# Fastest way to determine if molecule has resonance structures [closed]

I came across this old exam problem:

For which of the following choices, is every molecule a resonance hybrid with resonance structures?

A. $$\ce{SO2},$$ $$\ce{NH3},$$ benzene
B. $$\ce{SO3^2-},$$ $$\ce{NH3},$$ benzene
C. $$\ce{SO2},$$ $$\ce{NO3-},$$ benzene
D. $$\ce{SO3^2-},$$ $$\ce{NO3-},$$ benzene
E. $$\ce{SO3},$$ ethanol, benzene

So, in order to solve this problem I don't actually have to draw any Lewis structure and what not, I just have to determine which molecules have resonance structures. But what is the fastest way to do that?

How can I determine if a molecule has resonance without having to draw anything, if that's even possible? I hope someone can help me out.

• In the end, you do have to draw the molecules. – Jan May 25 at 10:27
• Incidentally, the answer is C although old textbooks might give it as D :D – Jan May 25 at 10:29

2. You have to compare $$\ce{SO2}$$ and $$\ce{SO3-}$$ because they distinguish A) from B) and C) from D).
3. You have to compare $$\ce{NH3}$$ and $$\ce{NO3-}$$ because the distinguish A) and B) from C) and D).
4. If you use this strategy, you miss out on the chance to figure out $$\ce{SO3}$$, ethanol and benzene, and you would learn less.
• Concerning 4: not really. For example, it could be that e.g. both $\ce{NH3}$ and $\ce{NO3-}$ have none and it could be that more than one line is correct. – Jan May 25 at 10:35
• @Carl - I can visualize them in my head because I already drew them "a couple" of times. $\ce{NH3}$ has just one none-hydrogen atom, so any resonance would have to invoke something unusual. I don't understand the question - what does "one or more resonance structures" mean? How can you have resonance with a single structure? – Karsten Theis May 25 at 10:45