2.13: Van der Waals Interactions
Atoms and molecules interact with each other through intermolecular forces. These electrostatic forces arise from attractive or repulsive interactions between particles with permanent, partial, or temporary charges. The intermolecular forces between neutral atoms and molecules are ion–dipole, dipole–dipole, and dispersion forces, collectively known as van der Waals forces.
Polar molecules have a partial positive charge on one end and a partial negative charge on the other end of the molecule, creating a dipole. In a polar molecule like water, the more electronegative oxygen atom bears the partial negative charge, and the less electronegative hydrogen atom carries the partial positive charge. The water molecules are attracted to each other because of the positive end of one water molecule and the negative end of another. This attractive force is called a dipole–dipole attraction.
An ion–dipole force is the electrostatic attraction between an ion and a dipole. These forces are common in solutions and play an important role in the dissolution of ionic compounds in water. When an ionic compound like KCl is added to a polar solvent like water, the ions in the solid separate and disperse uniformly. Ion–dipole forces attract the positive end of the polar water molecules to the negative chloride ions at the surface of the solid, and they attract the negative ends to the positive potassium ions. The water molecules surround individual K+ and Cl- ions, reducing the strong interionic forces that bind them together in a solid and dissolving them. The strength of ion–dipole interactions is directly proportional to i) the charge on the ion and ii) the magnitude of the dipole of polar molecules.
Dispersion forces are present in all condensed phases, regardless of the nature of the atoms or molecules composing the substance. These attractive forces are also called London dispersion forces in honor of German-born American physicist Fritz London who first explained them in 1928. The electrons of an atom or molecule are in constant motion at any moment in time, and an atom or molecule can develop a temporary, instantaneous dipole if its electrons are distributed asymmetrically. The presence of this dipole can, in turn, distort the electrons of a neighboring atom or molecule, producing an induced dipole. These rapidly fluctuating, temporary dipoles result in a relatively weak electrostatic attraction between the species—a so-called dispersion force. Dispersion forces are relatively weak and become significant only when the molecules come very close. Larger and heavier atoms and molecules exhibit stronger dispersion forces than smaller and lighter ones.
This text is adapted from Openstax, Chemistry 2e, Chapter 10: Liquids and Solids.