Melting and Boiling points of Hydrogen Fluoride (HF) is higher than Hydrochloric acid (HCl), Hydrobromic acid (HBr) and Hydrogen iodide (HI).
According to Fajan’s rule with the increase of size of anions, the tendency to be polarized increases. Because, as the size of anion increases, the distance between its nucleus and outermost electrons increases and hence the nucleus can attract the electrons less. So the cation can more easily deform the anion. Therefore in any group in the periodic table the tendency of the anion to be polarized increases from top to bottom. For example the radii of the halide ions are as follows.
F– (1.33A) < Cl– (1.81A) < Br (1.96A) < I (2.20A) and the tendency to be polarized increases in this order.
This is clearly demonstrated by the gradual decrease of melting and boiling points of Hydrogen halides Hf > HCl > HBr > HI
Again, due to the presence of hydrogen bond in Hydrogen Fluoride (HF) molecule, its melting and boiling points are higher.
Hence the intermolecular force between the molecules of HF compound increases which causes higher melting and boiling points.
But in case of HCl, HBr and HI, they can’t form hydrogen bond. Hence their melting and boiling points are lower than HF.