NH3, also known as Ammonia, is a polar molecule. While the molecule has a net charge of zero, it has an uneven distribution of shared electrons, resulting in positive and negative poles.
When ammonia forms, three hydrogen atoms form a covalent bond with a nitrogen atom. The molecule takes on a pyramidal shape, with the hydrogen atoms at the base and the nitrogen atom at the apex. The space between the apex and base is where the bond occurs. This pulls the hydrogen electrons away from base of the molecule, leaving that side with a positive pole. The extra electrons on the side of the nitrogen atoms forms a negative pole.