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# Practice determining oxidation states

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

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• Which elements show highest oxidation state in the perodic table ?
• 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.
• What is the oxidation state or oxidation number of iron in Fe3O4 (Ferrosoferric Oxide)?
• That compound has some Fe in the +2 state and rest in the +3 state.
• 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?
• 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
• How do you find out the oxidation numbers?
• 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.
• 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.
• 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.
(1 vote)
• Why is Magnesium oxidised in 3Mg+N2->Mg3N2?
• how to find the oxidation number of Fe2O6
• there is only two oxidation states for Fe, 2+ and 3+. Fe2O6 does not exist.
Fe(2+) O(2-) Fe2(3+) O3(2-)
• what is the oxidation number of vanadium in VO2+?
(1 vote)
• Why is oxygen always found in pairs in nature?