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How to Find Oxidation Number in Compounds and Ions (Worked Examples)

Chemistry · Redox Equilibrium · NEET

To find the oxidation number of one atom, write a simple equation: give each known atom its fixed value (H = +1, O = usually -2, Group 1 = +1), let the unknown atom be x, and make the total equal the charge (0 for a neutral compound, the ion charge for an ion). Then solve for x. Memory hook: "Known values in, unknown = charge minus the rest." This is one of the most repeated NEET redox skills, so getting the equation right every time earns easy marks.
Finding Oxidation Number: One EquationExample: S in H2SO4 (neutral, total = 0)H = +12 atoms → +2S = xunknownO = -24 atoms → -8(+2) + x + (-8) = 0x = +6 → S is +6Check: +2 + 6 - 8 = 0 ✓
The one-equation method: assign known values (H = +1, O = -2), call the target atom x, set the sum to the total charge (0 here), and solve. For H2SO4 this gives S = +6.

Your doubts, answered

What is the basic method to find an oxidation number?

Use one equation. Step 1: give every atom you already know its fixed value (H = +1, O = -2, F = -1, alkali metals = +1, alkaline earth = +2). Step 2: let the atom you want be x, and multiply each value by how many of that atom there are. Step 3: set the sum equal to the total charge: 0 for a neutral molecule, or the ion's charge for an ion. Step 4: solve for x. Example, SO4 2-: S + 4(-2) = -2, so S - 8 = -2, S = +6.

Do I set the total to 0 or to the ion charge?

For a neutral compound (no charge written), the sum of all oxidation numbers is 0. For an ion, the sum equals the charge on the ion. So for NO3-, N + 3(-2) = -1 gives N = +5. For a neutral molecule like HNO3, H + N + 3(-2) = 0 gives N = +5. Always match the right-hand side to the charge, or you get the wrong answer.

How do I find the oxidation number of carbon in CH4 and CCl4?

In CH4, hydrogen is +1 (H bonded to a non-metal), so C + 4(+1) = 0, giving C = -4. In CCl4, chlorine is -1, so C + 4(-1) = 0, giving C = +4. So carbon goes from -4 to +4. This exact change was asked in NEET 2020, so practise it.

Why does H2O2 give O = -1 instead of -2?

O = -2 is only the usual rule, not a law. In peroxides like H2O2 the two oxygens share a bond with each other (O-O), so each oxygen is -1. Check: 2(+1) + 2(O) = 0, so O = -1. Watch out for peroxides (H2O2, Na2O2), superoxides (KO2, O = -1/2), and OF2 (O = +2). NEET loves these exceptions.

How do I do CrO5 where the simple rule gives a wrong answer?

If you treat all 5 oxygens as -2 you get Cr = +10, which is impossible. CrO5 (chromium peroxide) actually has two peroxo O-O linkages: four oxygens are -1 and one is -2. So Cr + 4(-1) + 1(-2) = 0 gives Cr = +6. This was NEET 2019. When a normal calculation gives a silly value like +10, suspect a peroxo or superoxo bond.

How do I find oxidation number in a compound with more than one of the target atom, like N2O5?

Multiply the unknown by how many atoms there are. In N2O5: 2(N) + 5(-2) = 0, so 2N = 10, N = +5. The value is per atom, not for the pair. Same idea for Al2O3: 2(Al) + 3(-2) = 0, so Al = +3.

⚠️ The NEET trap
✗ In CrO5 all five oxygens are -2, so Cr = +10.
✓ CrO5 is chromium peroxide with two O-O (peroxo) linkages: four O are -1 and one O is -2, giving Cr = +6.
🧠 If your metal oxidation number comes out impossibly high (like +10 or +12), check for peroxide or superoxide oxygen before you trust it.

Real NEET questions

NEET 2020

What is the change in oxidation number of carbon in the following reaction? CH4(g) + 4Cl2(g) -> CCl4(l) + 4HCl(g)

A · -4 to +4 ✓
B · 0 to -4
C · +4 to +4
D · 0 to +4
Solution: In CH4, H is +1, so C + 4(+1) = 0 gives C = -4. In CCl4, Cl is -1, so C + 4(-1) = 0 gives C = +4. Carbon changes from -4 to +4, so option (a).
NEET 2018

The correct order of N-compounds in their decreasing order of oxidation states is:

A · HNO3, NH4Cl, NO, N2
B · HNO3, NO, NH4Cl, N2
C · HNO3, NO, N2, NH4Cl ✓
D · NH4Cl, N2, NO, HNO3
Solution: Find N in each: HNO3 -> N + 3(-2) + 1 = 0, N = +5. NO -> N + (-2) = 0, N = +2. N2 -> element, N = 0. NH4Cl -> N in NH4+ is -3. Decreasing order: +5 > +2 > 0 > -3, which is HNO3 > NO > N2 > NH4Cl, option (c).
NEET 2025

For KO2, H2O2 and H2SO4, the oxidation states of K in KO2, O in H2O2 and S in H2SO4 are respectively:

A · +1, -2, +4
B · +4, -4, +6
C · +1, -1, +6 ✓
D · +2, -2, +6
Solution: KO2 is a superoxide, K is always +1 (each O is -1/2). H2O2 is a peroxide, so O = -1. In H2SO4: 2(+1) + S + 4(-2) = 0 gives S = +6. So +1, -1, +6, option (c).

Solved Redox Equilibrium NEET PYQs

Try the real previous-year questions from this chapter — each with the answer and a full solution.

See all 22 Redox Equilibrium NEET PYQs ›
Next concept: Oxidation Number of Oxygen in Peroxides, Superoxides and OF2 →Keep learning — 2 minFeeling ready? Solve the Redox Equilibrium NEET PYQs ›Or practice on your phone — get the free MedicNEET app ›

Frequently asked

What is the sum of oxidation numbers rule?

In any neutral compound the oxidation numbers of all atoms add up to 0. In an ion they add up to the charge on that ion. This single rule lets you solve for any one unknown atom.

Which atoms have fixed oxidation numbers I can rely on?

Alkali metals (Group 1) = +1, alkaline earth metals (Group 2) = +2, fluorine = -1 always, hydrogen = +1 with non-metals (but -1 in metal hydrides), and oxygen = -2 except in peroxides (-1), superoxides (-1/2) and OF2 (+2).

Can an oxidation number be a fraction?

Yes. When the same element sits in different environments, the calculation can give an average that is a fraction, for example Fe in Fe3O4 is +8/3 and O in KO2 is -1/2. It is an average, not the value on every single atom.

How do I check if my oxidation number is correct?

Put it back into the sum. Add up every atom times its value and confirm the total equals 0 (compound) or the ion charge. If the total is off, or a metal comes out impossibly high, recheck oxygen for a peroxide or superoxide.

Does electronegativity matter when assigning oxidation numbers?

Yes, for bonds between different atoms the shared electrons are given to the more electronegative atom. That is why F is always -1 and why O is +2 in OF2 (F is more electronegative than O).

Related concepts:
Rules for Assigning Oxidation NumbersWhat Is Oxidation Number (Oxidation State)Oxidation Number of Oxygen in Peroxides, Superoxides and OF2Fractional and Average Oxidation States (Fe3O4, CrO5, S4O6)Balancing Redox Equations by Oxidation Number Method
Practise Chemistry for NEET:
Chemistry MCQs (free, chapter-wise)Redox Equilibrium PYQs