Central atom(N) in ammonia is having 1 lone pair and is having 3 bond pairs.

Central atom(O) in water  is having 2 lone pair and is having 2 bond pairs.

Thus, these 2 lone pairs on O- atom in water molecule repels the 2 bond pairs. And this repulsion is between lone pair and bond pair on O- atom of H2O is stronger than the repulsion is between lone pair and bond pair on N-atom of NH3.

Thus, the bond angle in H2O is less than NH3, even though they are having distorted tetrahedral structure.

Bond Angle in H2O and NH3: An Explanation

The bond angles in water (H2O) and ammonia (NH3) molecules are determined by the shape of their molecular orbitals and the repulsion between the electron pairs. Although both molecules have a distorted tetrahedral structure, the bond angle in water is smaller than in ammonia. This can be explained by examining the electronic and steric factors influencing the bond angles.

Electronic Factors:
The bond angles are influenced by the number of electron pairs around the central atom and their repulsion. Both water and ammonia have a central atom surrounded by four electron pairs, including two bonding pairs and two lone pairs.

1. Water (H2O):
- Oxygen (O) has six valence electrons. Two of these electrons participate in bonding with two hydrogen (H) atoms, forming two bonding pairs. The remaining four electrons form two lone pairs.
- The repulsion between the electron pairs causes the bonding pairs to push the lone pairs closer together, reducing the bond angle.
- The lone pair-lone pair repulsion is stronger than the bonding pair-bonding pair repulsion, resulting in a smaller bond angle of approximately 104.5° in water.

2. Ammonia (NH3):
- Nitrogen (N) has five valence electrons. Three of these electrons participate in bonding with three hydrogen (H) atoms, forming three bonding pairs. The remaining two electrons form a lone pair.
- The repulsion between the electron pairs causes the bonding pairs to push the lone pair closer, reducing the bond angle.
- The lone pair-bonding pair repulsion is weaker than the lone pair-lone pair repulsion, allowing the bonding pairs to spread out more, resulting in a larger bond angle of approximately 107° in ammonia.

Steric Factors:
Steric factors also play a role in determining bond angles. Steric factors refer to the repulsion between atoms or groups of atoms due to their size and shape.

1. Water (H2O):
- Oxygen is more electronegative than hydrogen, causing the oxygen atom to be slightly larger.
- The larger size of oxygen leads to increased steric repulsion between the bonding pairs, compressing the bond angle.

2. Ammonia (NH3):
- Nitrogen is less electronegative than oxygen, making the nitrogen atom smaller.
- The smaller size of nitrogen reduces the steric repulsion between the bonding pairs, allowing them to spread out more, resulting in a larger bond angle.

Conclusion:
In summary, the bond angle in water is smaller than in ammonia due to both electronic and steric factors. The presence of two lone pairs in water leads to stronger lone pair-lone pair repulsion, reducing the bond angle. Additionally, the larger size of oxygen compared to nitrogen increases the steric repulsion between the bonding pairs, further decreasing the bond angle. Ammonia, on the other hand, has one lone pair and a smaller central atom, resulting in weaker lone pair-bonding pair repulsion and reduced steric repulsion, leading to a larger bond angle.