Why do London forces occur?

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.