VSEPR identification of carbonate ion (CO3(2-))

[Yu Jin Kwon 3L]

Hello everyone!

So in lecture 20, Dr. Lavelle talked about how the carbonate ion is identified as a trigonal planar, with all bond angles being 120 degrees. I've attached a picture of the lewis structure for carbonate ion, and as you can see, there is a double bond there. I understand that Dr. Lavelle said that single, double, and triple bonds are considered single regions of electron density in VSEPR model, but just by looking at the structure as well, I can see that the double bonds are essentially four electrons that are right next to each other, whereas for lone pairs, the pairs are spread apart as far as possible (electron repulsion) and single bonds are two electrons (better than four total electrons like in double bonds). Thus, wouldn't double bonds experience more electron-electron repulsion, causing the bond angles to not be equal to 120 degrees (some larger than 120, some smaller than 120)?

Thank you!

[Stacey Phan 2I]

I believe Professor Lavelle just regarded them as regions of electron density which is the only thing considered in determining shapes.

[Arti_Patel_3H]

I believe Professor Lavelle just regarded them as regions of electron density which is the only thing considered in determining shapes.

This is what I interpreted from the lecture as well.

[ALee_1J]

To add to the answers above: Double bonds are indeed "shorter" than single bonds, but the structure you are looking at right now is a resonance structure so all of the bonds will be equal (ie. less than a single but more than a double bond). So I believe the angles will all be equal ( but I'm not sure if it will be exactly 120 degrees).

[Madison Muggeo 3H]

For determining the shape you only take regions of electron density into account even if it is a double bond, it is only one region of electron density so there are three total around the carbon atom.