London forces, also known as dispersion forces or induced dipole-induced dipole forces, are a type of intermolecular force that occurs between non-polar molecules. These forces arise due to the temporary fluctuations of electron density within a molecule, which results in the formation of temporary dipoles. The temporary dipoles can induce similar dipoles in neighboring molecules, leading to an attractive force between the molecules.
Let’s take an example to understand London forces better. Consider two non-polar molecules, such as chlorine (Cl2) and hydrogen (H2). Both these molecules have completely symmetric electron distributions and thus have zero permanent dipole moment.
However, due to the random motion of electrons within each molecule, there can be temporary fluctuations in electron density that create a temporary dipole moment. These temporary dipoles can induce similar dipoles in nearby molecules. In our example, the temporary dipole in one chlorine molecule can induce a dipole in another chlorine molecule or even a hydrogen molecule nearby.
The induced dipoles then attract each other and create an intermolecular force between the two molecules called London force. The strength of this force depends on the magnitude of the induced dipole moment and the distance between the two interacting molecules.
Another example where London forces play an important role is in hydrocarbons such as methane (CH4) and ethane (C2H6). These molecules have no permanent dipole moment due to their symmetric electron distribution but still experience intermolecular attraction due to London forces.
In larger hydrocarbon molecules such as pentane (C5H12), hexane (C6H14), heptane (C7H16), etc., London forces become progressively stronger as the number of electrons increases. This is because more electrons mean more opportunities for temporary fluctuations in electron density that result in larger induced dipoles and thus stronger intermolecular attraction.
In conclusion, London forces are an important intermolecular force that arises due to temporary fluctuations in electron density within non-polar molecules. These forces can induce dipoles in nearby molecules and lead to intermolecular attraction between the two molecules. The strength of London forces depends on the magnitude of the induced dipole moment and the distance between the two interacting molecules.