text: why does hbr have a higher boiling point than hcl? the order of boiling points of the hydrogen halides is as follows: hf > hi > hbr > hcl.  i understand that hf is higher than the rest because of hydrogen bonding that takes place in hf but does not in the others.  what i do not understand is why the order of boiling points of the other 3 are hi > hbr > hcl. to me, the opposite would make sense (i.e. hcl > hbr > hi), based on the following logic:  the electronegativity (en) on hydrogen is 2.2. the ens of cl, br and i are 3.2, 3.0, and 2.7, respectively. so the en differences of the h-cl, h-br and h-i bonds are 1.0, 0.8, and 0.5, respectively. therefore, each of these molecules are polar. however, as a greater en difference means a greater polarity, the polarity of hcl will be greater than hbr, which in turn will be greater than hi. 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)  also, the closeness of strengths means that london forces can become stronger than permanent dipole forces within reasonable logic. how easily is this achieved?   here are some b.ps in celsius obtained from wikipedia: i2: 184 hi: -35 br2: 58 hbr: -66 cl2: -34 hcl: -85  from 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.  the 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.  the 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.