Which of the molecules below must be shown with resonance structures?

I. C2H2
II. CO
III. CO2

Question

Which of the molecules below must be shown with resonance structures?

I. C2H2
II. CO
III. CO2

espex · Answer

The only one that could have a resonance structure is CO2. You could end up with a single/triple bond or double/double bond. The other two molecules do not have any other ways to bond.

anonymous · Answer

I disagree.  None of these molecules exhibit resonance.

espex · Answer

That wasn't one of the options, so despite the accepted structure, the only one that could produce a resonance has to be CO2.

anonymous · Answer

You can draw resonance structures for any of them.  That just means you can draw the same nuclear framework with a different arrangement of electrons.  No doubt in certain reactions these "minor resonance structures" are important.  But the question was whether any of the molecules "must" be shown with resonance -- and the answer is that none of them must be.

I can't help it that there isn't a correct answer to the question.  Pointing that out may have some utility, as the original author may realize he mistranscribed the question, for example.  Further, if I had set this exam, I would rather my students pointed out that the question is unanswerable or illogical on its face, rather than attempt to guess at some very peculiar interpretation of the question.  That demonstrates a better grasp of the concepts.

espex · Answer

You cannot draw a resonance structure for either the C2H2 molecule nor the CO molecule as the electron configuration affords you only one possible combination, the CO2 however does have the potential for several different structures.

anonymous · Answer

Ah, I get it.  You're requiring the octet rule for what you're willing to call a "resonance structure."  OK, fair enough.  And very likely the context in which the original question was meant.

anonymous · Answer

Since each hydrogen atom has just one electron, it can form only a single covalent bond. Therefore, the carbon atoms in C2H2 are connected by a triple-bond. The carbon and oxygen atoms in CO are joined by a triple-bond and in CO2 by double-bonds. In each case, the octet rule is completely satisfied without any electrons having to move within the molecule constantly.