I was wondering why H2O could form two hydrogen bonds per molecule, while NH3 and HF can only form one hydrogen bond per molecule. H2O has two lone pair electrons available and two H-atoms available for hydrogen bond formation; shouldn't this make it possible to form four hydrogen bonds (2 for the lone electrons + 2 for the hydrogen atoms)?.
For water, hydrogen bond is formed
between an -OH 'group'(I can't think of other expressions to say this)
AND a lone pair of electrons from another H
2O molecules.
In each water molecule, there are 2 'OH group' and two lone pair of electrons, or 2x('OH group + lone pair of electrons), so
on average, each water molecules can form two hydrogen bonds with each other.
Consider this diagram, you may think that a water molecule form 4 hydrogen bonds with other molecules. However, strictly speaking, not all the hydrogen bonds are 'form' the same water molecule, they actually come from other H
2O molecules.
Perhaps my wordings are not so specific and clear, but this is how I understand this problem.
Edit:
1. Resized image
2. The second time I opened your second link I can't see the picture. Can you post it again, using the 'attach' function in the 'Additional Options' under the posting area?