Overview
This lecture explains the difference between polar and non-polar molecules, the role of electronegativity, and why polarity is important in chemistry and life.
Molecular Polarity: Basics
- Molecules are categorized as polar or non-polar based on charge and molecular symmetry.
- Polar molecules have an uneven charge distribution, while non-polar molecules have a symmetric charge distribution.
- Water is polar; butter is non-polar, explaining why they don't mix.
Causes of Polarity
- Polarity requires: (1) asymmetrical electron distribution (charge asymmetry), and (2) molecular geometry that does not cancel out charge differences.
- Polarity cannot occur in molecules made of only one element due to equal electronegativity.
Electronegativity & Trends
- Electronegativity is how strongly an atom attracts electrons.
- It increases from left to right and decreases from top to bottom on the periodic table.
- A molecule is polar if the difference in electronegativity between atoms is 0.5 to 1.6.
Geometric & Charge Asymmetry
- Geometric symmetry can cancel polarity (e.g., COâ‚‚ or CHâ‚„ are non-polar despite polar bonds).
- Polar molecules require both a dipole (charge separation) and asymmetric geometry.
Dipole Notation & Partial Charges
- Dipole moments are indicated by an arrow toward the negative side; δ+ (delta plus) and δ– (delta minus) denote partial charges on atoms.
Intermolecular Forces & Solubility
- Polar molecules dissolve other polar or ionic substances (“like dissolves like”).
- Cohesive forces cause surface tension in liquids like water; non-polar oils disrupt this order, leading to separation.
- Water orients its partial charges to minimize system energy.
Hydrogen Bonding & Water’s Unique Properties
- Hydrogen bonds occur between the partially negative O and positive H in water molecules.
- Ice's structure (hydrogen bonding) makes it less dense than liquid water, causing it to float.
- Water has high specific heat capacity due to hydrogen bonding, making it stable and life-supporting.
Hybrid Molecules & Biological Applications
- Surfactants (like soaps) and fatty acids have both polar and non-polar regions, letting them interact with water and fats.
- Cell membranes use polar heads for interaction with water and non-polar tails to avoid dissolving.
Key Terms & Definitions
- Polar molecule — molecule with uneven charge distribution due to differences in electronegativity and asymmetric geometry.
- Non-polar molecule — molecule with even charge distribution, often due to symmetric arrangement or identical atoms.
- Electronegativity — tendency of an atom to attract electrons.
- Dipole moment — separation of positive and negative charges within a molecule.
- Hydrogen bond — weak attraction between partially positive hydrogen and partially negative atom (usually O or N) in different molecules.
Action Items / Next Steps
- Review examples of polar and non-polar molecules.
- Practice drawing dipole arrows and δ+/δ– notations.
- Read more on hydrogen bonds and their effects in biological systems.