H-O-H bond angle in H_2 O is 104.5^ and not 109^ 28^ because of

The hybridisation of oxygen in water molecule is $s p^3$. So, water molecule must be tetrahedral and have bond angle $109^{\circ} 28^{\prime}$. But due to the presence of two lone pair of electrons on oxygen atom (repulsion between lone pair-lone pair) the bond angle decreases about $104.5^{\circ}$ and the geometry becomes distorted tertrahedral or V-shaped.