Final answer:
The ammonium ion has polar bonds but because of its tetrahedral molecular structure, the individual bond dipole moments cancel out, leaving it with no net dipole moment.
Step-by-step explanation:
The ammonium ion contains polar bonds due to the difference in electronegativity between nitrogen and hydrogen atoms. Even though these bonds are polar, the ammonium ion has a tetrahedral molecular structure, which is highly symmetrical. Consequently, the individual bond dipole moments in the ammonium ion point away from the nitrogen atom toward the hydrogens symmetrically, leading to a cancellation of these local dipoles. This is why the ammonium ion has no overall dipole moment. A molecule like boron trihydride (BH3) demonstrates a similar concept as it is also symmetrical with bond dipoles canceling out. In contrast, a molecule such as difluoroamine (NHF2) with a trigonal pyramidal geometry is not symmetrical, and hence, it has a net dipole moment.