You can have a polarized ion. Think about the iodide ion, I-. If you average everything out, it's roughly spherical and pretty uniform. However, there is the possibility of a build-up of electrons at one side of the ion, leaving a slightly positively charged "gap" on the opposite side. The outer electrons in a large ion are not held as tightly because they are so far from the nucleus, therefore these polarizations are not entirely impossible. These polarized forms are fleeting, for sure, put they do represent a polar, monoatomic ion.
This is very similar to London dispersion forces in alkanes.