In this post I got an answer to the question "what is resonance".

What I understand overall is that "resonance is not something really happening physically, it is just an idea to make people understand a concept."

But a few doubts remain:

Previously I thought that due to the movements of delocalised electron(s), the pi bond and charge keep shifting rapidly, like various structures physically interchanging into one another very quickly as electrons keep revolving. That's why we use dotted lines to show that bonds can exist in two places at the same time.

But after reading the answer to my previous post I think I was wrong. Then please explain to me what those dotted lines actually mean. Are they a special type of bond or something else?

Does that mean that when we say, for example, that ozone has one double bond and two single bonds, that this is in fact not true!? Because in a hybrid structure we have one bond completely different in both structures, somewhat dotted and straight, with bond length inbetween.

Then, how is an electron shared in the case of a hybrid bond? Normally an octet (generally) is completed by a single bond (2 electron sharing) or a double bond (4 electron sharing). How many electrons are shared in a dotted bond, and how, if at all?

  • $\begingroup$ Have you considered browsing the site and looking for similar questions? Also, please check grammar etc to the extent possible before posting. $\endgroup$
    – Buck Thorn
    Sep 8, 2022 at 15:11

1 Answer 1

  • To address your first question:
    You (erroneously) believe that resonance structures are in dynamic equilibrium. Your description “the $\pi$ bond and charge keep shifting rapidly , like various structures are Physically Interchanging into one another very fastly” is exactly what Dynamic Equilibrium is. For eg. in the equilibrium $\ce{N_2 +3H_2\rightleftharpoons2NH_3}$, the nitrogen and hydrogen constantly combine to produce ammonia, while the ammonia also simultaneously decomposes to provide nitrogen and hydrogen gases. The rates of the forward and backward reactions are equal at equilibrium.
    Resonance forms are NOT in equilibrium. That is why we use $\leftrightarrow$, and not $\rightleftharpoons$ to denote resonance forms, to indicate that they are, indeed, equivalent forms of the same molecule. enter image description here
  • The dotted line refers to the fact that the properties of the bonds in the ozone molecule are intermediate to that of single and double bonds. From Wikipedia, BOTH O-O bonds are equivalent, i.e. have exactly the same properties (bond length, bond enthalpy etc). That wouldn’t be true if there actually were a single bond and a double bond. The dotted line shows that the electron density in each O-O bond in ozone is intermediate to that in single and double bonds. enter image description here
  • When we draw the above structure of ozone, we are drawing a Lewis structure. This is the best we can come up with to describe the behaviour of ozone (yes, this is not how ozone is actually). The charges that you see on the oxygen atoms are formal charges. They DO NOT actually appear on the atoms individually. The charges are just written to keep track of the valence electrons of the atoms when drawing the inaccurate description of the ozone molecule by the Lewis structure. The overall molecule is neutral, while the central atom acquires a slightly less electron density (called partial positive charge) and the side atoms acquire greater negative charge density.
  • $\begingroup$ Resonance is real Electron orbitals arrange to give maximum attraction to the various nuclei while minimizing electron repulsions. Our simple interpretations while pretty amazing in themselves might not be able to truly represent electron distributions. So we make up canonical contributing structures usually further from reality and worry about their contribution. $\endgroup$
    – jimchmst
    Sep 9, 2022 at 2:11

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