Lecture on London Dispersion Forces (LDF)

Overview

• Focus on London dispersion forces (LDF): one of three types of intermolecular forces (IMF), along with dipole-dipole forces and hydrogen bonding.
• IMFs are forces of attraction that bring separate molecules together.
• Goals: Understand the strength of LDF and compare molecules to determine which experiences stronger LDF.

Key Concepts

London Dispersion Forces (LDF)

• These are the only forces experienced by nonpolar molecules.
• Strength of LDF depends on polarizability, which is the ability to form temporary dipoles due to electron movement.

Example Molecules: F₂ and Cl₂

• Both are nonpolar molecules only influenced by LDF.
• Similarities: Both fluorine (F) and chlorine (Cl) are in group 17 (halogens) with 7 valence electrons.
• Differences: Chlorine has more protons and electrons than fluorine (Cl: 17, F: 9), leading to a larger electron cloud.

Molecular Size and Electron Cloud

• Chlorine's larger electron cloud allows greater movement of electrons within the molecule.
• More electrons in Cl₂ make it more polarizable, leading to stronger temporary dipoles.
• The electron cloud diagram for Cl₂ is larger than F₂, indicating more significant electron movement and stronger dipoles.

Temporary Dipoles and LDF Strength

• Temporary dipoles occur when electrons within a molecule shift to one side.
• Cl₂ exhibits stronger temporary dipoles compared to F₂ due to its larger number of electrons.
• Resulting in stronger London dispersion forces in Cl₂ than in F₂.

Implications of Polarizability

• More electrons = more polarizable = stronger partial charges = stronger IMFs.
• Summary: Molecules with more electrons form stronger attractions between separate molecules through LDF.

Practical Application: Boiling Points

• Boiling point relates to the energy required to break IMFs.
• F₂ boiling point: -188°C; Cl₂ boiling point: -34°C.
• The higher (less negative) boiling point of Cl₂ reflects stronger LDF due to more electrons.

Broader Trends in Halogens

• As we move from F₂ to I₂ in group 17, the number of electrons increases.
• Boiling points correlate with electron number:
  • F₂: -188°C
  • Cl₂: -34°C
  • Br₂: 59°C
  • I₂: 184°C
• More electrons mean higher boiling points because of stronger LDF.

Conclusion

• The molecule with the most electrons experiences stronger London dispersion forces.
• More electrons make a molecule more polarizable, leading to stronger intermolecular forces.