Hydrogen Bonding

Covalent bonding between hydrogen atoms and small highly electronegative atoms like oxygen, nitrogen, and fluorine, result in a secial type of bond among these molecules known as a hydrogen bond. Hydrogen bonds occur between molecules and so it is a fundamentally different type of bond than ionic or covalent bonds,that form between atoms.

When hydrogen bonding does occur, it greatly affects bulk properties.  Liquid water molecules make and break many hydrogen bonds as the molecules move around. For each molecule, the average number of hydrogen bonds is about 3.4. When water solidifies molecular movement is slowed. Many of the hydrogen bonds are broken as these molecules form ice crystals. As a result, the number of hydrogen bonds between molecules is reduced leaving the individual atoms much farther apart than they were in the liquid state.  The result is a lower density solid than liquid.


 

The hydrogen bonds between the water molecules on the left are shown as dashed lines. The molecules on the right are frozen and have assumed the characteristic shape of solid water crystals

These images are taken from the MathMol website where you will find a great deal more information about the water molecule.    

Hydrogen bonding significantly raises the boiling points of liquids in which it occurs.  Because of the increased energy that is necessary to separate hydrogen bonded molecules from each other, these liquids require much larger amounts of energy to pass from the liquid to the gaseous phase.