and the polarity of its bonds.
  • Determining polarity starts with drawing the Lewis structure to find the molecular shape.
  • Polarity involves checking if individual bond dipoles cancel or sum to a net dipole moment.

Examples of Molecular Polarity

  • Hâ‚‚: Both atoms are identical; bond is nonpolar, and the molecule is nonpolar.
  • HCl: Chlorine is more electronegative, pulling electrons; bond and molecule are both polar.
  • BF₃: Each B–F bond is polar, but the trigonal planar symmetry causes dipole moments to cancel; molecule is nonpolar.
  • COâ‚‚: Each C–O bond is polar, but linear shape causes dipoles to cancel; molecule is nonpolar.
  • Hâ‚‚CO (Formaldehyde): C=O bond is polar; C–H bonds are nearly nonpolar; molecule is polar due to net dipole moment.
  • CHâ‚‚Clâ‚‚ (Dichloromethane): Tetrahedral molecule with two C–Cl polar bonds; net dipole exists, so the molecule is polar.
  • CClâ‚„ (Carbon Tetrachloride): Four polar C–Cl bonds in a symmetrical tetrahedral structure; dipoles cancel, making molecule nonpolar.

Steps to Determine Polarity

  • Draw the Lewis structure.
  • Identify molecular shape.
  • Determine which bonds are polar or nonpolar.
  • Assess if bond dipoles cancel out (nonpolar) or add to a net dipole (polar).

Key Terms & Definitions

  • Polarity — The property of a molecule with an uneven distribution of electron density, leading to a dipole moment.
  • Dipole Moment — A measure of the separation of positive and negative charge in a bond or molecule.
  • Lewis Structure — A diagram showing the arrangement of electrons around atoms in a molecule.
  • Electronegativity — The tendency of an atom to attract shared electrons in a bond.
  • Nonpolar Molecule — A molecule with an even distribution of charge, so no net dipole moment.
  • Polar Molecule — A molecule with an uneven distribution of charge, resulting in a net dipole moment.

Action Items / Next Steps

  • Draw Lewis structures and determine polarity for:
    • CHCl₃ (trichloromethane/chloroform)
    • (NO_3^-) (nitrate ion)
    • SO₃ (sulfur trioxide)

Full transcript