Oxidation Number of Oxygen in Peroxides, Superoxides and OF2
Chemistry · Redox Equilibrium · NEET
Oxygen is usually -2, but there are three exceptions you must know for NEET. In peroxides (like H2O2, Na2O2) each O is -1. In superoxides (like KO2, RbO2) each O is -1/2. In OF2, oxygen is +2 because fluorine is more electronegative and takes the -1. Memory hook: "PerOne, SuperHalf, F-Flips" -> Peroxide -1, Superoxide -1/2, Fluorine flips O to +2.
Oxygen is -2 in normal oxides, but flips to -1 in peroxides, -1/2 in superoxides, and +2 in OF2 (because fluorine is more electronegative). Check the compound type before assuming -2.
Your doubts, answered
Why is oxygen -1 in peroxides and not -2?
A peroxide has an O-O single bond (the peroxide linkage). The two oxygen atoms share the bonding electrons equally, so neither O gets both electrons. Each O only gains one extra electron from the metal or hydrogen, giving it -1 instead of -2. Examples: H2O2, Na2O2, BaO2. Quick check for H2O2: 2(+1) + 2(x) = 0, so x = -1.
Why is oxygen -1/2 in superoxides like KO2?
A superoxide contains the superoxide ion O2^- , which has an overall charge of just -1 shared between TWO oxygen atoms. So each oxygen is -1/2. This is a fractional (average) oxidation state, which is allowed. In KO2, K is +1, so the O2 part must be -1, meaning each O = -1/2. Superoxides form with big alkali metals: K, Rb, Cs.
What is the oxidation number of oxygen in OF2? Why is it positive?
In OF2, oxygen is +2. Fluorine is the ONLY element more electronegative than oxygen, so in every F compound F is -1. With two F atoms at -1 each, oxygen must be +2 to make the molecule neutral: x + 2(-1) = 0, so x = +2. This is the one common case where oxygen is positive. (In O2F2, oxygen is +1.)
How do I know if a compound is an oxide, peroxide, or superoxide?
Look at the metal and the oxygen count. Normal oxide: O is -2 (Na2O, CaO). Peroxide: has O2^2- , O is -1 (Na2O2, H2O2, BaO2). Superoxide: has O2^- , O is -1/2 (KO2, RbO2, CsO2). Trick: small alkali metals (Li, Na) mostly give oxides/peroxides; large ones (K, Rb, Cs) give superoxides. Never assume O = -2 in these compounds.
What is the oxidation number of K in KO2? Is it +4?
No. K is +1 in KO2, never +4. This is a famous NEET trap (asked in 2022). Potassium is an alkali metal, so it is ALWAYS +1. Since KO2 is a superoxide, the two oxygens carry -1 total (each O = -1/2), which balances the +1 of potassium. If you wrongly treat O as -2, you get a fake +3 or +4 for K, which is impossible.
⚠️ The NEET trap ✗ In KO2, treat oxygen as the usual -2, so K = +4 (or +3). ✓ KO2 is a superoxide, so each O = -1/2 (O2 part = -1). K is +1, as all alkali metals always are. 🧠 NEET 2022 asked exactly this: the WRONG statement was 'oxidation number of K in KO2 is +4'. Alkali metal = always +1. If you get a weird value for the metal, the oxygen is not -2.
Real NEET questions
NEET 2025
Consider the compounds KO2, H2O2 and H2SO4. The oxidation states of the underlined elements (K in KO2, O in H2O2, S in H2SO4) in them are, respectively:
A · +1, -2 and +4
B · +4, -4 and +6
C · +1, -1 and +6 ✓
D · +2, -2 and +6
Solution: KO2 is potassium superoxide, so K is +1 (each O is -1/2, not asked here). H2O2 is a peroxide, so O is -1. In H2SO4: 2(+1) + S + 4(-2) = 0, giving S = +6. So the values are +1, -1 and +6, which is option (c). Note how the same question uses two of our exceptions (superoxide and peroxide) at once.
NEET 2022
Identify the incorrect statement from the following:
A · Alkali metals react with water to form their hydroxides
B · The oxidation number of K in KO2 is +4 ✓
C · Ionisation enthalpy of alkali metals decreases from top to bottom in the group
D · Lithium is the strongest reducing agent among the alkali metals
Solution: KO2 is potassium superoxide. Potassium is an alkali metal, so it is always +1, never +4. The superoxide ion O2^- carries an overall -1 charge, so each O is -1/2 and K is +1. Statement (b) is wrong, so it is the answer. The other three statements are correct alkali-metal facts.
NEET 2019 Odisha
The oxidation state of Cr in CrO5 is:
A · -6
B · +12
C · +6 ✓
D · +4
Solution: CrO5 (chromium peroxide, written CrO(O2)2) has two peroxo O-O linkages. In those four peroxo oxygens O = -1, and the one doubly-bonded oxygen is -2. Let Cr = x: x + 4(-1) + 1(-2) = 0, so x = +6. If you wrongly count all five oxygens as -2 you get a fake +10. Spotting the peroxo bonds (O = -1) is the whole trick.
Solved Redox Equilibrium NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Oxygen is -2 in most compounds, but there are exceptions: peroxides (O = -1), superoxides (O = -1/2), and fluorine compounds OF2 (O = +2) and O2F2 (O = +1). In elemental O2 and O3 the oxidation number is 0.
What is the oxidation number of oxygen in H2O2?
It is -1. H2O2 is hydrogen peroxide, which has an O-O peroxide bond. Using 2(+1) + 2(x) = 0 gives x = -1 for each oxygen.
Which metals form superoxides?
The larger alkali metals: potassium (KO2), rubidium (RbO2) and caesium (CsO2). In all of them each oxygen is -1/2 and the metal is +1.
Why can oxygen be positive only with fluorine?
Fluorine is the most electronegative element, more electronegative than oxygen. So in OF2, fluorine pulls the shared electrons and takes -1, forcing oxygen to be +2. Oxygen is positive only when bonded to fluorine.
Can an oxidation number be a fraction like -1/2?
Yes. In superoxides each oxygen is -1/2, which is an average (fractional) oxidation state. It happens because the -1 charge of the O2^- ion is shared between two oxygen atoms. Fractions are allowed in oxidation-state work.
