Hydrogen bonding strongly influences the properties and structure of the water molecule, where hydrogen is bound to electronegative oxygen.
The oxygen centre polarizes electron density towards itself, and we could represent this polarization by #""^(delta+)H-O^(delta-)-H^(delta+)#. In macroscopic solution, these dipoles align between different molecules, and in aggregate they constitute a potent intermolecular force. Hydrogen bonding is largely responsible for the high boiling points of water, and ammonia, and hydrogen fluoride, in comparison with the homologous hydrides. Compare the boiling point of water, with that of #H_2S#, #-60# #""^@C#, where hydrogen bonding is not so extensive.
I should add a final definition: #"electronegativity"# refers to the ability of an atom involved in a chemical bond to polarize electron density towards itself. Small, 2nd row elements, to the right of the Periodic Table, i.e. oxygen, fluorine, nitrogen, tend to be highly electronegative.
