Chemistry · Redox Equilibrium · NEET
The oxidation number of a single atom is always a whole number in reality. The fraction only appears when a formula contains several atoms of the SAME element that are actually in different whole-number states. When you apply the normal formula (sum of all oxidation states = charge), you get one average value spread over all those atoms. So +8/3 for Fe in Fe3O4 is not the state of one iron atom - it is the average of the three iron atoms present.
Take Fe3O4. Oxygen is -2, and there are 4 of them, giving -8. The whole compound is neutral, so the 3 Fe atoms together must be +8. Divide: 3x = +8, so x = +8/3 (which is about +2.67). This is an AVERAGE. In real Fe3O4 (which is FeO.Fe2O3) there is one Fe(+2) and two Fe(+3): (2 + 3 + 3)/3 = 8/3. Same answer, and it explains why the value is fractional.
This is the biggest NEET trap here. If you wrongly treat all 5 oxygens as -2 you get x = +10, but chromium can never be +10 (Cr only has 6 valence electrons). CrO5 is chromium PEROXIDE, structure CrO(O2)2. It has 4 peroxide oxygens (O-O bonds, each O is -1) and 1 double-bonded oxygen (-2). So x + 4(-1) + 1(-2) = 0, giving x = +6. Always check for peroxide/superoxide before assuming oxygen is -2.
In S4O6 (the tetrathionate ion S4O6^2-), oxygen is -2 x 6 = -12, and the ion charge is -2. So 4 sulphurs = -2 - (-12) = +10, giving average x = +10/4 = +5/2 (about +2.5). Structurally it is a mix: two central S atoms are 0 and two outer S atoms are +5, and the average of 0, 0, +5, +5 is +10/4 = +5/2. This is why it is fractional.
You get a fraction ONLY when the same element appears multiple times in different real oxidation states (like Fe3O4, S4O6, Fe2O3 stays whole, but C3O2 gives fractions). If every atom of that element is chemically identical, the answer is a whole number. Rule of thumb: fractions come from mixed states of ONE element, not from strange bonding of oxygen.
No. Whether you use the true whole-number states or the fractional average, the total electron change over the whole molecule is the same. The average is a shortcut. For NEET, average values are perfectly fine for comparing oxidising power or ordering states, but remember they are averages when a question asks about a single atom.
The oxidation state of Cr in CrO5 is:
Consider KO2, H2O2 and H2SO4. The oxidation states of K (in KO2), O (in H2O2) and S (in H2SO4) respectively are:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. No single iron atom carries +8/3. It is the average of one Fe(+2) and two Fe(+3). The formula gives you the average because it cannot tell the atoms apart.
Chromium has only 6 valence electrons (3d5 4s1), so its maximum possible oxidation state is +6. Any answer above +6 for Cr is automatically wrong. CrO5 has peroxide oxygens, giving +6.
It is +5/2 (about +2.5). Two central sulphurs are 0 and two outer sulphurs are +5, and their average is +10/4 = +5/2.
Yes. CrO5, Fe3O4, KO2 (superoxide) and mixed-state compounds have appeared. Knowing peroxide (-1), superoxide (-1/2) and averages saves you from the common traps.
If the same element appears more than once and the total does not divide evenly by the number of those atoms, the state is fractional. It signals the element sits in two or more different real states.
