"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?

  • 1
    $\begingroup$ Kinetic energy is about moving the molecule as a whole, and not the electron clouds inside it. The latter is almost unaffected by temperature. $\endgroup$ Commented Feb 13, 2017 at 19:30
  • $\begingroup$ why doesn't temperature affect the electrons movement/speed? @IvanNeretin $\endgroup$
    – Bayan
    Commented Feb 13, 2017 at 19:59
  • 3
    $\begingroup$ @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. $\endgroup$
    – TAR86
    Commented Feb 13, 2017 at 20:17
  • $\begingroup$ Related chemistry.stackexchange.com/questions/74780/… $\endgroup$ Commented May 19, 2017 at 4:06

1 Answer 1


When we lower the temperature of a substance, we are decreasing the average molecular kinetic energy (they're slowing down). As mentioned in the comments on the question, this doesn't really affect the electron cloud.

So, the substance has been cooled down and molecules are moving more slowly. At this point, it's not that the dispersion forces because stronger, but rather that they become comparatively stronger. While in the gaseous phase, the molecules have velocities of several hundred meters per second. At those speeds, London forces don't really have much of an effect. But, as the molecules are cooled and slowed down (this seems to suggest this is around 10m/s), London forces can cause more interactions between molecules. If the substance is cooled far enough, those forces are able to overcome the kinetic energy of the molecules and they condense into a liquid. As the cooling continues, the molecules move around less and less until they take on a definite shape, at which point it is considered a solid.

The primary issue with your intuition is merely understanding what cooling the substance down really does.


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