Could you please explain how the bent molecular geometry of water is due to hydrogen bonding? I was under the impression that it was because of the lone pairs (VSEPR Theory).

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    $\begingroup$ Your impression is correct. The bend in the water molecule doesn't have anything to do with hydrogen bonding, as evinced by the fact that isolated water molecules retain their bent geometry. $\endgroup$ – Richard Terrett Dec 23 '12 at 0:34

Your understanding is correct. The usual explanation given in introductory classes is VESPR theory. The oxygen atom has four bond-like "things" coming out of it. Two of those are the covalent bonds with the hydrogen bonds. The other two are the two lone pairs. VESPR theory calls this a steric number of four.

The slightly more advanced explanation comes from molecular orbital theory. Oxygen's valence electrons are all $2\text{p}$ electrons; hydrogen's electron is $1\text{s}$. The only way to mix those orbitals to get enough bonding orbitals is by using two $\text{sp}^3$ hybrid orbitals to generate the two bonds. The $\text{sp}^3$ orbitals are arranged in a tetragonal geometry.

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