Question

Can a polyatomic ion have no dipole moment?

Answer 1

Yes if all atoms within the molecule are the same, which in reality means no.

Explanation:

If two (or more) different kinds of atoms are bonded you will always experience a polarity of the bond which in turn will lead to a dipole moment.

In practice, these dipole moments can sometimes be so small that they are for the most part considered negligible. Sometimes, however, their effect is great - water is a great example of this.

Polyatomic molecules never really have only one kind of atom in them (usually that is diatomic molecules) and so usually do have a dipole moment. If however, all bonding atoms were the same or overall electron distribution was ‘equal’ then yes, it is possible for there to be no dipole moment.