oxidation state of S2O3 2-

I would ask you a question first.

Why fluorine always has a oxidation number -1?
As fluorine is a most electronegative atom in the periodic table.

So, u may find that why O atom in OF2 has a oxidation number +2.

For other cases, such as H2O.
O is more electronegative than H and thus the O atom attracts the bond pair electrons from H. Theorectically, O atom now has 8 electrons in its outershell to form O^2-while H atom now has no electrons in its outermost shell to form H+.

So, in this case, S2O3^2-, two S atoms are attached to each other.

Considering the electronegativity, They are the same and thus have the oxidation umber 0.

For the central S atom, four O atoms are attached to it and O is more electronegative than S. Thus, oxidation number of central S atom is +4.

2011-06-13 16:16:32 補充：
I'm sorry for my inappropriate answer

For your case, it may be assumed that all of the valence electron of central S atom are donated to the 3 oxygen atoms in S2O3^2-

Therefore, the O.S of the central S atom becomes +6.

2011-06-13 16:21:40 補充：
Now, consider about the ionic charge 2-
u can see that the central S atom and the 3 oxygen atoms are stable as they had 8 electrons which obeys the octet rule

And thus, the O.S of the upper S atom is -2 as it accepts the ionic charge 2-

其實我係唔明點解個答案講寫 +6 同 -2 都得