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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.

enter image description here

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

enter image description here

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?

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    $\begingroup$ Apparently, your teacher is not aware that sulfur can expand its octet. Your reasoning and answer seem correct, although strictly speaking, if overall bond order was asked, the result would be 4 and not 4/3. $\endgroup$
    – Sam202
    Nov 21, 2022 at 19:36
  • $\begingroup$ @Sam202 Hm not really (even though this theory is disproved/debated or w/e) he just claimed that an octet is more "stable", so sulfur must have only 3 single bonds. He ignored my argument of formal charges, and never mentioned anything about expanded octets. Also i thought bond order was number of bonds / bonding sites which would be 4/3? $\endgroup$
    – Max0815
    Nov 21, 2022 at 21:08
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    $\begingroup$ Bond order of the sulfur-oxygen bond in $\ce{SO3^{2-}}$ is 4/3, but overall bond order of $\ce{SO3^{2-}}$ is 4. The way you stated the question suggests the latter was being asked. $\endgroup$
    – Sam202
    Nov 21, 2022 at 21:24
  • $\begingroup$ In this case, the octet rule predominates. $\endgroup$ Nov 22, 2022 at 0:25
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    $\begingroup$ The problem is that electrons are never taught rules, so they figure it out themselves… $\endgroup$
    – Jon Custer
    Nov 22, 2022 at 13:09

1 Answer 1

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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.

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