Question

What is the name of `S_2O_3`?

Answer 1

`S_2O_3^(2-)` is the `"thiosulfate anion"`. Note that this has a formal, double negative charge.

Explanation:

In a thiosulfate group one sulfur atom has formally replaced one of the oxygen atoms of sulfate dianion, `SO_4^(2-)`.

In sulfate, the oxidation number of `S=+VI`. We could imagine that in thiosulfate, the central `S` also has this oxidation state, and the terminal sulfur has an oxidation state of `-II`, i.e. the same oxidation state as the oxygen atoms, which are in the same Group as sulfur. The average `(6-2)/2=+II` is the same as we would get if we assumed the sulfurs were in equivalent oxidation states.