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I am doing a multiple choice question for which one of the possible answers is, for the Lewis structure of NO3- (one of the oxygens is double bonded to the nitrogen and the other two are single bonded),

"one of the N-O bonds is shorter than the other two".

Apparently this is not the right answer, but my understanding was that a double bond is shorter than a single bond. I just googled it and I'm reading that the more bonds there are the shorter the bond length. So shouldn't that answer be correct?

Thanks!

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    $\begingroup$ Resonance, resonance... all three bonds are equal. $\endgroup$ – orthocresol Dec 1 '18 at 21:37
  • $\begingroup$ Could you explain that? Is it just that the "double bond" moves around equally around the N? $\endgroup$ – M Do Dec 1 '18 at 21:39
  • $\begingroup$ It's not really a duplicate (IMO), but it contains the answer to this question nevertheless. $\endgroup$ – Martin - マーチン Dec 2 '18 at 20:11
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The main reason for your answer being wrong is the phenomenon of resonance in nitrate ion dur to presence of a conjugated system. The resonance is shown in the figure below. Dur to resonance none of the cannonical structures explain the bond length in nitrate except the resonance hybrid ( which happens to be the last drawn structure). Due to resonance all bond lengths would be the same enter image description here

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