NH4+, commonly known as the ammonium ion, is considered to be polar. This polarity is primarily due to the molecular geometry and the presence of a positive charge.
The ammonium ion has a tetrahedral shape, with one nitrogen atom centrally located and four hydrogen atoms arranged around it. Because nitrogen is more electronegative than hydrogen, there is a slight difference in charge distribution. However, the symmetrical arrangement of hydrogen atoms around the nitrogen atom means that the dipole moments of the N-H bonds cancel each other out to some extent.
Despite the cancellation, the presence of a positive charge and the asymmetry created by the nitrogen atom being more electronegative implies that NH4+ has polar characteristics. In solution, NH4+ interacts with polar solvents like water, further supporting its classification as a polar ion.