Question

What is the difference between oxidation number and oxidation state?

Answer 1

Oxidation number might be easier to understand at the high school level, but oxidation state and oxidation number are synonymous in many cases.

The only time I can think of where they are different are in coordination complexes.

In `["Ti"("CO")_6]^(2-)`, or hexacarbonyltitanate(II), the oxidation state is `-2` but the oxidation number is `"II"`. The "-ate" indicates the negative overall charge of the complex.

On the other hand, `["Ti"("CO")_6]^(2+)`, or hexacarbonyltitanium(II), would have the same oxidation number but a `+2` oxidation state.

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OXIDATION STATE EXAMPLE

The oxidation state is the hypothetical charge if the bond that the atom makes with a second atom is 100% ionic in character, and if after accounting for electronegativities, the more electronegative atom in the pair has the more negative oxidation state.

It isn't ever exactly the case for a real bond to have 100% ionic character (i.e. to transfer 100% of the electron density into another atom's valence orbitals), but oxidation states turn out to be a good charge accounting scheme.

Consider the following `E^@` vs. `"pH"` (Pourbaix) diagram.

For example, if we are at `"pH"` `2`, then we can apply some positive voltage to `"Fe"^(3+)` at `"1 atm"` pressure, `"273.15 K"`, and constant `"pH"` as follows:

`\mathbf(stackrel(color(blue)(+3))("Fe"^(3+))(aq) + 4stackrel(color(blue)(+1))("H")_2stackrel(color(blue)(-2))("O")(l)-> stackrel(color(blue)(+6))("Fe")stackrel(color(blue)(-2))("O"_4^(2-))(aq) + 8stackrel(color(blue)(+1))("H"^(+))(aq) + 3e^(-))`

`E_"ox"^@ > "0.8 V"`

(We started in the `"Fe"^(3+)` region on the upper left side of the graph, and we moved upwards to oxidize.)

For this half-reaction, we've oxidized iron(III) `(stackrel(+3)("Fe"))`to iron(VI) `(stackrel(+6)("Fe"))` in acidic conditions.

So, iron attains a `+6` oxidation state, and its oxidation number is represented as `"VI"`.

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OXIDATION NUMBER VS. STATE EXAMPLE

One difference I can find is in `["Ti"("CO")_6]^(2-)`. This is called hexacarbonyltitanate(II).

The oxidation number of titanium here is written as `"II"`, but the oxidation state of titanium is actually `-2` (making it a `d^6` metal). The negative overall charge of the complex is conveyed by the "-ate" in the name, and the negative oxidation state is deduced.

This compound has a `t_(2g)^6` configuration, being low spin, and is a stable 18-electron complex (as it should be for a coordination complex with six `pi`-acceptor ligands).