Explain what gives rise to London forces and when they occur.

London forces, also known as dispersion forces or Van der Waals forces, are weak attractive forces between non-polar molecules or atoms. These forces are caused by the temporary fluctuation in the electron distribution around atoms or molecules, which leads to a short-lived dipole formation.

In an atom or a non-polar molecule, the electrons are in constant motion around the nucleus. At particular moments, these electrons can be unevenly distributed, creating a temporary, localized charge imbalance. This leads to the formation of a temporary dipole, which induces a dipole in neighboring atoms or molecules. The two dipoles are attracted to each other, leading to the London forces. This phenomenon is referred to as "instantaneous dipole-induced dipole interaction."

London forces occur between non-polar molecules and atoms, as well as between polar molecules when the dipolar interactions are not strong enough to dominate the behavior. These forces are generally weak, but they can become significant between large molecules or in substances with a high molecular weight. They explain why non-polar substances, like noble gases and many organic compounds, can exist as liquids or solids at low temperatures. In summary, London forces arise due to temporary fluctuations in electron distribution which creates temporary dipoles that interact with neighboring molecules. These forces generally occur between non-polar molecules and atoms, as well as between polar molecules when the dipolar interactions are not dominant.