Chemistry · Redox Equilibrium · NEET
KO2 is a superoxide, built from the superoxide ion O2^- (not O^2-). Potassium is a Group 1 metal and is fixed at +1. To make the compound neutral, the two oxygen atoms together must carry -1. So charge per oxygen = -1 divided by 2 = -1/2. Oxygen only takes its normal -2 in simple oxides like Na2O or MgO, not here.
Oxidation state is an average charge spread over the atoms of one element in a species. In O2^-, the single extra -1 charge is shared equally by two oxygen atoms, so the average per atom is -1/2. Fractional oxidation states are allowed and just mean the atoms are averaged (same idea appears in Fe3O4 where Fe averages +8/3).
They differ in the oxygen oxidation state. Oxide (like Na2O): O is -2. Peroxide (like Na2O2, H2O2): O is -1, from the O2^2- ion. Superoxide (like KO2): O is -1/2, from the O2^- ion. The metal stays at its normal value (+1 for K/Na) in all three; only the oxygen changes.
Larger cations (K+, Rb+, Cs+) have low charge density, so they stabilise the large, low-charge superoxide ion O2^-. Small Li+ forms mainly the oxide, Na+ forms the peroxide, while K, Rb and Cs form stable superoxides (KO2, RbO2, CsO2). This is why KO2 exists and is used to release O2 in breathing equipment.
Yes, but do not force oxygen to -2. Set K = +1 and let each O = x: (+1) + 2x = 0, so 2x = -1 and x = -1/2. If you wrongly assume O = -2 you would get K = +4, which is impossible for potassium. The trick is to fix the metal (+1) and solve for oxygen, not the reverse.
Identify the incorrect statement from the following:
Consider the compounds KO2, H2O2 and H2SO4. The oxidation states of the underlined elements (K in KO2, O in H2O2, S in H2SO4) respectively are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Potassium is +1 and each oxygen is -1/2. KO2 is potassium superoxide, and the O2^- ion holds a -1 charge shared over two oxygen atoms.
KO2 is a superoxide (contains O2^- with oxygen at -1/2). A peroxide contains O2^2- with oxygen at -1, like Na2O2 and H2O2.
No. Potassium is a Group 1 alkali metal with only one valence electron, so it is always +1 in compounds. A +4 answer for K in KO2 is the deliberate trap.
Oxidation state is an average charge per atom. The single -1 charge of O2^- is shared equally by two oxygens, giving -1/2 each. Fractions are valid whenever a charge is averaged over identical atoms.
KO2 reacts with CO2 and moisture to release O2, so it is used in breathing masks, submarines and space missions. This is a redox use, but for NEET the key point is K = +1 and O = -1/2.
