Escola Secundária de Loulé, Portugal
Water molecule has an angular geometry due to the existence of two lone pairs of electrons around de oxygen atom.
The pairs of electrons have a repulsive effect over each other because they have the same charge. To minimize these repulsions one should expect that the four pairs of electrons around oxygen would have the maximum separation between themselves, which should be achieved by having a tetrahedral distribution around the central atom. This would mean that the angles between them are of 109.5º.
But this is not what happens. The repulsions of the lone pairs over the bond electrons are bigger than the repulsions between the bonding pairs. This gives the distribution of the electron pairs around oxygen the geometry of a distorted tetrahedron which has the consequence that the angle between chemical bonds is 104.5º.
Waters bond length is 95.7 pm (1 pm = 1 x 10-12 m).
Oxygen is more electronegative than hydrogen. This means that the electrons of the chemical bonds will be in average closer to the oxygen atom. As a consequence of the unequal share of the electron pairs there is a zone with negative charge density around oxygen and one can say that the region around the hydrogen atoms has a positive charge density. The fact that the molecule is angular allows us to think that it has two "sides": one with the hydrogen atoms and one with the oxygen atom. From this point of view waters molecule is an electric dipole with the negative pole closer to the oxygen atom and the positive pole closer to the hydrogen atoms.
Due to waters molecule polarity this substance has important physical properties as the capability of solving polar or ionic substances because it establishes strong intermolecular interaction with them.
On the other hand it cannot solve non polar substances because the intermolecular interactions with the molecules of these substances are much weaker than the interactions that water molecules establish with each other.
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