What is the reason for water molecules $\ce{H_2O}$ being asymmetric while other molecules like $\ce{CO_2}$ being completely symmetric? See the pictures below:

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In both case there is an electronegative difference between the atoms in the molecule. I have read that the angle in the water molecule is always a specific value. Is this a special thing for water or is there a common mechanism I should know?

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    $\begingroup$ That's because of the lone pairs of oxygen. We have the notorious VSEPR, the mathematical hybridization, and the ultimate molecular orbital theory to explain this. Have you studied any of them? $\endgroup$ – M.A.R. Jun 8 '15 at 18:13
  • $\begingroup$ Related: Why is H₂O a dipole? $\endgroup$ – user7951 Jun 8 '15 at 18:39