There are 2 resonance structures for ozone.
Unlike sulfur dioxide, selenium dioxide is polymeric, so if you drew resonance structures for charged forms there would be an awful lot!
Ozone has two resonance structures.
[O=O-O] <--> [O-O=O]
3(three)
2
4
The sulfate ion has four resonance structures.
3 O-C=O O=C-O O-C-O O O ll O
There is only one resonance structure for BCl3. Boron is attached by three single bonds to the three Chloride atoms and there are three lone pair electrons around the chlorine atoms. There are no resonance structures because they are all single bonds.
there are 2
4
4
The sulfate ion has four resonance structures.
2 or more
one
2 or more
3
4
1
1
1
There are three possible, equally distributed, resonance structures of nitrate (NO3-).Cf. Related links for the possible Lewis structure of them, on the lowest half of that page.