Water is Wierd!

Amazingly one of the most amazing compounds on our planet is also one of the most common, water! Found naturally in all three states on this planet it is in fact an oddity. Looking at other hydrogen containing compounds of the group 6 block we would expect it to boil at around -100oC. We would expect it to boil at this temperature simply because of the intermolecular interactions called Van Der Waals’ forces (or more specifically instantaneously induced dipole – dipole interactions)

This strange anomaly also occurs between molecules with N-H Bonds and F-H bonds as can be seen on this graph. you can clearly see that other than N, O and F the general trend is for the boiling point to rise down these groups, however N, O and F are really strange

We all know that water boils nearly 200oC higher at 100oC. The big question is WHY!?

There must be something holding them together to keep the molecules from being far away enough to exist as a gas. This is called a Hydrogen bond. Hydrogen bonds are very special as they are much stronger interactions and only under certain conditions.

  1. A Hydrogen must be bonded to a small, highly electronegative element (These are only N, O or F)
  2. This will result in a strong permanent dipole between the hydrogen atom and the N, O or F atom.
  3. The now δ+ hydrogen becomes attracted to a lone pair on a δ N, O or F.

There are loads of other posts on hydrogen bonding and water on my site. I was searching through the older posts and discovered this one from my old molecule of the month section which I would recommend all have a read of.