Tetrathionate ion
Since each of the two terminal sulphur atoms is connected to two oxygen atoms by a double bond and one oxygen atom by a single bond, therefore, the oxidation state of each of these terminal sulphur atoms is +5.
Since two central sulphur atoms are linked to each other by a single bond and each sulphur is further attached to similar species on either side, the electron pair forming the S – S bond remain in the centre and hence each of the two central sulphur atoms has an oxidation state of zero.
However, the average oxidation state of the sulphur atoms is
