Given problem:
S₂O₈²⁻
Find the oxidation number of S;
Oxidation number presents the extent of oxidation of each atom of elements a molecular formular or formula unit or an ionic radical.
For radicals:
"the algebraic sum of all the oxidation numbers of all atoms in an ion containing more than one kind of atom is equal to the charge on the ion "
S₂O₈²⁻; oxidation number of O is usually -2
2(S) + 8(-2) = -2
2S - 16 = -2
2S = -2 + 16
2S = 14
S = +7
The oxidation state of S in the radical is +7