This question already has an answer here:

Yesterday, I came across $\ce{SO2}$ and was supposed to write the reduction and oxidation half reactions. However, I do not understand the reason why S is assigned the oxidation number of 4+. Oxygen shares 4 electrons with S in total, and oxygen is more electronegative than S. I don't get how we somehow "assigned" +4 to S. Shouldn't S also have a negative charge? I am so confused and need help. What is the difference between the oxidation charge and ionic charge?


marked as duplicate by Mithoron, Jon Custer, Tyberius, A.K., Todd Minehardt Nov 28 '18 at 23:02

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • 1
    $\begingroup$ What would you suggest that we assign it instead? $\endgroup$ – Ivan Neretin Nov 27 '18 at 5:35
  • $\begingroup$ We do it for our own convenience, there is no deeper reason. $\endgroup$ – Ivan Neretin Nov 27 '18 at 7:06
  • $\begingroup$ electronegativity? @IvanNeretin $\endgroup$ – ten1o Nov 27 '18 at 7:07
  • 1
    $\begingroup$ Yes, electronegativity does play a role, but why? That's a matter of convenience. We imagine that the more electronegative atom "owns" the shared electrons, and the other one kinda "borrows" them. $\endgroup$ – Ivan Neretin Nov 27 '18 at 7:10
  • $\begingroup$ Maybe you forgot an important property/definition for a while, that the sum of all oxidation numbers gives a total charge, i.e. 0 for neutral molecules. $\endgroup$ – mykhal Nov 27 '18 at 15:55

Strictly speaking, oxidation states should only be used with ionic compounds. However, this concept can be interesting in organic or semi-organic molecules as long as you understand that atoms actually don't have such a black and white oxidation state.

The oxidation state is attributed according to the relative electronegativity of 2 atoms (the most electronegative takes a -1 state and the other a +1 state) and they sum up if an atom has more than one neighbors. For example:

In CCl4, there are 4 C-Cl bonds which you need to consider. Every time, the chlorine atom will get a -1 oxidation state, while the central carbon atom will get 4 times a +1 oxidation state, which gives +4 at the end.

For multiple bonds, you need to consider each single bond individually. In an S=O bond, there are 2 single bonds and each of them gives a -1 state to the oxygen (hence a +1 state for the sulphur).

Combining the two examples, you should see why in SO2 sulphur has a +4 oxidation state and both oxygens have a -2 state.


Not the answer you're looking for? Browse other questions tagged or ask your own question.