Does formal charge or the octet rule take precedence over one another?

Question

In my AP chem test, I had a question that asked me to find the bond order of the ion $\ce{SO3^2-}$.

To draw the lewis structure, I determined that there are a total of $6+3\cdot 6+2=26$ electrons, and the sulfur had $\frac{26-8\cdot3}{2}=1$ lone pairs.

When we measure formal charges, every atom has one, and I have been told that this means the given lewis structure is inherently unstable and will attempt to form additional bonds. I use a pair on the oxygen to form a double bond, which eliminates the formal charges on the oxygen and sulfur, giving me the three (what i presume to be more stable) resonance structures

Hence the bond order is $\frac43$.

However, my teacher told me this was wrong. Apprently, due to the octet rule, the first lewis structure is the correct one where there are just three single bonds, and it's actually more stable since it satisfies the octet rule.

This makes me confused though. Which is the actual correct lewis structure (well, correct in a sense that the majority of the resonance structures exist in that form. I know that at least some aka a minor minor amount of $\ce{SO3^2-}$ would probably exist in the single bond form) that you would find bond order with?

I think this in the end comes down to whether or not formal charge or octet rule takes precedence, and based on everything I can find online, it seems formal charge does. So why does my chemistry teacher insist that the octet rule does? Am I misunderstanding something?

Answer 1

I believe the confusion stems from the false assumption that conventional Lewis structures are actually representative of the electronic structure of a molecule. They are not intended to be so. Rather, a Lewis structure is intended to provide enough information that a competent chemist can understand the structure of a molecule. As a trivial example, consider the Lewis structure of benzene, which uses alternating double and single bonds. Or the fact that Lewis structures only use integer charges.

So, if the question were to "draw the Lewis structure of $\ce{SO3^2-}$", then following the conventions of Lewis structures with regard to charge vs octet would make sense. Those conventions are that in first row elements (eg N, C, O), the octet rule takes precedence over reducing formal charge. For example, it is correct to have a positive charge on N in $\ce{HNO3}$ rather than having five N-O bonds. However, in second row elements like sulfur, the priority reverses, and the convention is to break the octet rule in order to minimize formal charge. For example, sulfuric acid is drawn with six S-O bonds.

But all of that is moot, since the question was about the bond order, not about the Lewis structure. For such a question about the actual electronic structure, you should use the structure that is closest to our best understanding of the actual molecule. In the case of $\ce{SO3^2-}$, it is now understood (and has been for decades) that $d$ orbitals participate negligibly in bonding, so the structure with formal charge that obeys the octet rule is a more accurate representation, and thus the bond order deduced from it is more accurate than a bond order deduced from the conventional Lewis structure.