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# Practice determining oxidation states

Determining oxidation numbers in magnesium oxide and magnesium hydroxide.  Created by Sal Khan.

## Want to join the conversation?

• Which elements show highest oxidation state in the perodic table ?
(20 votes)
• Iridium is the only element known (and this was only recently demonstrated) to achieve the +9 oxidation state (in the cation IrO₄⁺). There are seven elements that can achieve +8 (Ru, Xe, Os, Ir, Pu, Cm and Hs).

Since the results for the +9 oxidation state in Ir were first published just a few weeks ago (October 23, 2014) most sources you see will say that +8 is the highest oxidation state.

The lowest known oxidation state is −4, which only Group 14 elements are known to achieve.
(60 votes)
• What is the oxidation state or oxidation number of iron in Fe3O4 (Ferrosoferric Oxide)?
(7 votes)
• That compound has some Fe in the +2 state and rest in the +3 state.
(18 votes)
• How does Mg lose two electrons to have a 2+ charge in Mg(OH)2, if both elements in OH have full valence shells. What happens to those two Mg electrons?
(6 votes)
• First, keep in mind that oxidation numbers are NOT charges. The oxidation numbers are statements about what the charge on the atom would be if all of its bonds were 100% ionic.

Thus, in Mg(OH)₂ you have two separate things going on.
First you have O and H covalently bonded to each other with a negative charge (taken from Mg) and you have two sets of O and H.
So, we have two OH⁻ anions. They got their extra electrons from the Mg, so Mg has a charge of +2, so it is a cation.
Thus, the second thing occurring is that we have an ionic bond between Mg⁺² cation and the two OH⁻ anions. That is why we have Mg(OH)₂

Thus, the oxidation states in Mg(OH)₂ are
Mg +2
O -2
H +1
(20 votes)
• How do you find out the oxidation numbers?
(2 votes)
• My Chemistry teacher taught us to assume that Oxygen is 2-, and Hydrogen is 1+. You always know the Charge of the compound, such as HO 's charge is 1-. So if Oxygen is 2-, and Hydrogen is 1+, then the charge is 1-. For something like MgO, where the charge is neutral. You can assume that Oxygen is 2-, and so Mg must be 2+ to keep the compound neutral.
(13 votes)
• Why is Magnesium oxidised in 3Mg+N2->Mg3N2?
(5 votes)
• About the second example: Mg(OH)2

Looking at the OH group separately. The video states O has a negative 2 oxidation state (unspoken thumb rule). That's assuming oxygen grabs two electrons which it doesn't looking at the OH group alone. There's only one electron to grab (from the lone H).

Looking at the OH group alone, oxygen gets -1 oxidation state, H gets +1 oxidation state. +1 -1 = 0 (since it's not an ion it adds up to zero).

But each OH group has a spare space on each oxygen to grab another electron so each oxygen in each OH group can get to -2 oxidation state by grabbing another. And this they do by grabbing one each of the 2 from the Mg.

Stating of the OH group that O gets -2 oxidation state is confusing since doesn't happen without taking the Mg into account.
(5 votes)
• Your analysis is not entirely correct, more specifically your statement in the second paragraph on OH: you miss that OH actually has a negative charge, so it is really (OH)- ion. Oxygen will always have a -2 oxidation state and hydrogen will always have a +1 oxidation state. Hopefully this also better explains the bonding to Mg.
(2 votes)
• how to find the oxidation number of Fe2O6
(2 votes)
• there is only two oxidation states for Fe, 2+ and 3+. Fe2O6 does not exist.
Fe(2+) O(2-) Fe2(3+) O3(2-)
(3 votes)
• what is the oxidation number of vanadium in VO2+?
(1 vote)
• Why is oxygen always found in pairs in nature?
(2 votes)
• It's diatomic, which means it will be found as O2. It's unstable when it's just by itself, and to become stable it shares its electrons with another oxygen atom.
(2 votes)
• When would I find atoms with oxidation number of 0 in a compound?
(2 votes)
• when its a compound
a compound that doesnt have any superscript would have oxidation number of 0
(1 vote)

## Video transcript

Now that we know a little bit about oxidation and reduction, what I want to do is really just do an exercise to just make sure that we can at least give our best shot at figuring out the oxidation states for the constituent atoms that make up a compound. So, for example, here I have magnesium oxide, which is used in cement. It has other applications. And this is magnesium hydroxide, which is actually used in antacids. It's used in deodorant. And what I want you to think about, and I encourage you to pause this video right now, is given these two molecules, these two compounds, and what we know about the periodic table, try to come up with the oxidation states for the different elements in each of these compounds. So I'm assuming that you've given a go at it. Now let's try to work through this or think through this together. So first of all, magnesium. Magnesium right over here. We see it's group two. It's an alkaline earth metal. It has two valence electrons. It's not that electronegative. We've already seen that something in this group right over here with two valence electrons, it's likely to give them away. So if it were to form ionic bonds, or if it were to be ionized, it's likely to lose two electrons. If you lose two electrons, you would have a plus 2 charge. So magnesium would typically have a plus 2 oxidation state. On the other side of the periodic table, oxygen, group seven. It has six valence electrons. It's very electronegative, so electronegative that oxidation is named for it. It likes to take electrons from other elements. And oxygen in particular likes to take two electrons. So it's not unusual to see, actually anything in this group, but especially oxygen, taking two electrons from something else. If you take two electrons, and you started off neutrally, or you started in a neutral state, it's not unusual to see oxygen at a negative 2 oxidation state. So given that, it seems like this could work out. Magnesium could have a positive 2 oxidation state. And actually when you write it as a superscript here, the convention is to write the positive after the 2. And oxygen would have or could have a negative 2 oxidation state. And this makes sense relative to the overall charge of the molecule. Positive 2 plus negative 2 is going to be 0. And that makes sense. This thing overall is a neutral molecule. And not only in this case is the oxidation state a hypothetical ionic charge, if these were to be ionic bonds, this actually is an ionic compound. Oxygen actually does take two electrons. And magnesium actually does give away two electrons. So in this case the oxidation state is actually describing what is happening ionically. Now let's think about this one right over here, magnesium hydroxide. Well, just like before, magnesium typically has an oxidation state, likes to give away its electrons. So it could have an oxidation state of positive 2, which would imply that the entire hydroxide anion-- And let's just say hydroxide for now. Well I'll say hydroxide anion. I kind of gave it away a little bit-- that this hydroxide, or this part of the molecule, the right-hand part of what I've written here, for this whole thing to be neutral, it should have a negative 2 oxidation state. Now how does that make sense? Well we have two hydroxides here. Notice this subscript right over here. So if each of those hydroxides has a negative 1 charge, or a negative 1, I guess you could say, total oxidation state, then when you take two of them together, they would net out against the magnesium. And that does seem to make sense. If oxygen has a negative 2 oxidation state, hydrogen has a positive 1 oxidation state. Each hydroxide part of this molecule is going to have a net oxidation state of negative 1. But then you have two of them. So the net oxidation for this part of the molecule or the compound is going to be negative 2 nets out with the positive 2 from magnesium. So once again, it makes sense.