# Why does oxygen in OH- have an oxidation state of 2-?

I am watching videos on Khan Academy in order to go over redox reactions (a really weak area for me.) In Sal's video, "Oxidation state trends in periodic table" at 7:10, he says:

If you had to pretend this was an ionic bond, then maybe this hydrogen would fully lose an electron, so it would get an oxidation state of plus 1. It would be oxidized by the oxygen.

And the oxygen actually has fully gained one electron. And if we're forced to think about this is an ionic bond, we'd say it fully gains two electrons. So we'd have an oxidation state of negative 2.

I'm having a hard time understanding how he is able to deduce that oxygen has an oxidation state of 2- given the above reasoning. He does add that the states add up to the overall charge which is helpful, but I think I need a little more clarity than that. My reasoning for oxygen being 2- is that "it usually is because my textbook says it usually is," which is probably an even worse explanation.

Could anyone clarify this for me?

• As Jan has said, oxidation state is the charge on an atom when the bonds are "cleaved heterolytically in such a way that the electronegative partner gets both electrons". Oxygen is the second most electronegative (after fluorine), and it forms two bonds, so it gets a $-2$ charge.
– DHMO
Commented Jan 14, 2017 at 5:36
• Haha, thanks for quoting my answers =) I feel happy now!
– Jan
Commented Jan 14, 2017 at 20:05
• Well the oxygen in $\ce{OH}$ doesn't have a -2 charge, but the oxygen in $\ce{OH^-}$ does.
– MaxW
Commented Jan 26, 2017 at 21:45

First step: draw the Lewis structure of $\ce{OH-}$ with all lone pairs. You should be getting three lone pairs on oxygen and one $\ce{O-H}$ bond.

Second step: ‘[cleave] [all bonds] heterolytically in such a way that the electronegative partner gets both electrons.’[1] You should now have eight electrons around the oxygen and zero on hydrogen.

Third step: count the electrons on both (or all) atoms and check what the difference is to their elemental valence electron count. Oxygen formally has eight electrons on it which is two more than the six it would in its elemental state. The two extraneous electrons give an oxidation state of $\mathrm{-II}$. Likewise, hydrogen has lost its electron for an oxidation state of $\mathrm{+I}$.

Control step: add up the oxidation states of all atoms. If you did everything correctly, they will add up to the entire charge of the ion. Here, this works out:

$$(-2) + (+1) = -1$$

• All the numbers refer to number of valence electrons (so we are not counting the electrons in the inner shell).
– Karsten
Commented Mar 26 at 22:25