[smghz]

How many resonance structures does [IO3]- have? Here it says three, but I counted four. In fact, I've done the work. Am I doing wrong? thanks

[sjb]

On the document you linked it gives some rationale. In fact iodate (like many other molecules) has a lot more than the resonance structures mentioned. Some however are quite high energy and contribute little to the overall hybrid.

[Vidya]

On the document you linked it gives some rationale. In fact iodate (like many other molecules) has a lot more than the resonance structures mentioned. Some however are quite high energy and contribute little to the overall hybrid.

Yes  sjb is right and not all structures on the document are important because either they have too  many formal charges or negative charge on central atom. Three important structures which are main contributor for the resonance hybrid are with negative charge on one oxygen atom.