Dispersion Force



 

Learning Objective

  • Discuss the characteristics of dispersion forces.

Key Points

    • London dispersion forces are weak intermolecular forces and are considered van der Waals forces.
    • Temporary dipoles can occur in non-polar molecules when the electrons that constantly orbit the nucleus occupy a similar location by chance.
    • Temporary dipoles can induce a dipole in neighboring molecules, initiating an attraction called a London dispersion force.

Terms

  • London dispersion forcesA weak intermolecular interaction arising from induced instantaneous dipoles in molecules; part of the Van der Waals forces.
  • dipoleAny molecule that has both slight positive and negative charges on either end.
  • Van der Waals forcesThe sum of the attractive or repulsive forces between molecules (or between parts of the same molecule) other than those due to covalent bonds, or the electrostatic interaction of ions with one another, with neutral molecules, or with charged molecules.

Temporary Dipoles

Temporary dipoles are created when electrons, which are in constant movement around the nucleus, spontaneously come into close proximity. This uneven distribution of electrons can make one side of the atom more negatively charged than the other, thus creating a temporary dipole, even on a non-polar molecule. The more electrons there are in an atom, the further away the shells are from the nucleus; thus, the electrons can become lopsided more easily, and these forces are stronger and more frequent. These intermolecular forces are also sometimes called “induced dipole-induced dipole” or “momentary dipole” forces.

London Dispersion Forces

Although charges are usually distributed evenly between atoms in non-polar molecules, spontaneous dipoles can still occur. When this occurs, non-polar molecules form weak attractions with other non-polar molecules. These London dispersion forces are often found in the halogens (e.g., F2 and I2), the noble gases (e.g., Ne and Ar), and in other non-polar molecules, such as carbon dioxide and methane. London dispersion forces are part of the van der Waals forces, or weak intermolecular attractions.

Interactive: Charged and Neural AtomsThere are two kinds of attractive forces shown in this model: Coulomb forces (the attraction between ions) and Van der Waals forces (an additional attractive force between all atoms). What kinds of patterns tend to form with charged and neutral atoms? How does changing the Van der Waals attraction or charging the atoms affect the melting and boiling point of the substance?

Interactive: Comparing Dipole-Dipole to London DispersionInvestigate the difference in the attractive force between polar and non-polar molecules.

Interactive: Factors Affecting London Dispersion AttractionsExplore the role of size and shape in the strength of London dispersion attractions.

Van der Waals forces help explain how nitrogen can be liquefied. Nitrogen gas (N2) is diatomic and non-polar because both nitrogen atoms have the same degree of electronegativity. If there are no dipoles, what would make the nitrogen atoms stick together to form a liquid? London dispersion forces allow otherwise non-polar molecules to have attractive forces. However, they are by far the weakest forces that hold molecules together.

Liquid nitrogenWithout London dispersion forces, diatomic nitrogen would not remain liquid.