To my knowledge, resonance happens where more than one lewis stucture can represent the compound with no shift in sigma bonds

so, suppose in a simple conjugated system even like 1-3 pentadiene , two forms are well known, i want to know if a third form where each atom has completely filled or completely empty p orbitals should also be taken into consideration, as technically, there is nothing wrong with it, I accept that it's "contribution" will be negligible, but it should be counted in all POSSIBLE resonating structures


similar argument here, a lot more can be created if we allow charge separation of pi bond

EDIT: The question is whether there is a convention of some sort which tells us which ones to count which ones not (if we cant tell the magnitude of difference in stability of the structure)

  • $\begingroup$ These resonance structures are valid but they usually provide a very small contribution to the overall structure. $\endgroup$
    – bon
    Commented Feb 1, 2016 at 21:41
  • $\begingroup$ I wrote that :) $\endgroup$
    – Mrigank
    Commented Feb 1, 2016 at 21:42
  • $\begingroup$ So what is your question? What you have written is correct. $\endgroup$
    – bon
    Commented Feb 1, 2016 at 21:45
  • $\begingroup$ in a question where it asks us to tell the total number of resonating structure, theoretically, not only the ones in majority, so is there a convention which limits certain kind of structure to not be included? $\endgroup$
    – Mrigank
    Commented Feb 1, 2016 at 22:10
  • $\begingroup$ also, chemistry.stackexchange.com/questions/15665/… here, how a sigma bond can be broken in resonating structures? i thought that it was a necessary condition taht only pi electrons move $\endgroup$
    – Mrigank
    Commented Feb 1, 2016 at 22:37

1 Answer 1


Resonances structures are a model of the quantum mechanical orbital mixing. To my mind they are a complete farce. They are also quite useful

A model is a simplification of a problem, usually based on ignoring negligible factors. So this question really distills down to what is negligible and what isn't?

That depends on roughly two factors:

  1. The conditions of reaction.

Are the conditions strongly ionic or acidic? Then charge separation may become relevant.

  1. The rigor of the problem.

Is this for a high school course? A Ph.D. thesis? A bar-room chat with your mates?

Unfortunately, like most of chemistry, there is not an absolute answer here.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.