oxidation no. of S in S2O3 2-

'coz Sally made mistake.
yes, thiosulphate is like sulphate with one of oxygen replaced by sulphur. terminal sulphur attacted to central sulphur with double bond, while central sulphur has =O, -O(-) and -O(-).

however, we look at the oxidation number in some other way.
when considering oxidation number, we look at the number of bonds as well as difference in electronegativity among them.
look at terminal sulphur -- two bonds with another sulphur. however, they have same electronegativity, so neither one draws electrons from another (the bonds are non-polar). thus it doesn't "gain" or "lose" any electrons, and O.N. remains zero.

for central sulphur, it has 2 bonds to terminal sulphur and totally 4 bonds to oxygen. as oxygen is more electronegative than sulphur, in each bond oxygen will draw the bonding electrons to them.
(remember that a covalent bond is formed by sharing an electron from each bonding atom -- in this case, one from oxygen and one from sulphur.)

as there're four S-O bonds, central sulphur "loses" 4 electrons, so its O.N. is +4.


Sally just missed considering the electronegativity difference between S and O, and incorrectly think that the terminal-S retains the O.N. of original oxygen.

thiosulphate derived from sulphate. When one of the oxygen (double bonded to S) of a sulphate ion is replaced by a sulphur atom, the ion becomes thiosulphate. So,there is two kind of S atoms in a thiosulphate ion, one is the sulphur atom that replacing the oxygen atom and thus has an oxidiation number of -2. Another is the central S atom with oxidation number +6, calculated as follows:
-2-(-2x4) =+6
The average oxidation number of these 2 S atoms if +2 which agree with that directly calculated by inspection of the formula.