[positiveion]

On a previous thread, a member posted a really really clear explanation of oxidation states:

To determine oxidation states, you consider that atoms with higher electronegativity keep electrons for themselves (gain electrons) and lower electronegative atoms lose electrons. Then, you look at how many electrons have been gained, how many lost.

For example, water H2O.
Oxygen is more electronegative than hydrogen, so oxygen "keeps" electrons all for itself: O2- and 2 H+.
Oxygen has gained 2 extra electrons, so its oxidation state is -II.
Each hydrogen has lost 1 electron, so theirs oxidation states are +I.

Second example, OH-.
Oxygen already has one electron, which explains the - charge on this anion. Then, it grabs another electron from the hydrogen  right arrow 2 electron gained. Oxygen has oxidation number -II.
Similarly, hydrogen is +I.

But there is one thing that I don't understand. For OH-, why does oxygen already have one electron?

[Borek]

Think in terms of water molecule - it is neutral, right? You remove H⁺, right? Whatever is left must be -1, right?

But don't waste your time trying to put too much thinking into oxidation numbers. I have repeated it on many occasions - they don't reflect any real property of the matter. You can't determine them experimentally, you can just calculate them using some set of rules. Trying to find an "explanation" is just a waste of time.

Rules used follow some simple logic describing very basic properties of atoms, but they are simplified to the point were they have lost any physical meaning.

[positiveion]

:S

I just find it really difficult to grasp the concept of oxidation states and I really want to understand them. I understand that they don't reflect any real properties and that they are purely hypothetical, but there has to be explanations. These rules wouldn't just be randomly assigned...

//

So what is it that you are saying? That OH is just H2O with an H removed?

But what if it wasn't an H ion that was part of it, what if there was something with a greater electronegativity - a fluorine atom?

[Borek]

I just find it really difficult to grasp the concept of oxidation states and I really want to understand them. I understand that they don't reflect any real properties and that they are purely hypothetical, but there has to be explanations. These rules wouldn't just be randomly assigned...

No, they are not randomly assigned, but they are built around oversimplified assumptions that you already know. There is really no insight nor deep chemistry knowledge that you can gain from chasing details.

So what is it that you are saying? That OH is just H2O with an H removed?

OH^- is just a H₂O with H⁺ removed, pay attention to charges.

But what if it wasn't an H ion that was part of it, what if there was something with a greater electronegativity - a fluorine atom?

F is always -1 in compounds, so if removed it would left OH⁺. But as I told before trying to 'understand' such structures assigning ON is a waste of time. They can be understand in terms of http://en.wikipedia.org/wiki/Molecular_orbital_theory or http://en.wikipedia.org/wiki/Valence_bond_theory - these are much closer to reality.