Notes on London Dispersion Forces and Intermolecular Forces

Temporary Dipole

  • Definition: A temporary dipole occurs in a neutral atom when the distribution of electrons is uneven, leading to one side having more electrons than the other.
  • Characteristics of Temporary Dipoles:
    • One side becomes partially negative (electron-rich) and the other side becomes partially positive (electron-deficient).
    • The dipole effect is transient as electrons quickly redistribute back to an even state.
  • Key Point: A temporary dipole is a dipole formed from distortion of the electronic cloud of a neutral atom for a brief period.

Induced Dipole

  • Definition: An induced dipole is formed when a neutral atom is brought near a temporary dipole, causing the neutral atom's electron cloud to distort due to the electric field.
  • Mechanism:
    • The positive side of the temporary dipole attracts the electrons of the neutral atom, pulling its electron cloud toward it.
    • This results in the neutral atom becoming polarized with a partially negative charge on the side closer to the positive pole and a partially positive charge on the opposite side.
  • Key Characteristics:
    • Polarized neutral atom created due to the influence of a nearby temporary dipole.

London Dispersion Forces (LDF)

  • Definition: London dispersion forces are weak, temporary intermolecular forces that arise from the attraction between a temporary dipole and an induced dipole.
  • Terminology:
    • Also known as Van der Waals forces.
  • Nature of LDF:
    • These forces exist between all types of molecules, including both polar molecules (e.g., hydrogen fluoride) and non-polar molecules (e.g., carbon dioxide).
  • Dominance:
    • London dispersion forces are the primary intermolecular forces in non-polar molecules.
    • Examples: LDF are particularly significant in halogens such as chlorine, bromine, and iodine.

Examples of London Dispersion Forces in Halogens

  • Boiling Points of Halogens:
    • Fluorine: -188 °C
    • Chlorine: -34 °C
    • Bromine: 59 °C
    • Iodine: 114 °C
  • Reason for Different Boiling Points:
    • Fluorine (9 electrons) has weaker LDF than iodine (53 electrons), which leads to stronger LDF and higher boiling points in larger atoms.
  • Trend in Group Properties:
    • As you move down Group 7 in the periodic table, the number of electrons increases.
    • Increased electrons lead to stronger London dispersion forces and consequently higher boiling points down the group.

Summary of Key Takeaways

  • London dispersion forces are weak and occur between temporary and induced dipoles.
  • They are present in all molecular types, more dominant in non-polar compounds.
  • The strength of LDF varies with the number of electrons in the molecules, affecting physical properties such as boiling points.