There are two types of Van der Waals forces: London dispersion and dipole-dipole interactions. Van der Waals and other intermolecular forces like hydrogen bonding have observable effects like boiling and melting points.
London dispersion:
London dispersion is most easily observed in nonpolar molecules, and it’s sometimes called dipole-induced dipole. Electrons aren’t exactly point particles; they’re closer to clouds of probability. Electron density in a neutral compound can fluctuate, and that fluctuation can result in temporary polarization. The images below show what happens:
Electron-Density fluctuations
Stage 1
In stage 1, the electron density is pretty evenly distributed in both uncharged molecules.
Stage 2
In stage 2, the electron density of the molecule on the left fluctuated, resulting in partial charges developing on opposite sides of the molecule. We know electrons repel each other, and the extra electron density is repulsive to the neighboring molecule’s electrons. This creates a dipole.
Stage 3
Stage 3 shows the attraction between the partial negative charge and the induced partial positive. This attraction (between two nonpolar molecules) is sometimes referred to as the Debye force.
Bonus Biology knowledge: geckos are actually able to walk on featureless surfaces like glass thanks to London dispersion forces. Look at that! Just another example of nature being super cool. Here’s a poorly drawn gecko for your viewing pleasure:
Poorly drawn gecko
Dipole-Dipole interactions:
Dipole dipole interactions are found in polar molecules, i.e. those with permanent dipole moments. These interactions are generally stronger than London dispersion because of the permanence of the dipole. The attractive force between two permanent dipoles is sometimes referred to as the Keesom force. Let’s look at acetone:
Acetone dipole-dipole
Acetone has an electronegative oxygen attached to a relatively electropositive carbon, and this bond results in a polar bond. The oxygen develops a partial negative while the carbon develops a partial positive. Since there’s more than one acetone molecule in solution, they can actually stack like in the image above. The oxygens will match up with the carbons according to polarity.
