Here's why:
* Electronegativity Difference: Oxygen (O) is more electronegative than nitrogen (N). This means that the oxygen atom in water attracts the shared electrons in the O-H bonds more strongly than the nitrogen atom in ammonia attracts the shared electrons in the N-H bonds. This creates a larger partial negative charge on the oxygen atom in water compared to the nitrogen atom in ammonia.
* Molecular Geometry: Both water and ammonia have a bent molecular geometry due to the presence of lone pairs on the central atom. However, the lone pair on the nitrogen atom in ammonia is smaller and less electron-dense compared to the lone pairs on the oxygen atom in water. This contributes to a weaker dipole moment in ammonia.
* Dipole Moment: The dipole moment, which is a measure of the polarity of a molecule, is higher for water (1.85 D) than for ammonia (1.47 D). This indicates that the water molecule has a stronger separation of charge and is therefore more polar.
In summary:
While both ammonia and water are polar molecules, water is more polar due to its higher electronegativity difference, larger lone pairs, and stronger dipole moment.
