Question

Why do London forces occur?

Answer 1

Random fluctuations in the electron cloud of one atom/molecule induce a spontaneous dipole in an adjacent molecule, leading to a temporary attractive force between the two induced dipoles.

Explanation:

London forces are also known as Van der Waals forces or instantaneous dipole-instantaneous dipole interactions. They are present in every substance, and provide the intermolecular forces in non-polar substances.

Because electrons are constantly moving, there may be a moment when a molecule has a temporary charge imbalance across it. Since electrons repel other electrons, this charge imbalance (known as a 'dipole') can induce another dipole in a neighbouring molecule. The `delta-` end of one will be attracted to the `delta+` end of the other (here `delta` just means the charge is very small), causing a temporary attraction.

London forces are very weak because they only last for a very brief period of time, but due to the incredibly large number of electrons in a substance they have a significant effect. They are stronger if there are more electrons present (for example Iodine, `I_2`, has stronger intermolecular forces than `Br_2`, which has stronger forces than `Cl_2`). They are weaker than interactions between permanent dipoles and induced or permanent dipoles, and a lot weaker than hydrogen bonds.