Science Forums 2

[HydrogenBond]

The angle discussed is true for an isolated water molecule. The unbonded electrons repel each other, forcing the hydrogen to get a little closer than a tetrahedron. The extra strain put on the hydrogen is a reflection of the higher electronegativity of oxygen. Oxygen wants the electrons more and isn't going to make it easy for hydrogen.

When you talk about hydrogen bonding angle, this is different, since it involves more than one water molecule. The tetrahedral of OH- bonds does form in some versions of ice because water will expand as ice. The hydrogen gets the better tetrahedral angle but loses something by being pushed further away during expansion. Oxygen is still being stubborn with electrons, so it gives and takes.

The thing about hydrogen bonds is that a hydrogen bond is maximized if it form a nice straight line with the shared electrons on another water. 180degrees. The tetrahral in ice allows all the hydrogen bonds to line up in straight lines. Beiing pushed away to reflect expansion, puts them at the correct distance for partial covalent bonding to also form. This is how the oxygen gets even and assert its higher electronegativity.

In liquid water, the tetrahedral state is not as common. Water forms all type of random and extended structure. But hydrogen just can't stay very long in the tetrahedral. With water denser than ice, the oxygen can't push hydrogen away for the tetrahedral to be stable, so hydrogen assumes placement that is less then a perfect tetrahedral. What this does, is make the average hydrogen bond angle with the shared electrons different than 180, with bond strength falling off rapidly with deviation from linearity. This is another trick by oxygen to assert its higher electronegativity.