What is the oxidation number for S in Na2S4O6 ?

Average oxidation number of S :

Na₂S₄O₆ is a neutral compound, and thus the sum of oxidation number is 0.
(Total oxidation numbers of Na) + (Total oxidation numbers of S) + (Total oxidations of O) = 0
(+1) × 2 + (Average oxidation number of S) × 4 + (-2) × 6 = 0
(Average oxidation number of S) × 4 = 10
Average oxidation number of S = 2.5


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Oxidation numbers of individual S atoms :

Refer to the S₄O₆²⁻ in the diagram below.

Consider the 2 S atoms in red.
Each of the S atoms in red is attached to 2 S atoms.
Hence, oxidation number of each S atom in red = 0

Consider the part of -SO₃⁻.
Since the part has a change of -1, the total oxidation numbers = 0
(Total oxidation numbers of O) + (Oxidation number of S in blue) = -1
(-2) × 3 + (Oxidation number of S in blue) = -1
Hence, oxidation number of S in blue = +5

Answers :
There are two types of S atoms having different oxidation numbers.
The oxidation of each S atom attached to 2 other S atoms = 0
The oxidation of each S atom attached to 1 S atom and 3 O atoms = +5

+5