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# Unusual oxygen oxidation states

Determining oxidation numbers in hydrogen peroxide, H₂O₂, and oxygen difluoride, OF₂.  Created by Sal Khan.

## Want to join the conversation?

• So what about LiO2? What oxidation state does oxygen have, -.5?
• Yes. In LiO₂, Li has a +1 oxidation state. The two oxygens each have a -½.

You should be aware that all superoxides are radicals that are extremely reactive. Lithium superoxide (that is the name of LiO₂) is exceptionally extremely reactive. To the best of my knowledge, it has never been confirmed to exist at temperatures above 15 K (I did see a paper once about LiO₂ possibly existing momentarily at a temperature as high as 40 K, but I don't know whether that was ever confirmed.)
• In hydrogen peroxide the oxygen has one oxidation state right but the oxygen is bonded with another oxygen do that doesnt count as another oxidation state ?
• When an element is bonded to another atom of itself, that contribute 0 to its oxidation state. So, in other words, you just ignore the bond an atom makes to another atom of itself. So, in hydrogen peroxide we have:
H-O-O-H. The O-O bond doesn't count. So each O is bonded to one H. Since H has an oxidation state of +1, O must cancel that out, so its oxidation number is −1.
• Sal says that with oxidation states we write the sign after the number (2+ or 2-), but according to my textbook oxidation states should be written as +2 and -2.

Which way is correct?
• I was always taught to use the +2 and -2 notation (i.e. number after the sign) in order to distinguish oxidation states from actual ionic charges. So when someone looks at your chemical equation they know it's an oxidation state you have written and not an ionic charge on the molecule.
• How many oxidation states are there besides -2 and -1 ?
• There are a whole range of oxidation states that can range from about -7 to +7. It is rare that you go above 7. Chromium, a transition metal, with my knowledge, has the most oxidation states, which range I believe from +7 to -2.
• is it same for every compound that one element has to become electro+ while the other element has to become electro-
• No, that is not always the case. It is very common, but is not necessarily the case all of the time.

Remember, the oxidation states are ASSIGNED numbers, they are not necessarily actual charges that are physically present.
• what is the difference between a superoxide and peroxide and how to determine whether an oxide is superoxide or peroxide?
• At a very basic level, in a peroxide, the oxygen has an oxidation number of -1, where in a superoxide, the oxidation number of oxygen is -1/2. To determine whether the anion is a superoxide or peroxide, use the total oxidation number of a compound and the known oxidation numbers of other elements. For example, in KO2, we know that the oxidation number is 0, and K is +1; So we can do 1 + 2O = 0, simple algebra tells us that the oxidation number of the Oxygens is -1/2, which means it is the superoxide anion.
• what does covalent bond mean?
• A covalent bond is a bond formed between two non metal atoms where they share a bonding pair of electrons.

However you can have a dative covalent bond where one atom shares both of the bonding electrons
• at why is hydrogen more electronegative
(1 vote)
• You misheard Sal, he said hydrogen is less electronegative. Hydrogen is less electronegative than oxygen because oxygen has six electrons in its outer shell and so really really wants more electrons to become stable...wheras hydrogen doesn't want to gain electrons as much, so in the covalent bond the oxygen 'hogs the electrons' they spend more time near the oxygen and less time near the hydrogen.

Sal explain this really well in this video.https://www.khanacademy.org/science/chemistry/periodic-table-trends-bonding/v/other-periodic-table-trends
• At , if Florine is more electronegative than oxygen, why not we consider it as standard? That is, why don't we say 'Floridised' instead of 'Oxidised' ?
• Our atmosphere is 20% oxygen and almost all metals are found naturally in compounds with oxygen (ie oxidised) on Earth.

The term was coined before we knew that oxygen was not the only element that could do this.
• so the oxidation number for an element is not absolute? i.e. O can be 2- or 1- or whatever the heck it wants?
if so, then how are we supposed to work out what its oxidation number is in any given question?
(1 vote)
• Oxidation numbers can vary quite a lot for some atoms, but are very consistent for others.

Oxygen is actually among the most reliable atoms, as far as oxidation number. It nearly always counts for 2-, except when it's part of a peroxide, when it is instead 1-.
Fluorine has a 1-, with no exceptions that I know of. Other halogens also tend to have 1-, unless there is oxygen involved, as in polyatomic ions like chlorate and chlorite.

This makes sense if you think about electronegativity. Halogens are pretty electronegative, and tend to pull an electron toward them. But if you compare the actual EN values, oxygen is actually more electronegative than anything but fluorine, and tends to pull electrons away from even chlorine and the rest.
Fluorine is always 1-, because it pulls hardest, and only wants one electron, while oxygen pulls almost as hard, and wants two electrons, so it is almost always 2-.

It's also important to remember that in a pure element, the oxidation number is 0, by definition. So, in O2, both oxygen atoms have oxidation number 0, just as both fluorines in F2 have 0, and so on.