Sulfate ion class example clarification

[Emily Ngo 1E]

Hi,
During Professor Lavelle's lecture today, he went over the lewis structure for SO4 2-. Can someone please explain to me why the structure of sulfate where sulfur has two double bonds and 2 single bonds was the most desirable? How is it possible for the sulfur atom to have 12 e- around it when the octet rule states that atoms prefer to have 8 e- in its valence shell.

[Kaira Shibata 1E]

Hi,
This is the most stable as there is a formal charge of 0 for the sulfur and then a formal charge of -1 for the two oxygens with a single bond.
Regarding how sulfur can have 12 electrons, he said he would explain that later. It has something to do with the d-block I think.

[ashna kumar 3k]

We always want the structure of the Lewis Structure with the lowest formal charges, which came from the diagram he drew in class. In regards to how sulfur can have 12 e-, we must remember that the octet rule is more of a guideline and some atoms can have an expanded octet. Because it was lowering the formal charge of sulfur, and thus giving it lower energy, there were more than 8 electrons around the sulfur. This is possible due to the fact that we are moving down the periodic table and thus have more orbitals that can hold more electrons. Since S is in the 3s block, where n=3, l can equal 0,1 or 2 meaning we have the 3d subshell for electrons to occupy, allowing for more than 8 electrons just in the s and p blocks.

Hope this helps!

[SarahOMalley1D]

Hi! To summarize and add a bit more context, we want the formal charge of each atom to be as close to zero as possible. A formal charge that is close to zero means a more stable molecule. The Lewis structure with two double bonds has a lower overall formal charge (adds to -2) when compared to the original structure. The original structure's formal charge also did not make sense because the O atoms display a negative formal charge while the S atoms displayed a positive formal charge. These positive and negative formal charges would electrostatically attract.

[Julia Todorov 2F]

This is the most stable because the closer each atom's formal charge is to zero the less energy it has. Sulfer is not like oxygen, nitrogen, carbon, and flourine in the sense that they must have 8 electrons no more no less since but sulfer can have more. Basically the closer an atoms formal charge is to zero, the better, so that is why the model of SO4 2- with 2 double bonds is the most stable

[Reagan Feldman 1D]

To add, it is possible for S to have more than 8 valence electrons because atoms in period 3 or higher have valence shells in d-orbitals, which can accommodate for more than 8 electrons.

[Omar Alkhalili 1J]

Hello! I believe that Sulfur can have more since it has the d-orbital to use due to its atomic number and placement on the periodic table. Since the formal charge is closer to 0 and thus more stable when it has more valence electrons, that is the ideal scenario. Hope this helps!

[Do Yeun Park]

Hi!
This is because having the double bond between O and S is the most stable. When you are doing Lewis Structures and Formal charges, you want to have 0 formal charges per element if possible. Also, S can have 12 e- because it's at level 3, meaning there are d-orbitals to fill up the electrons.