what is the oxidation number of S in 2S2O3^2-?

(1) The average oxidation number of S in S₂O₃²⁻ :

S₂O₃²⁻ is a polyatomic ion carrying 2 negative charges.
Total oxidation number = Charges on the ion
(Average oxidation number of S)×2 + (Oxidation number of O)×3 = -2
(Average oxidation number of S)×2 + (-2)×3 = -2
(Average oxidation number of S)×2 = +4
Average oxidation number of S = +2


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(2) The different oxidation numbers of the two S atoms in S₂O₃²⁻ :

Refer to the structure of S₂O₃²⁻ ion below :

The green line divides the S=S bond into two sides.

Consider the left hand side :
There is no charge on the left hand side.
The oxidation number of the blue S = 0

Consider the right hand side :
Total oxidation numbers = Charges on the right hand side
(Oxidation number of the blue S) + (Oxidation number of O)×3 = -2
(Oxidation number of the blue S) + (-2)×3 = -2
Oxidation number of the blue S = +4