van der Waals interactions occur because, at any given instant, there is a greater electron density on one side of the molecule than on the other.
There are three types of van der Waals forces:
- Dipole-dipole forces
- Dipole-induced dipole forces
- London dispersion forces
Dipole-dipole forces occur between molecules that have permanent dipoles.
Hydrogen bonds are also van der Waals forces. They are justespecially strong dipole-dipole forces.
Dipole-Induced Dipole Forces
A dipole-induced dipole attraction is a weak attraction that results when a polar molecule induces a dipole in an atom or nonpolar molecule.
For example, an oxygen molecule is nonpolar.
When a polar molecule like water approaches, the electron cloud on the O₂ molecule responds and the O₂ molecule develops a dipole.
London Dispersion Forces
In a symmetrical molecule like hydrogen, the electrons are in constant motion. At any one instant, they might find themselves at one end of the molecule, making that end δ⁻. The other end then becomes δ⁺.
An instant later, the electrons may have moved to the other end, reversing the polarity of the molecule.
This "sloshing around" of the electrons causes fluctuating dipoles even in symmetrical molecules.
Imagine a molecule with a temporary polarity approaching a nonpolar molecule.
As the right hand molecule approaches, the positive end of the left hand molecule attracts its electrons. This sets up an induced dipole in the approaching molecule.
The molecules arrange themselves in such a way that the δ⁺ end of one attracts the δ⁻ end of the other.
This attraction is the van der Waals force.