The electronegativity of nitrogen (N) is 3.0, while the electronegativity of hydrogen (H) is 2.1. This means that N has a stronger attraction for electrons than H. As a result, the electron pair in the N-H bond is pulled closer to N, creating a partial negative charge on N and a partial positive charge on H. This results in a polar bond.

The polarity of the N-H bond also gives rise to the overall polarity of the NH3 molecule. The nitrogen atom has a partial negative charge, while the three hydrogen atoms have a partial positive charge. This creates a net positive charge on the hydrogen side of the molecule and a net negative charge on the nitrogen side. This makes NH3 a polar molecule.