Redraw resonance

[bree_wray_14a]

Hi could someone explain why we must redraw bonds and change them into double bonds and remove valence electrons. I know that we do this because it was unstable but why must we Draw a stable one?

[Anna Furton]

Hi Bree,

We try to draw structures in their most stable state because naturally, this is the base condition of the molecule that has the lowest energy. It would require extra energy for the molecule to maintain opposing charges in atoms right next to each other, so they automatically share electrons to correct their instability and fall to their lowest energy state. This is how double bonds form between the atoms sharing their valence electrons to complete their outer shells, and therefore exist in the lowest energy state possible.

I hope this helps!

[KyleNagasawaDisc3C_Chem 14B2022W_]

Hi Bree,

A big point of today's lecture was that even if a given species fulfill the octet rule, there is no guarantee that the drawing is an accurate depiction of a compound's structure. There is the additional rule that IN ADDITION to a species' atoms fulfilling octet (for 2nd-period p-block elements), the most stable species in a nonpolar environment have the lowest formal charge on their atoms (assuming there is the capability for effective p-orbital overlap to form double bonds, with a common example that breaks this rule being the dipolar form of DMSO). In class, Doctor Lavelle redrew the sulfate ion, which we previously drew with 4 sets of S-O single bonds, to a structure with 2 S=O bonds and S-O bonds. This allowed there to be the least amount of formal charge (compared to the 2+ charge on S and 1- on each O). The newer representation only has two 1- charges, one on each of two oxygen atoms. This decreased sequestering of charge allows a more stable structure.

I hope this helps!

[SarahOMalley1D]

Hi! That was a great conceptual explanation. On the more numerical side of things, we want the formal charge of the molecule to be as close to zero as possible. The redrawn Lewis structure had a lower overall formal charge, so it is a more stable molecule. More stable molecules have lower energy and are less reactive, which is something that all molecules seek to be.

As for information on expanded octets and their relationship to s and p orbitals, I believe that Professor Lavelle will be going over some of this on Wednesday. If you are confused, I would wait until then and then maybe read over the textbook/do the textbook problems.

[Ivy Nguyen 3I]

Hi! I think all the previous responses are correct and go into great detail and even your understanding that they are unstable is great. Both forms of the lewis structure (unstable or stable -- like SO4 2-) are correct but we want to draw the stable ones because they are lower energy. Dedrawing bonds as double bonds and removing electrons is not necessarily the way to draw every stable form of molecules; sometimes it will require a triple bond, etc. Hope that helped!