Why does hydrogen iodide has a higher boiling point than hydrogen chloride?
The reason is that the London dispersion forces are greater for HI, than for HCl. London dispersion forces are determined by the polarizability of the molecule, which in turn is determined by the NUMBER of electrons and the VOLUME over which the electrons are spread. To clear up a few misconceptions that are reflected in some other answers: ALL intermolecular forces are classified as van der Waals forces. Van der Waals forces include hydrogen bonding, dipole-dipole attractions AND London dispersion forces. London dispersion forces are NOT determined by the mass of the molecule. While a mass increase often accompanies an increase in the number of electrons, it is the polarizability of the molecule which determines the strength of the London dispersion forces and that is determined by the number of electrons and the volume of the molecule.
