Chemistry · Redox Equilibrium · NEET
It is a made-up (hypothetical) charge on an atom. We pretend every bond in the molecule is ionic, meaning the shared electrons fully belong to the more electronegative atom. The charge left on each atom after this pretend sharing is its oxidation number. It is just a bookkeeping number that helps us track which atom lost or gained electron control. This matters for NEET because almost every redox question starts by finding oxidation numbers.
Yes. For NEET they mean the same thing and are used interchangeably. NCERT uses both words. So if a question asks for the 'oxidation state of Cr' or the 'oxidation number of Cr', you do the exact same calculation. Do not waste time worrying about a difference; there is none at this level.
Real (ionic) charge is what an ion actually carries, like Na+ is really +1. Oxidation number is a pretend charge we assign even inside covalent molecules where no real ions exist. Example: in CH4, carbon is not a real -4 ion, but its oxidation number is -4 because carbon is more electronegative than hydrogen, so we give C all four shared electron pairs. For a simple monatomic ion, the oxidation number and the real charge are the same.
Yes, all three. Zero: any free element like O2, Cl2, or Na metal has oxidation number 0. Negative: oxygen is usually -2 and chlorine in salts is -1. Fractional: sometimes an average is not a whole number, for example the oxidation state of oxygen in the superoxide KO2 is -1/2, and Fe in Fe3O4 averages +8/3. NEET does test these odd cases, so do not assume it must be a whole number.
Because it is the easiest way to spot oxidation and reduction. If an atom's oxidation number goes up, it is oxidised (lost electrons). If it goes down, it is reduced (gained electrons). Without oxidation numbers you cannot easily see this inside covalent molecules. Many NEET questions like 'which reaction is not redox' are solved just by checking if any oxidation number changes.
The more electronegative atom in each bond gets both shared electrons. So in a C-H bond, carbon (more electronegative) takes the pair, making C more negative and H more positive. In an O-H bond, oxygen takes the pair. This single idea is the reason oxygen is usually -2 and hydrogen is usually +1. Once you learn the rules, you rarely have to think about electronegativity directly, but this is why the rules exist.
What is the change in oxidation number of carbon in the reaction CH4(g) + 4Cl2(g) -> CCl4(l) + 4HCl(g)?
The correct order of N-compounds in their decreasing order of oxidation states is:
For the compounds KO2, H2O2 and H2SO4, the oxidation states of the underlined elements (K in KO2, O in H2O2, S in H2SO4) are respectively:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
No. Usually it is a whole number, but for some species it is an average that comes out as a fraction, such as -1/2 for O in KO2 or +8/3 for Fe in Fe3O4. These average values are still counted as correct oxidation states in NEET.
Always zero. This includes single atoms like He and molecules of one element like H2, O2, Cl2, S8, and metals like Na or Fe in their pure form. Zero simply means the atom has not lost or gained any electron control.
Write the sign before the number: +2, -1, +6. This is different from real ionic charge, which is often written with the sign after, like Ca2+. NEET solutions follow the sign-first style for oxidation numbers.
If the oxidation number increases during a reaction, that atom is oxidised. If it decreases, that atom is reduced. For example, carbon going from -4 in CH4 to +4 in CCl4 means carbon is oxidised.
Because potassium is a group-1 metal and can only be +1. In KO2 the oxygen is the special part as a superoxide (each O is -1/2). Always fix the metal's known value first, then solve for the unusual atom.
