text: you should know that London dispersion forces as well as dipole-dipole interactions between polar molecules are both termed as van der Waals forces in some chemistry environments. this is because instantaneous and permanent dipole interactions have little appreciable difference in strength when compared to other forces that also influence mp/bp like hydrogen, covalent or ionic bonds. this is why the relatively tiny HF can disobey the entire trend(because it has that extra oomph)\n\nalso, the closeness of strengths means that London forces can become stronger than permanent dipole forces within reasonable logic. how easily is this achieved? \n\nhere are some b.ps in celsius obtained from wikipedia:\nI2: 184\nHI: -35\nBr2: 58\nHBr: -66\nCl2: -34\nHCl: -85\n\nfrom here you can observe that for the halogen molecules, bps drop drastically due to an equally drastic fall in Mr and electron cloud size. however, for the hydrogen halides, the decrease is only slight.\n\nthe current logic is that london forces may exceed the polar forces because Mr is allowed to increase greatly across many periods while electronegativity is limited to a small range on the Pauling scale.\n\nthe only way for polar forces to greatly exceed london forces woiuld be if the number of halogen atoms is allowed to increase, thereby increasing electronegativity, for example adding an additional bromine atom on a 1-bromomethane rather than merely changing 1-bromomethane to 1-chloromethane.