# London/dispersion forces

"London (dispersion) forces are responsible for the fact that non-polar substances can be condensed to form liquids and sometimes solids at low temperatures"._

I learned that what results in London fores between molecules is the "cloud of electrons" that results in an instantaneous dipole and induced dipole, which eventually results into forming these weak London forces._

The aforementioned statement suggests that as we lower the temperature we form a solid or a liquid meaning that the inter-molecular forces (London forces in this case)became stronger._

My question is: if we lower the temperature we subsequently decrease the kinetic energy of the electrons, i.e. we decrease the frequency/probability of these "electron clouds" moving to one side of the molecule and forming instantaneous and induced dipole, which means that London forces should be weaker and not stronger to form a liquid or a solid.

This is how I thought of it, can anyone suggest what's wrong with my way of thinking of it? Is it that what we relate to the strength of the London forces is how long the electrons cloud stay on one side rather than how often that happens?

• Kinetic energy is about moving the molecule as a whole, and not the electron clouds inside it. The latter is almost unaffected by temperature. – Ivan Neretin Feb 13 '17 at 19:30
• why doesn't temperature affect the electrons movement/speed? @IvanNeretin – Bayan Feb 13 '17 at 19:59
• @Bayan Consider the forces acting on a helium atom. There is gravity, which is really tiny (see chart on strength at en.wikipedia.org/wiki/Fundamental_interaction) and the mentioned dispersion forces, which are also rather small but more importantly, short-ranged ($r^{-6}$). These forces need to be counteracted by kinetic energy. Now consider that electrons feel the electromagnetic force, which is way stronger. So, when going to high temperatures, at some point, electron "movement" is affected by the temperature - eventually, you get a plasma. But that means really high temperature. – TAR86 Feb 13 '17 at 20:17
• – Pritt says Reinstate Monica May 19 '17 at 4:06