Why does argon have a lower melting point than chlorine?
Or, to put it another way, why is the MP or Cl2 higher than for Ar. First of all, if we look at the halogens and the inert gases, the melting points of both Cl and Ar are “right where they should be”. MP increases as we go down each column. And that is do to the increasing attraction between the molecules of the halogens and the atoms of the inert gases. The attractions are due to van der Waals forces, which in turn, depend on the number of electrons and the volume over which they are spread. The greater the number of electrons and the greater the volume, the greater the polarizability of the molecule and the greater the van der Waals forces. Now, back to Cl2 and Ar. Cl2 has more electrons and they are spread over a larger volume than does Ar. Therefore, the van der Waals forces are stronger for Cl2 than for Ar, and there is more intermoleuclar attraction between Cl2 molecules, and as a result, a higher melting point for Cl2.
