Drawing Lewis Structures with the smallest formal charge

[Sujin Lim 1H]

In lecture today, the professor drew an alternate lewis structure for sulfate saying that the original one we drew last week was not as stable because the formal charges for each atom was not the smallest they could be. However, is there a guideline that we should be following in order to get the structure that has the minimal formal charge? Or are we just supposed to guess the best alternate structure and check the formal charges after? I was a little confused on how the professor knew to create the two double bonds between sulfur and oxygen.

[Allison Li 2F]

Hi! Sulfur is an exception because it can have six bonds as opposed to a max of four for other elements. That is how the professor knew to put the two double bonds for sulfur and the two oxygens.

[Ethan Hung 2A]

I'm not certain about this one so someone can correct me if I'm wrong, but there aren't specific guidelines besides the ones we've covered in class like using lowest electronegativity atoms as the central atom, filling the octet rule where possible, etc. For formal charges, you can kind of cross-check them as you go about building your lewis structure, and in general, you want atoms with high electronegativity to take negative formal charges (most frequently this is oxygen) and to avoid separation of formal charges where possible. If you're not as familiar with building lewis structures, I think guessing the best possible structure and then checking formal charges is a valid method.

For the SO₄^2- example given in class, the sulfur atom specifically is able to access the d-orbitals where other atoms like oxygen and carbon are not, so after building the lewis structure with two double bonds, this structure is still valid (and I think Prof. Lavelle will cover it more next lecture). Hope that helped :)

[Kimia Rategh 2A]

I also think there aren't any specific guidelines in creating Lewis structures with the smallest formal charges, however I do remember professor mentioning that some atoms can take more than the octet because they start filling their d sub-shells and therefore can take more than the previously stated 8 electrons. Therfore, if the molecule you are drawing a lewis structure can take more than 8 electrons, I think you can make is so that it does by assigning double or triple bonds to make the formal charges of the the individual atoms decrease and achieve a more stable structure. Hope this helps!

[Helen Ringley 2E]

You can tell which ones should be double or triple bonds by looking at how many electrons they have. We want to decrease the number of lone pairs in order to decrease formal charges. Also, after a while of working with lewis structures, we will start to recognize which atom-atom combinations tend to form double or triple bonds.