What causes temporary fluctuations in electron density, leading to London forces?

Temporary fluctuations in electron density are caused by the random movement of electrons within an atom or molecule.

In more detail, electrons in an atom or molecule are constantly in motion. This motion is random and unpredictable, which means that at any given moment, there may be more electrons on one side of the atom or molecule than on the other. This creates a temporary, fluctuating dipole, with a slightly negative charge on one side and a slightly positive charge on the other.

These temporary dipoles can induce dipoles in nearby atoms or molecules, causing them to also become temporarily polarised. The slightly negative side of one atom or molecule will be attracted to the slightly positive side of another, creating a weak intermolecular force known as a London dispersion force, or simply a London force.

It's important to note that these forces are very weak and short-lived, as the electron distribution is constantly changing. However, they can become significant in large, heavy atoms or molecules, where there are many electrons that can move around.

London forces are a type of van der Waals force, named after the Dutch scientist Johannes Diderik van der Waals, who first proposed their existence. They are the weakest of the intermolecular forces, but they are present in all atoms and molecules, regardless of whether they are polar or non-polar. This is because they are caused by the random movement of electrons, which is a fundamental property of all matter.

In summary, temporary fluctuations in electron density, caused by the random movement of electrons, lead to the creation of temporary dipoles. These dipoles can induce dipoles in nearby atoms or molecules, leading to the formation of London forces. Despite their weakness, these forces play a crucial role in the behaviour of atoms and molecules, influencing properties such as boiling point, melting point, and solubility.