# What does fully oxidized mean? [closed]

I am confused as to the meaning of fully oxidized, in the context of environmental chemistry (what is water soluble). While I believed that CO$_2$ was fully oxidized after reading http://hyperphysics.phy-astr.gsu.edu/hbase/Chemical/redoxea.html, I also read that SO$_2$ is a reducing agent. Additionally, looking at the electronegativities of carbon versus sulfur, they are the same. Thus, what types of chemicals would fully oxidized refer to?

## closed as too broad by Todd Minehardt, airhuff, Jon Custer, Jan, Klaus-Dieter WarzechaMar 15 '17 at 3:50

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• Highest oxidation state – Mithoron Mar 14 '17 at 19:19
• A couple notes: carbon and sulfur do have similar electronegativities, but that is not relevant to their possible oxidation states. Also, $\ce{SO2}$ is amphoteric in that it can act as an oxidant or reductant. The S in $\ce{SO2}$ is in the +4 oxidation state, but S can exist in oxidation states from +6 to -2. So, if $\ce{SO2}$ becomes oxidized to $\ce{SO3}$ for example (S goes from +4 ox state to +6 ox state), then $\ce{SO2}$ indeed would have acted as the reducing agent for that reaction (S was oxidized, another reactant was reduced). – airhuff Mar 14 '17 at 20:35

It comes down to the oxidation number of the element in the compound. From the link you provided, $\ce{CH4}$ has carbon in its most reduced form, with an oxidation number of $-4$. Carbon can't become any more reduced because a $-5$ state would imply that carbon had $9e^-$ in its valence shell, which would be highly unfavorable. Similarly, $\ce{CO2}$ has carbon in its most oxidized form, with an oxidation number of $+4$. Carbon can't become any more oxidized because a $+5$ state would imply that carbon lost an electron from its inner shell, which would be highly unfavorable.