Textbook problem 2F.3

[DArcy Perlman 1C]

Textbook problem 2F.3 asks how many sigma and pi bonds there are in SO2. The answer given is 2 sigma bonds and 2 pi bonds because of the resonance between the 2 SO2 Lewis structures, with the double bond character shared among both bonds. Why doesn't the lewis structure for SO2 have 2 double bonds and therefore 2 pi bonds? Can't sulfur have an expanded octet and wouldn't that structure have the fewer formal charges than SO2 with one double bond? Also, it's still considered 2 pi bonds if the double bond character is shared among two bonds?

[Jonathan Sautter 1J]

The answer does show the structure with 2 double bonds. Recall that 1 double bond has 1 sigma and 1 pi bond. Thus the structure with 2 double bonds will have 2 sigma and 2 pi bonds.

[BB Dis 1H]

Will they tell us when they want the sigma and pi bonds for the resonance structure too? How do we know which structure to go with if they don't specify?

[amar_151]

Will they tell us when they want the sigma and pi bonds for the resonance structure too? How do we know which structure to go with if they don't specify?

To answer your question, Bryn, Dr. Lavelle always wants us to find the resonance structure that minimizes formal charge unless told otherwise because this is the most stable structure.

[Aneesha_Nema_3C]

I don't know if you meant that the answer given was 2 σ-bonds; 1 π-bond and that it clarifies that there is also a resonance structure that includes 2 σ-bonds and 2 π-bonds. SO2's Lewis structure is having sulfur be double bonded to one oxygen atom and single bonded to another, which would mean that there would be 2 σ-bonds and 1 π-bond, however it is also possible to have sulfur be double bonded to both oxygen atoms because, as you said, sulfur can have an expanded octet. This structure would have 2 σ-bonds and 2 π-bonds.