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We know that the molecule of H₂O is V-shaped. This is what makes it a dipole.

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But why is that? I mean, if the hydrogens have a partial positive charge, then they should try to get away from each other, until they are diametrically opposite to the oxygen molecule. But that doesn't happen. Why is this?

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    $\begingroup$ If you're interested at why molecules have a given shape, read up on VSEPR theory (Valence shell electron pair repulsion), which is the typical undergraduate-level model for understanding/explaining these things. $\endgroup$
    – F'x
    Aug 25, 2013 at 10:07

4 Answers 4

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The oxygen atom fills its octet rule by forming two bonds. It shares 1 electron in a covalent bond with each hydrogen and has 4 remaining valence electrons. It is sp$^3$ hybridized and has the 4 non-bonding electrons in two lone pairs. An sp$^3$ hybridized atom has four attachment points spaced approximately 109$^\circ$ apart and has the shape of a pyramid with a triangular base. See this image for an example. You don't typically see the water molecule drawn in 3 dimensional space with the lone pairs of electrons, all you see is a planar molecule with H-O-H bond angle of ~109$^\circ$.

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It is not the + charges on hydrogens, but the negative charges on the O-H bonds, and on the O it self that is dominant in determining the molecule's shape. There are four non-bonding electrons on O -two lone pairs- on top of the two O-H bonds.

So you should think of four groups around O, not two.

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I mean, if the hydrogens have a partial positive charge, then they should try to get away from each other.

Well, this is not a complete answer to your question, but this could be a factor I guess.

Bond pair – lone pair repulsion is always greater than bond pair – bond pair repulsion

So what I want to say is that the repulsion between the O---H bond and the lone pair is so heavy such that it is really hard for two hydrogen atoms to get away each other as you are expecting to happen.

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In the bonding of Oxygen with Hydrogen, Oxygen ends up with 2 extra electrons, giving it what is called the "bent" shape. This is because the attachments and the electrons all repel each other but the electrons are not drawn except in a Lewis Dot structure such as the one here: http://en.wikipedia.org/wiki/File:Water-2D-flat.png

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    $\begingroup$ Oxygen has four extra electrons: two lone pairs. $\endgroup$
    – hBy2Py
    Dec 12, 2016 at 22:19

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