Chemistry · Redox Equilibrium · NEET
Apply them in this priority order and the answer comes out right almost every time: (1) A free element (uncombined) has oxidation number 0, e.g. Na, O2, P4, N2 are all 0. (2) A monatomic ion equals its charge, e.g. Na+ is +1, Cl- is -1, Al3+ is +3. (3) Group 1 metals are always +1, Group 2 metals always +2. (4) Fluorine is always -1 in every compound. (5) Hydrogen is +1 (except -1 in metal hydrides like NaH). (6) Oxygen is -2 (except -1 in peroxides, -1/2 in superoxides, and positive with fluorine like OF2). (7) The sum of all oxidation numbers equals 0 in a neutral molecule, and equals the ion charge in an ion. You use rule 7 to solve for the unknown atom.
Write the sum rule and plug in known atoms. In H2SO4, H = +1 and O = -2. The molecule is neutral, so total = 0. So 2(+1) + S + 4(-2) = 0, which gives +2 + S - 8 = 0, so S = +6. Always fix the atoms you know (H, O, Group 1, F) first, then let the unknown atom be x and solve. This exact method gave S = +6 in a 2025 NEET question.
Oxidation number counts electron sharing between DIFFERENT atoms. In a free element like O2, N2 or Na metal, every bond is between two identical atoms, so no atom pulls electrons more than the other. Nothing is transferred, so the value is 0. This is why O2, P4, S8, Cl2 and any pure metal are all 0, no matter how many atoms are in the molecule.
Hydrogen is -1 only in metal hydrides, where it is bonded to a metal (NaH, CaH2, LiH). Oxygen breaks its -2 rule in three cases you must memorise for NEET: peroxides like H2O2 and Na2O2 (O = -1), superoxides like KO2 (O = -1/2), and compounds with fluorine like OF2 (O = +2, because F is more electronegative). These exceptions are a favourite NEET trap.
For an ion, all the oxidation numbers add up to the CHARGE of the ion, not zero. Example: in SO4^2-, O = -2, so S + 4(-2) = -2, which gives S = +6. In Cr2O7^2-, 2Cr + 7(-2) = -2, so 2Cr = +12, Cr = +6. Forgetting to use the ion charge (using 0 instead) is the most common mistake students make.
What is the change in oxidation number of carbon in the following reaction? CH4(g) + 4Cl2(g) -> CCl4(l) + 4HCl(g)
The correct order of N-compounds in their decreasing order of oxidation states is:
Consider 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.
Yes. When identical atoms in a compound are in different environments, the average can be a fraction. For example, oxygen in KO2 (superoxide) is -1/2, and iron in Fe3O4 is +8/3 on average. NEET does test these average or fractional states, so do not reject a fraction as wrong.
No. Valency is the combining capacity and has no sign; oxidation number has a positive or negative sign and can be zero or a fraction. For example, in H2O2 oxygen has valency 2 but oxidation number -1.
Follow the priority order: free element, then monatomic ion, then Group 1 and Group 2 metals, then fluorine, then hydrogen, then oxygen, then the sum rule last. Fluorine outranks oxygen, and oxygen outranks the sum rule, which is why OF2 gives oxygen +2.
Almost every redox question, and many inorganic questions, start by asking you to assign oxidation numbers. If you get the rules wrong here, you get disproportionation, balancing, and agent-identification questions wrong too. It is a small topic that unlocks a large chunk of marks.
