Number of Resonance Structures

$FC=V-(L+\frac{S}{2})$

Raashi Chaudhari 3B
Posts: 64
Joined: Wed Sep 30, 2020 9:31 pm

Number of Resonance Structures

How do we know how many resonance structures there are for one molecule? My TA said we need to draw all the Lewis Structures to determine which structure is the most stable (which would be the best answer), but how do we know how many structures we need to draw?

Mikayla James 2C
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Re: Number of Resonance Structures

Hi! From my understanding, there's no easy way to know or calculate how many resonance structures there are for a molecule, you just have to practice knowing how to identify them when drawing the Lewis structure. Hopefully, we will be given relatively simple ones if asked on the midterm, like C4H4, but I think a very basic rule for identifying resonance structures would be if there is a lone pair or a single bond next to a double bond (someone correct me if I'm wrong) where the bonds can be switched without changing the number of electrons. As for the most stable resonance structure, I believe that that would be the structure with the lowest formal charge. Hope this helps!

Alessia Renna 1D
Posts: 51
Joined: Wed Sep 30, 2020 9:36 pm

Re: Number of Resonance Structures

A resonance structure means that you can draw two or more Lewis structures for a molecule. You can do this when you are able to change the locations of the electrons within the structure (I.E. if there are two O molecules in a structure and one has a double bond, you can form the resonance structure by making the single bonded O a double bond and the double bonded O a single bond).

Racquel Fox 2I
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Joined: Wed Sep 30, 2020 10:11 pm

Re: Number of Resonance Structures

I know that resonance structures are any molecule with 2 or more Lewis dot structures, but I'm pretty sure we wouldn't get a really hard, time-extensive one on the midterm, so I wouldn't worry too much about that :)

Raashi Chaudhari 3B
Posts: 64
Joined: Wed Sep 30, 2020 9:31 pm

Re: Number of Resonance Structures

Mikayla James 2C wrote:Hi! From my understanding, there's no easy way to know or calculate how many resonance structures there are for a molecule, you just have to practice knowing how to identify them when drawing the Lewis structure. Hopefully, we will be given relatively simple ones if asked on the midterm, like C4H4, but I think a very basic rule for identifying resonance structures would be if there is a lone pair or a single bond next to a double bond (someone correct me if I'm wrong) where the bonds can be switched without changing the number of electrons. As for the most stable resonance structure, I believe that that would be the structure with the lowest formal charge. Hope this helps!

Thank you so much!! For the formal charge can't you also determine which is the most stable by the structure that has the least amount of charges within the molecule?

Tobie Jessup 2E
Posts: 64
Joined: Wed Sep 30, 2020 10:02 pm

Re: Number of Resonance Structures

Mikayla James 2C wrote:Hi! From my understanding, there's no easy way to know or calculate how many resonance structures there are for a molecule, you just have to practice knowing how to identify them when drawing the Lewis structure. Hopefully, we will be given relatively simple ones if asked on the midterm, like C4H4, but I think a very basic rule for identifying resonance structures would be if there is a lone pair or a single bond next to a double bond (someone correct me if I'm wrong) where the bonds can be switched without changing the number of electrons. As for the most stable resonance structure, I believe that that would be the structure with the lowest formal charge. Hope this helps!

This was super helpful thank you!

605291562
Posts: 67
Joined: Wed Sep 30, 2020 10:07 pm

Re: Number of Resonance Structures

I don't think you need to worry too much about having to draw all the resonance structures for an exam, as long as you know how to identify them and then compare (for example which is the most stable etc. like the sapling problem) I think you will be in a good spot.

Mikayla James 2C
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Re: Number of Resonance Structures

Raashi Chaudhari 2D wrote: Thank you so much!! For the formal charge can't you also determine which is the most stable by the structure that has the least amount of charges within the molecule?

Yep! The most stable structure is the one with the least amount of charges on each atom within the molecule (I might have mistyped that on my last comment) and I also believe that if you end up with two structures that have the same amount of charges, then the structure with the negative charge on the most electronegative atom is the more stable one.