Overview
This lecture explains how to determine if a molecule is polar or nonpolar, focusing on electronegativity, bond polarity, molecular geometry, and dipole moments.
Polar and Nonpolar Molecules
- Polar molecules have regions with partial positive and negative charges due to unequal electron sharing.
- Nonpolar molecules share electrons equally and have no separation of charge within the molecule.
- A molecule made from one type of element (e.g., O₂, N₂) is always nonpolar.
- Hydrocarbons (molecules with only carbon and hydrogen) are always nonpolar because the C-H bond is nonpolar.
Determining Polarity: Steps and Examples
- Check electronegativity difference; a bond is polar if the difference is 0.5 or greater.
- Example: In HF, fluorine is more electronegative than hydrogen, so HF is polar.
- If a molecule has only polar bonds, check geometry to determine if dipole moments cancel.
- Tetrahedral molecules like CF₄ are nonpolar despite having polar bonds because dipole moments cancel.
- Water (H₂O) is polar due to bent geometry causing dipole moments to add up rather than cancel.
Dipole Moments and Molecular Geometry
- Dipole moment: Arrow points from partial positive to partial negative within a bond.
- If all dipole moments in a molecule cancel (net dipole moment = 0), the molecule is nonpolar.
- If there is a net dipole moment (arrows don't cancel), the molecule is polar.
More Examples: CO₂ vs. SO₂
- CO₂ is linear; dipole moments are equal and opposite, so it's nonpolar.
- SO₂ is bent; dipole moments partially add up, so it's polar.
Key Terms & Definitions
- Polar molecule — a molecule with positive and negative ends due to unequal electron sharing.
- Nonpolar molecule — a molecule with no charge separation; electrons are shared equally.
- Electronegativity — the ability of an atom to attract electrons in a bond.
- Dipole moment — a measure of charge separation in a bond or molecule.
Action Items / Next Steps
- Practice drawing Lewis structures and identifying dipole moments for various molecules.
- Memorize rules: single-element molecules and hydrocarbons are nonpolar.
- Review electronegativity values and practice predicting bond polarity.