Debates about an exact model of the chemical bonding of the H₂0 molecule have been running for more than 50 years.

There are two main models, the Molecular Orbital Theory and the Valence Bond Theory (described at Wikipedia).

Neither theory (both of which which rely on quantum physics rather than classical mechanics), is sufficient to explain the observed properties of the water molecule.

Molecular Orbital Theory vs. Valence Bond Theory has been a topic of debate since the early to mid 1900s. Despite continued heated debate on which model more accurately depict the true bonding scheme of molecules, scientists now view MO and VB theories as complementary and teammates.
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The truth is, both theories are equally important in understanding chemical bonding that while neither theory is completely comprehensive, the two together nonetheless provides a in-depth model for chemical bonds.

Source : Wikipedia

Note : The observed 'bond angle' of the asymmetric H₂0 molecule is measured at 104.45° - but current theories calculate it at 109.5° - again hinting at a lack of a full understanding of the physical processes involved.

Also see : Water clustersplugin-autotooltip__plain plugin-autotooltip_bigWater clusters

Water has a long list of 'anomalous' physical (and chemical) properties (list here, archived from London Southbank University). Most are now fairly well understood - but there is currently no overall model to explain all of its unusual properties

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