Here's a breakdown:
Water (H₂O):
* Oxygen has two lone pairs of electrons and two bonding pairs with hydrogen atoms.
* Lone pairs exert a stronger repulsive force than bonding pairs due to their greater electron density and less localized nature.
* This repulsion forces the bonding pairs closer together, resulting in a smaller bond angle of 104.5°.
Ammonia (NH₃):
* Nitrogen has one lone pair and three bonding pairs with hydrogen atoms.
* The lone pair again pushes the bonding pairs closer together, reducing the bond angle to 107°.
In summary:
The presence of lone pairs on the central atom in water and ammonia leads to a decrease in the bond angle compared to the ideal tetrahedral angle. This is because lone pairs have a greater repulsive force than bonding pairs, forcing the bonding pairs closer together.
