London Dispersion Forces (College Board AP Chemistry)

• Intermolecular forces are weak forces that exist between electrically neutral molecules
• All intermolecular interactions are electrostatic
  • This means that they involve attractions between positive and negative species, much like ionic bonds
• Intermolecular forces are generally weaker than intramolecular attraction forces like ionic bonds, covalent bonds and metallic bonds
• Intermolecular forces are responsible for the physical properties of covalent molecules including:
  • Boiling point
  • Melting point
  • Density
  • Viscosity
  • Solubility
• For example, the stronger the intermolecular force, the higher the boiling and melting point of the molecule

Intermolecular Forces Vs Intramolecular Forces

Weak intermolecular forces exist between molecules while stronger intramolecular forces exist between the atoms that make up the molecules

• London dispersion forces are a weak type of intermolecular forces produced by small instantaneous dipoles that occur in nonpolar molecules
• Instantaneous dipoles are the result of an electron's motion in one atom affecting another atom
• The average distribution of electrons in a nonpolar molecule is symmetrical
  • This means that they lack a permanent dipole moment
• However, at any instant due to their motion, these electrons become unevenly distributed
  • This means that one side develops a partial negative charge, δ- and the other side develops a partial positive charge, δ+
• The instantaneous dipole that develops in one molecule then influences another molecule resulting in a weak attraction

London Dispersion Forces

A snapshot of the average and instantaneous distribution of electrons in an atom to demonstrate the development of London dispersion forces.

• The strength of London forces is influenced by polarizability
• Polarizability describes the ease of charge distortion in a molecule and the consequent development of an instantaneous dipole
• The greater the polarizability, the more easily the electron cloud can be distorted to give an instantaneous dipole
  • This results in a stronger dispersion force
• Polarizability increases with increasing number of electrons in a molecule
  • Hence, the strength of dispersion forces increases with increasing atomic or molecular size
• For example, the boiling points of halogens and noble gases increase as you go down the group, where dispersion forces are the only forces responsible for this property

Boiling Points of Halogens & Noble Gases

The boiling point of halogens and noble gases increases with the number of electrons

• For molecules of about the same molecular mass, polarizability is influenced by branching/compactness
  • The more branched the molecule, the less polarizable the molecule is and the smaller its dispersion force
• For example, the compounds pentane, 2-methylbutane, and 2,2-dimethylpropane all have the same molecular formula, C₅H₁₂, and same molecular mass, 72.15 g mol^-1
  • However, they differ in the arrangement of their atoms
  • Pentane has no branching and is the least compact
  • 2-methylbutane has one branch making it more compact than pentane
  • 2,2-dimethylpropane has two branches making it more compact than 2-methylbutane
  • This increase in the branching/compactness results in a decrease in boiling point as you move from pentane to 2-methylbutane to 2,2-dimethylbutane

Branching & Polarizability

The effect of branching on the strength of dispersion forces for molecules of the same molecular size.