hw ch3. 57

[Chels Zh 1D]

Reading the solution of Q57, for sulfite ion, SO3 2-, since both the structure followed the octet rule and the structure with expanded octet have the equal charge, what does it mean by "the structures with expanded octets have lower formal charges"?

[Kyle Sheu 1C]

Even though both structures have the same overall charge, we should take a deeper look at the formal charges.
For the Lewis structure that follows the octet rule, the FC of S is +1, while the FC of each O is -1.
For the 3 Lewis structures which do not conform to the octet rule, the FC of S is 0, the FC of the double bonded O is 0, and the FC of the single bonded O's is -1.

Overall, the formal charges for the structures with the expanded octet are lower, thus resulting in a more stable structure.