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Which compound has resonance structures?

A. $\ce{C6H12}$
B. $\ce{CH3CHO}$
C. $\ce{NaBr}$
D. $\ce{Na2CO3}$

The answer key says D is correct, but I do not understand why $\ce{Na2CO3},$ which is an ionic compound between the polyatomic ion carbonate and sodium, can have resonance structures.

The other choices are equally confusing.

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  • 3
    $\begingroup$ Draw the structure of the carbonate ion, and you'll see. $\endgroup$ – Ivan Neretin Aug 13 at 5:26
  • $\begingroup$ Arguably, A and B both have resonance structures, although those structures are not as good compared to what you could draw for D. $\endgroup$ – Zhe Aug 14 at 2:11
  • $\begingroup$ @Zhe what are the resonance structures in A and B? I can only see possible hyper conjugation in B but nothing in A $\endgroup$ – user78585 Aug 14 at 13:06
  • $\begingroup$ Technically speaking, you can just break a carbon-hydrogen or carbon-carbon bond and create an ion pair. You might do this to describe hyperconjugative stabilization of a cation or a 1,2-group shift. Like I said, it's not as good of a resonance structure as for D. Note that I didn't make this suggestion for C because that's almost purely an electrostatic interaction. But if you wanted to go nuts, draw a covalent bond between those two atoms. It is a resonance structure for the standard charge separated ion pair. $\endgroup$ – Zhe Aug 14 at 17:00
  • $\begingroup$ @Zhe ok I get your point $\endgroup$ – user78585 Aug 15 at 5:52
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Resonance Structures of Carbonate Ion

As you can see in this structure 3 equivalent resonance structures are possible by simple conjugation between negative charge and the π bond. Hence being covalent is not a condition for possible resonance even compounds with some ionic character can show resonance besides all compounds have some ionic and covalent nature both present.

In the other options there is no scope for resonance as none of the resonance conditions are formed

Source- https://commons.m.wikimedia.org/wiki/File:Carbonate-ion-resonance-2D.png

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