[maccha]

Can NO2+ have resonance structures? I seem to be able to draw them but my textbook says they can't..

[Yggdrasil]

There are no major resonance structures.  The only ones you could draw would involve two atoms carrying charges which is unfavorable compared to the case where only one atom is charged.

[maccha]

Okay thanks. I seem to be confused about the definition of resonance structures then.. I assumed it was all reasonable chemical species, but is it just when the most favourable structure can have different bonds in different places?

[Yggdrasil]

It's a matter of practicality.  A certain molecule can have a bunch of different resonance structures, but only a few of those resonance structures will actually contribute to the resonance hybrid and affect the properties of the molecule.  Do we count the structures that contribute very little to the resonance hybrid?  If we want to be technically correct, then maybe yes.  If we want to be practical, then the answer is probably not.

[maccha]

Okay that really clarifies things, thank you so much!