Question

rank the strenght of oxidants Fe3+, Cr2O7(2-), S4O6(2-)

Answer 1

What you need to do is look at the oxidation state of the central atom. In the case of Fe+3, it's +3. In the case of the dichromate anion the Cr is in the oxidation state of +7, and in the case of the tetrathionate anion two of the sulfurs are in an oxidation state of 0 and two in an oxidation state of +5.

Since Cr is in the highest oxidation state, this is the one that will most want to be reduced (gain electrons). So it will be the strongest oxidizer.