Expanded Octet

[paulacamara1E]

In the last lecture one of the TAs was explaining the concept of expanded octects in the elements of the 3rd row of the periodic table or below. The example that was given was with SO4. I do not really understand why double bonds were formed between the S and two of the Os to make it more stable but the same was not done with the other two oxygens. Could someone further explain this concept and example?

[Chem_Mod]

Either 1. you would not be minimizing the number of formal charges in the Lewis Structure if you add more double bonds (to all O). or 2. You end up with a -1 formal charge on the S rather than the O. It is better to have -1 charge on the O because O is more electronegative than S.

[Nienjou Claire 1E]

Regarding the expanded octet, it's just that elements that are below the 3rd row have a d orbital or more that can bond if needed