Lecture Notes on Polar and Nonpolar Molecules

Introduction to Molecules

• Molecules are diverse and varied.
• Human tendency to classify helps make sense of complexity.

Polar vs. Nonpolar Molecules

• Polar molecules have asymmetrical charge distribution.
• Nonpolar molecules have symmetrical charge distribution.
• Preference expressed for polar molecules due to their interesting properties.

Examples of Polar and Nonpolar Substances

• Butter (nonpolar) vs. Water (polar):
  • Nonpolar substances resist mixing with polar substances.
  • Demonstrated inability to wash butter (nonpolar) with water (polar).

What Makes a Molecule Polar?

1. Asymmetrical Electron Distribution:
   • Requires different elements with varying electronegativities.
   • Electronegativity: tendency of element to attract electrons.
   • Periodic trend: increases across periods, decreases down groups.
   • "Upper right" elements like Fluorine have high electronegativity.
2. Geometrical Asymmetry:
   • Linear molecules like CO2 may have polar bonds but are nonpolar overall due to symmetry canceling out charge asymmetry.
   • Need a dipole moment: charge separation within the molecule.

Polarity and Dissolving

• Like Dissolves Like: polar solvents dissolve polar solutes.
• Polar molecules align positive to negative at low energy states, forming structures like liquid crystals.
• Surface tension is due to cohesive forces among polar molecules.
• Nonpolar substances like oil don't mix well in polar environments, e.g., water.

Hydrogen Bonding

• Important for water’s properties, like enabling life.
• Creates unique phenomena:
  • Ice floats due to hydrogen bonds increasing volume, decreasing density.
  • Specific heat capacity of water is high due to energy required to manipulate hydrogen bonds.

Hybrid Molecules

• Have both polar and nonpolar regions:
  • Examples: Surfactants in soaps.
  • Fatty acids in cell membranes have polar heads and nonpolar tails, aiding in structural integrity in aqueous environments.

Conclusion

• Polar molecules require both charge asymmetry and geometric asymmetry.
• Electronegativity differences lead to charge asymmetry.
• Water’s polarity underpins its ability to support life and dissolve various substances.

Acknowledgments

• Written by Hank Green.
• Chemistry consultants: Dr. Heiko Langner and Edie Gonzalez.
• Production credits include Nicholas Jenkins, Michael Aranda, Thought Cafe.