I was reading about the London dispersion forces and it is written that it arises due to an asymmetric distribution of charges in an atom at an instant which gives rise to a temporal dipole and this temporal dipole then induces a dipole in nearer atoms and thus they experience the intermolecular forces.

This site says that $Cl_2$ and $Br_2$ can exist as solid because of this force at lower temperature and can exist in liquid form too.

So , my question is that since this force is due to temporal dipole moment then why doesn't the state of the compound or molecule (held together by dispersion forces) keep on oscillating between solid , liquid and gaseous phase ?

One thing which I think for the above question is that we need a repulsive force to separate two closer molecules and this is the reason why we don't notice oscillations . Am I right ?

Okay if my intuitions are correct then again there is a question.

If the forces are temporal , then surely the strengths of the compound in solid phase must be changing i.e. if it is harder at an instant then after sometime it must be softer and can be easily converted into gas with a negligible effort or with a single blow of hand or even with a beam of photons.

But I don't think we notice such strange things in our daily life.

So why don't we notice such transformations ?


1 Answer 1


The dipole interactions may oscillate (at very high frequencies) but they average out over time (and billions of atoms) into a steady force. The ability of this force to hold atoms together against thermal agitation generally depends on the temperature. A change of state is associated with a significant amount of energy (the heat of fusion or vaporization) and time. You wont see it occurring in a random manner.

  • $\begingroup$ Of course, any fluctuation that happens to occur at the surface that allows a molecule to detach and depart into the void is irreversible and doesn't average out. This is the origin of evaporation/sublimation into the gas state. We don't see the corresponding solid→liquid fluctuations at the surface because the positive surface tension of liquids requires a large number of molecules to undergo the phase transition essentially simultaneously to nucleate a droplet. $\endgroup$ Nov 2, 2020 at 0:32
  • $\begingroup$ @R.W. Bird you didn't answer my second question I guess . $\endgroup$
    – Ankit
    Nov 2, 2020 at 7:16
  • $\begingroup$ @R.W. Bird can you edit your answer for the second part of my question ? $\endgroup$
    – Ankit
    Nov 10, 2020 at 3:19

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