Resonance Structures

[Shibhon_Shepard]

Can someone please give a formal definition to what resonance structures are? and why we use them?

[Tam To 1B]

Resonance is a blending of structures.
For example, a nitrate ion lewis structure depicts N as the central atom with bonds to three O atoms. Nitrate has one double bond and two single bonds. The double bond is connected to an oxygen, however, it can be connected to any of the three O atoms.
This makes it a resonance because all three structures are interchangeable and the nitrate ion is depicted as a blend of all three Lewis structures rather than having just one Lewis structure.
The molecule is regarded as a blend of structures to show that the electrons are delocalized in a set of resonance structures.

[Shibhon_Shepard]

That makes so much more sense! thank you!

[Cade Okohira 4K]

I was wondering if for resonance structures do you need to have the same element of atom surrounding the central atom? And can there be more than one double bond for resonance to occur?

[Lucy Agnew 3E]

I was wondering if for resonance structures do you need to have the same element of atom surrounding the central atom? And can there be more than one double bond for resonance to occur?

In all the examples we have looked at and some I have looked at online, there seems to be only one double bond in the structure for resonance to occur. But im sure there are some exceptions!

[Jovian Cheung 1K]

I was wondering if for resonance structures do you need to have the same element of atom surrounding the central atom? And can there be more than one double bond for resonance to occur?

For Q1, I'd like for someone to answer this too. So far, I've never come upon a case where I have to consider placing the double bond between the central atom and a different element. But it's definitely true that resonance structures should have the same atomic arrangements. I think if the elements surrounding the central atom are different, then formal charges will be involved. Hopefully someone can provide a better explanation!

For Q2, yes, resonance occurs even in molecules with multiple double bonds! Check out the resonance structures of benzene :-)

[Jovian Cheung 1K]

That makes so much more sense! thank you!

Also note that in the "real" structure (in the real world), the molecule's bonds would be an actual blend of the bonds involved in the resonance structures! So their bond lengths etc. would also be the average. (E.g. the midterm question on resonance structures)

This is possible because for multiple bonds, the second/third bond is actually a pi bond which involves delocalisation of electrons. (https://socratic.org/questions/what-is- ... ed-pi-bond)

Hope this helps! :o)