Oxygen is sp3 hybridised in H2O molecule. Two hybrid orbitals are occupied by lone pairs and two are used in sigma bonding with hydrogen atoms. Now, the expected tetrahedral angle is 109.5° but the actual bond angle observed is 104.5°. This because from VSEPR theory, we know that lone pair-lone pair repulsions are greater than bond pair-bond pair repulsions and the same situation we observe here. As a result, bond angle in water is 104.5°.
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