When we consider the hydrides of group VA, VIA, VIIA then NH3, HF and H2O show maximum boiling points in the respective series. The reason is the enhanced electronegative character of N, O and F. That is why, water is liquid at room temperature, but H2S and H2Se are gases. Comparatively high boiling points of SbH3, HI and H2Te are again due to high polarizabilities of Sb, I and Te.
These series show that the boiling point of water is extremely high as compare to boiling point of HF. Fluorine is more electronegative than oxygen. So, we should expect H-bonding in HF to be stronger than that in water and as a result the boiling point of HF should be higher than that of H2O. However it is lower and the reason is that the fluorine atom can make only one hydrogen bond with electronegative hydrogen of a neighboring molecule but water can form two hydrogen bonds per molecule, as it has two hydrogen atoms and two lone pairs on oxygen atom.
Ammonia can form only one hydrogen bond per molecule as it has only one lone pair.
