Ammonia (NH3) is a polar molecule. A polar molecule is one that has a non-uniform distribution of electron density, resulting in a partial positive charge on one end (the hydrogen atoms) and a partial negative charge on the other end (the nitrogen atom).
In the case of ammonia, the nitrogen atom is more electronegative than the hydrogen atoms. As a result, the nitrogen atom attracts the shared electrons in the ammonia molecule more strongly, leading to a partial negative charge (δ-) on the nitrogen atom and partial positive charges (δ+) on the hydrogen atoms.
The polarity of ammonia makes it capable of forming hydrogen bonds. Hydrogen bonding occurs when a hydrogen atom bonded to a highly electronegative atom (such as nitrogen, oxygen, or fluorine) interacts with another electronegative atom in a different molecule. In the case of ammonia, the nitrogen atom can form hydrogen bonds with other electronegative atoms, such as oxygen, in other molecules.
Hydrogen bonding is a special type of intermolecular force that is stronger than typical dipole-dipole interactions. It contributes to the unique properties of substances that exhibit hydrogen bonding, such as higher boiling and melting points, greater solubility in water, and increased viscosity.
So, to summarize, ammonia is a polar molecule capable of forming hydrogen bonds due to the electronegativity difference between nitrogen and hydrogen atoms.