# What is the name of S_2O_3?

${S}_{2} {O}_{3}^{2 -}$ is the $\text{thiosulfate anion}$. Note that this has a formal, double negative charge.
In a thiosulfate group one sulfur atom has formally replaced one of the oxygen atoms of sulfate dianion, $S {O}_{4}^{2 -}$.
In sulfate, the oxidation number of $S = + V I$. We could imagine that in thiosulfate, the central $S$ also has this oxidation state, and the terminal sulfur has an oxidation state of $- I I$, i.e. the same oxidation state as the oxygen atoms, which are in the same Group as sulfur. The average $\frac{6 - 2}{2} = + I I$ is the same as we would get if we assumed the sulfurs were in equivalent oxidation states.