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In the course reader around page 73, there is an example of resonance for sulfur dioxide (SO2). It is a set of three resonance structures. In the first one, all three atoms have a formal charge of 0, but in the second and third structures, oxygen has a formal charge of -1 and sulfur has a formal charge of 1. The first structure has two double bonds, while the other two have one double bond and one single bond. My question is, if we were asked to draw the Lewis Structure for SO2 on an exam, would we have to draw all three of those resonance structures? Or would we only have to draw them if it was specified in the question? Also, should we still do resonance even though the first structure is the most stable structure? Can someone please explain it to me?
You would only draw all of the resonance structures if you are asked to. Otherwise, if you were asked just to draw one structure, it would be best to draw the most stable one, but as long as you show that you can draw the Lewis Structure, I think you should be fine, so don't worry, the question should specify what it wants you to do.
You only need to draw all of the resonance structures if the question asks you to. If the question asked for the most stable resonance structure, until you draw all of them and calculate all the formal charges, you wouldn't necessarily know that the first one you draw is the most stable. I suppose if the FC was 0 for all of the atoms then you could assume so, but it's still probably best to draw out each Lewis structure and do the FC calculations in order to compare and show how you know that.
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