London Dispersion Forces Lecture Notes

Introduction to London Dispersion Forces

• Present in every atom and molecule.
• Predominant in nonpolar molecules.

Electron Distribution and Polarization

• Nonpolar atoms have evenly distributed electrons.
• Electrons are constantly moving, leading to temporary polarization.
• Temporary dipoles form due to uneven electron distribution.
• A temporary dipole can induce a dipole in neighboring atoms.

Mechanism of London Dispersion Forces

• Temporary dipole in one atom can induce a dipole in another atom.
• Resulting induced dipole creates a temporary attraction.
• Interaction is short-lived and depends on the number of electrons.

Factors Affecting London Dispersion Forces

• Polarizability: Ease of electron cloud distortion.
  • Larger atoms with more electrons are more polarizable.
  • Example: Iodine is more polarizable than Fluorine.
• Number of Electrons: More electrons increase polarizability and LDF.

Trends in London Dispersion Forces

• Noble gases (Helium, Neon, Argon, Krypton) show increased LDF with more electrons:
  • Helium: 2 electrons, Neon: 10 electrons, Argon: 18 electrons, Krypton: 36 electrons.
• Boiling points rise with increased LDF:
  • Helium: -269°C, Neon: -249°C, Argon: -186°C, Krypton: -153°C.

Examples and Comparisons

• Halogen Molecules: Fluorine, Chlorine, Bromine, Iodine.
  • Iodine has the highest boiling point (184°C) due to most electrons and LDF.
• Boiling Points: Related to states of matter:
  • Solids (high boiling point), Gases (low boiling point).
• Molecular Structure Influence:
  • Pentane (straight chain) vs. Neopentane (branched):
    • Pentane has higher boiling point (36°C) vs. Neopentane (9.5°C).

Key Takeaways

• Increase in surface area increases LDF and boiling point.
• Straight chain molecules generally have higher boiling points than branched ones.
• More electrons in nonpolar molecules increase LDF strength and boiling points.