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I was given this question and don't understand why the two sulphur atoms have different oxidation states - and how to actually figure it out. Could someone please explain it to me? I've attached the reasoning that was provided but it isn't very clear to me. I understand that oxygen is usually -2 (with exceptions) and that the molecule itself should have a net charge of -2 but I thought that then meant that each sulphur atom would have an oxidation state of +2. Any help would be greatly appreciated!!

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    $\begingroup$ If you count oxidation state formally, than it is done by moving all electrons of a shared electron pair to the more negative atom or dividing it between atoms if they have the same oxidation state. This, however, gives rise to question for which resonance structure one calculates oxidation state, which might give different values for thiosulfate. On another hand, thiosulfate is isoelectronic to sulfate, so it is natural to use same oxidation numbers for same positions as in sulfate. This approach is often used for organization of compounds into groups. $\endgroup$
    – permeakra
    Dec 11, 2023 at 12:26

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If the last paragraph in your quotation is meant to be the book's answer, I would stain that text with my intensely colored, anthocyanin-rich cranberry jam*. I would find either $-1$ and $+5$ or $0$ and $+4$ more defensible.

-1 and +5

In this model we use the raw electronegativities of the atoms to render the sulfur-sulfur bond nonpolar, so the outer sulfur atom would have the same $-1$ oxidation state as it would in the disulfide ion $\ce{S2^{2-}}$. The central sulfur atom, losing two electrons to each oxygen atom to satisfy $-2$ for the latter, ends up at $-1+6=+5$.

0 and +4

Here we take a slightly more advanced approach, arguing that the sulfur-sulfur bond is inductively polarized by the oxygen atoms withdrawing electronic charge from the central sulfur. In response the sulfur-sulfur bond has its electrons polarized towards the central atom as if that atom were more electronegative. Thereby the outer sulfur is rendered with an oxidation state of $0$ and the inner one with $-2+6=+4$. This assignment has the advantage of dovetailing with a well-known chemical property of thiosulfate: a generally redox-inert acid such as hydrochloric acid causes it decompose to elemental sulfur (oxidation state $0$) and sulfur dioxide (sulfur oxidation state $+4$).


*Cranberries are actually white on the inside when fresh. The bulk color turns red only when the anthocyanin dye, which is not colored in its reduced form, is exposed to air as the cranberries are crushed or popped.

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  • $\begingroup$ Lol yes I would love to stain the textbook too. Thank you for the response, that makes a lot more sense to me!! $\endgroup$
    – Koko Faen
    Dec 11, 2023 at 2:10
  • $\begingroup$ @KokoFaen If the answer helped, please accept it by clicking on the checkmark. $\endgroup$ Dec 11, 2023 at 3:05
  • $\begingroup$ There's one in every crowd. Would be good to know why the downvote, even if that is as much of a luxury as a free lamborghini. $\endgroup$ Dec 11, 2023 at 13:05
  • $\begingroup$ @OscarLanzi maybe they don't like cranberry jam ;) $\endgroup$ Dec 11, 2023 at 15:20
  • $\begingroup$ @NilayGhosh that 's because they're thinking of the canned stuff. Make your own and then you'll like it. $\endgroup$ Dec 11, 2023 at 15:23

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