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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.

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  • $\begingroup$ In the end, you do have to draw the molecules. $\endgroup$ – Jan May 25 at 10:27
  • $\begingroup$ Incidentally, the answer is C although old textbooks might give it as D :D $\endgroup$ – Jan May 25 at 10:29
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  1. You don't have to do anything for benzene because it occurs in every choice.
  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.
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    $\begingroup$ 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. $\endgroup$ – Jan May 25 at 10:35
  • $\begingroup$ When you say "compare" do you indicate that I have to draw them? On a different note, are you danish? Your name seems danish. $\endgroup$ – Carl May 25 at 10:38
  • $\begingroup$ @Jan Yes, I'm not saying it's a perfect strategy. In fact, I think it is a bad strategy for multiple reasons. $\endgroup$ – Karsten Theis May 25 at 10:41
  • $\begingroup$ @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? $\endgroup$ – Karsten Theis May 25 at 10:45
  • $\begingroup$ You are right. I made a mistake copying the problem. I edited the question. $\endgroup$ – Carl May 25 at 10:46

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