Intermolecular Forces:

London Dispersion Forces (van der Waals).

• Weak interactions caused by momentary changes in electron density in a molecule.

• They are the ONLY attractive forces present in nonpolar compounds.

• Examples:

  • Hydrocarbons

  • Polyethylene (plastic bags)

  • Gases in the atmosphere

The surface area of a molecule determines the strength.

• The larger the surface area, the larger the attractive force between two molecules, and the stronger the intermolecular forces.

• The shape also affects how many forces can occur.

If a molecule has a kink/bend in to it can not get as close to other molecules as a long/straight molecule can.

• The kink decreases the number of forces that can occur.

Dipole-Dipole Forces

• Are the attractive forces between the permanent dipoles of two polar molecules.

• Adjacent molecules align so that the partial positive and partial negative charges are in proximity.

Hydrogen Bonding

• Typically occurs when a hydrogen atom bonded to O, N, or F, is electrostatically attracted to a lone pair of electrons on an O, N, or F atom in another molecule.

Intermolecular Forces

1. van der Waals forces (London forces)

2. dipole-dipole interactions

3. hydrogen bonding