1) Free element: 0. 2) Monatomic ion: charge. 3) O: -2 (peroxide -1, OF₂ +2). 4) H: +1 (-1 in metal hydride). 5) Sum = charge of species. 6) Group I/II metals: fixed +1/+2.
-- NCERT Class 11 Chemistry, Ch. 7, p. 240Oxidation Number
8 MCQs9-step worked example
Source: NCERT Redox Reactions and ElectrochemistryPYQ coverage: NEET 2020, 2022, 2025, 2026Official key: NTA-verifiedLast updated: 25 Sep 2026
Oxidation Number, explained for NEET
Oxidation number assignment is a bookkeeping system — not a measure of actual charge on an atom. NEET regularly tests whether you can assign oxidation numbers correctly in compounds, ions, and polyatomic species. The confusion almost always comes from exceptions to the default rules, and from elements that take multiple oxidation states in the same compound.
The core rules (NCERT Class 11 Chemistry, Chapter 7):
- Free element: oxidation number = 0 (Na, O₂, P₄, S₈ — all zero).
- Monoatomic ion: oxidation number = ion charge (Na⁺ = +1, Cl⁻ = −1).
- Oxygen is −2, EXCEPT in peroxides (−1), superoxides (−1/2), OF₂ (+2), and O₂F₂ (+1).
- Hydrogen is +1 with non-metals, −1 with metals (NaH, CaH₂).
- Fluorine is always −1 (most electronegative element — no exceptions).
- Sum rule: oxidation numbers in a neutral compound sum to 0; in a polyatomic ion, sum to ion charge.
Where aspirants lose marks:
- Peroxide vs. oxide confusion. Seeing "Na₂O₂" and assigning O as −2 gives Na as +2 (wrong — Na is +1, O is −1 in the peroxide).
- Fractional oxidation states. In Fe₃O₄, average iron oxidation state is +8/3. NEET may ask you to identify this or recognise that Fe₃O₄ is a mixed oxide (FeO + Fe₂O₃, so Fe is both +2 and +3).
- Elements in multiple states. In Na₂S₄O₆ (tetrathionate), the four sulfur atoms are not all equivalent — two are 0, two are +5, but the average is +2.5. Questions may test whether you treat them as equivalent or identify the structural reality.
- OF₂ and O₂F₂. Fluorine's −1 rule overrides oxygen's −2 default. In OF₂, oxygen is +2. Many aspirants default to O = −2 here and get a nonsensical fluorine value.
The method that doesn't fail: Start by assigning atoms with fixed oxidation states (F, alkali metals, alkaline earths). Then assign H and O using their rules and exceptions. Solve for the unknown using the sum rule. Always verify the sum.
Can you answer these Oxidation Number MCQs?
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
MCQ 1Easy RecallPractice
What is the oxidation number of each atom in a free element (e.g., O₂, P₄, S₈)?
Show answer and why every option is right or wrong
Answer: B. By definition, every atom in a free (uncombined) element has an oxidation number of zero, regardless of how many atoms are in the molecule (NCERT Class 11 Chemistry, Chapter 7, page 240).
Why A is wrong: A is wrong because +1 is the oxidation number assigned to hydrogen in most compounds, not to free elements.
Why C is wrong: C is wrong because −1 applies to fluorine in compounds, not to atoms in their elemental state.
Why D is wrong: D is wrong because the rule is universal — all free elements have oxidation number 0, with no element-specific exceptions.
MCQ 2Easy RecallPractice
What is the oxidation number of fluorine in all its compounds?
Show answer and why every option is right or wrong
Answer: C. Fluorine is the most electronegative element and always takes an oxidation number of −1 in every compound, with no exceptions (NCERT Class 11 Chemistry, Chapter 7, page 240).
Why A is wrong: A is wrong because −2 is the default oxidation number for oxygen, not fluorine.
Why B is wrong: B is wrong because 0 applies only to fluorine in its elemental form (F₂), not in compounds.
Why D is wrong: D is wrong because fluorine never takes a positive oxidation number in any compound — it is always −1.
MCQ 3Easy RecallPractice
What is the oxidation number of hydrogen in calcium hydride (CaH₂)?
Show answer and why every option is right or wrong
Answer: A. In metal hydrides, hydrogen is −1 (not its usual +1 with non-metals). Ca is +2, each H is −1, and the sum is 0 (NCERT Class 11 Chemistry, Chapter 7, page 240).
Why B is wrong: B is wrong because +1 is hydrogen's oxidation number with non-metals (e.g., HCl, H₂O), but in metal hydrides like CaH₂ the metal donates electrons to hydrogen, making H negative.
Why C is wrong: C is wrong because 0 applies only to hydrogen in its free elemental form (H₂), not when bonded in a compound.
Why D is wrong: D is wrong because +2 is calcium's oxidation number in this compound, not hydrogen's.
MCQ 4Direct ApplicationPractice
What is the oxidation number of oxygen in Na₂O₂?
Show answer and why every option is right or wrong
Answer: D. Na₂O₂ is a peroxide. Na is +1 (alkali metal), so total positive charge = +2. For the sum to be zero with two oxygen atoms: 2x = −2, so x = −1. Oxygen is −1 in peroxides (NCERT Class 11 Chemistry, Chapter 7, page 240).
Why A is wrong: A is wrong because −2 is oxygen's default in most compounds, but Na₂O₂ is a peroxide where oxygen has a −1 oxidation state. Assuming −2 gives Na = +2, which is incorrect for sodium (trap: peroxide vs. oxide confusion).
Why B is wrong: B is wrong because +1 is the oxidation state of oxygen in OF₂, not in peroxides. Na₂O₂ has oxygen in the O₂²⁻ ion with each O at −1.
Why C is wrong: C is wrong because −1/2 is the oxidation state of oxygen in superoxides (e.g., KO₂), not in peroxides like Na₂O₂ (trap: confusing peroxide and superoxide classifications).
MCQ 5Direct ApplicationPractice
What is the oxidation number of oxygen in OF₂?
Show answer and why every option is right or wrong
Answer: B. Fluorine is always −1. In OF₂, two fluorines contribute −2 total. For the molecule to be neutral, oxygen must be +2. The fluorine rule overrides oxygen's usual −2 default (NCERT Class 11 Chemistry, Chapter 7, page 240).
Why A is wrong: A is wrong because −2 is oxygen's default, but when bonded to fluorine (more electronegative), oxygen must take a positive oxidation state. Applying −2 to O here gives F = +1, which violates fluorine's invariable −1 rule (trap: applying the oxygen default without checking for fluorine).
Why C is wrong: C is wrong because +1 would require F to have an oxidation number of −1/2 each, violating fluorine's fixed −1 rule. The sum check: +1 + 2(−1) = −1 ≠ 0.
Why D is wrong: D is wrong because −1 applies to oxygen in peroxides (e.g., Na₂O₂), not in OF₂ where fluorine forces oxygen positive.
MCQ 6Direct ApplicationPractice
What is the oxidation number of Cr in K₂Cr₂O₇?
Show answer and why every option is right or wrong
Answer: D. K = +1 (×2 = +2), O = −2 (×7 = −14). Let Cr = x. Sum: 2 + 2x − 14 = 0 → 2x = 12 → x = +6. Chromium is +6 in dichromate (NCERT Class 11 Chemistry, Chapter 7, page 240).
Why A is wrong: A is wrong because +7 is the oxidation state of Mn in KMnO₄, not of Cr in K₂Cr₂O₇; the charges give 2x = 12, so x = +6 (trap: permanganate/dichromate swap).
Why B is wrong: B is wrong because +12 treats both Cr atoms as a single unit. Oxidation number is per atom; the total contribution of both Cr atoms is +12, but each individual Cr is +6.
Why C is wrong: C is wrong because +3 is the oxidation state of Cr in Cr₂O₃ or Cr³⁺ salts, not in dichromate. Using +3 here gives a sum of 2 + 6 − 14 = −6 ≠ 0.
MCQ 7Concept TrapPractice
Fe₃O₄ can be written as FeO·Fe₂O₃. What are the oxidation states of iron in Fe₃O₄?
Show answer and why every option is right or wrong
Answer: A. Fe₃O₄ is a mixed oxide: FeO (Fe = +2) and Fe₂O₃ (Fe = +3). So one iron is +2 and two are +3. The average (+8/3) is mathematically correct but does not reflect the actual structural reality (NCERT Class 11 Chemistry, Chapter 7, page 240).
Why B is wrong: B is wrong because while +8/3 is the arithmetic average oxidation state, iron does not actually exist in a fractional oxidation state. Fe₃O₄ is a mixed oxide with distinct Fe²⁺ and Fe³⁺ sites (trap: confusing average oxidation number with actual oxidation states).
Why C is wrong: C is wrong because the stoichiometry is FeO·Fe₂O₃ — one FeO unit (one Fe²⁺) and one Fe₂O₃ unit (two Fe³⁺), not the reverse.
Why D is wrong: D is wrong because if all three were +3, total iron contribution = +9, and total oxygen = −8, giving a sum of +1 ≠ 0. The compound requires both +2 and +3 iron.
MCQ 8CalculationPractice
In the reaction 2KMnO₄ + 3H₂SO₄ + 5H₂C₂O₄ → 2MnSO₄ + K₂SO₄ + 8H₂O + 10CO₂, what is the change in oxidation number of Mn per atom?
Show answer and why every option is right or wrong
Answer: C. In KMnO₄: K = +1, O = −2 (×4 = −8), so Mn = +7. In MnSO₄: SO₄²⁻ has charge −2, so Mn = +2. Change: +7 → +2, a decrease of 5. This is a 5-electron reduction per Mn atom (NCERT Class 11 Chemistry, Chapter 7, page 240).
Why A is wrong: A is wrong because +4 is the oxidation state of Mn in MnO₂ (the product in neutral medium), not in MnSO₄. In acidic medium (H₂SO₄ present), Mn goes all the way to +2 (trap: confusing MnO₂ product of neutral medium with MnSO₄ product of acidic medium).
Why B is wrong: B is wrong because Mn starts at +7 in KMnO₄, not +4. The +4 state (MnO₂) doesn't appear in this reaction at all.
Why D is wrong: D is wrong because +6 is the oxidation state of Mn in manganate (MnO₄²⁻), the product in strongly basic medium. This reaction is in acidic medium — Mn reduces fully to +2.
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How do you solve a Oxidation Number question? A worked example
- 1
Given
• Compound: Na₂O₂• Na is an alkali metal (Group 1)• The compound is a peroxide (contains O₂²⁻ ion)
- 2
Required
Oxidation number of Na and O in Na₂O₂.
- 3
Concept
Alkali metals always have oxidation number +1. Oxygen is −1 in peroxides (not the default −2). The sum of all oxidation numbers in a neutral compound must equal zero.
- 4
Formula / Rule
Sum rule: Σ(oxidation numbers × atom count) = 0 for a neutral compound.
- 5
Assignment
• Na: +1 (alkali metal rule, no exceptions)• O: −1 (peroxide exception to the −2 default)
- 6
Verification
Sum = 2(+1) + 2(−1) = +2 − 2 = 0 ✓
Note: The subscript 2 on both Na and O are exact counting integers — they do not affect any significant-figure analysis. - 7
Final answer
Na = +1, O = −1 in Na₂O₂.
- 8
Common trap
Assigning O = −2 (the default) gives Na = +2, which is impossible for sodium — Na is ALWAYS +1 in compounds. If your sum rule gives Na anything other than +1, re-check whether the compound is a peroxide or superoxide.
- 9
Similar NEET-style question
Assign oxidation numbers to all elements in KO₂ (potassium superoxide). [Answer: K = +1, O = −1/2. Verification: +1 + 2(−1/2) = 0 ✓]
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What to remember before solving Oxidation Number questions
More in Redox Reactions and Electrochemistry: 4 exam traps and mistakes · 5 formulas · 2 question patterns from its other lessons.
Oxidation Number questions from past NEET papers
4 questions from NEET 2020, 2022, 2025, 2026. Answers verified against NTA official keys.
NEET 2026
The correct decreasing order of oxidation state of the underlined atom in each molecule is
1P₄O₁₀ > SO₃ > H₂O
2N₂O₅ > Al₂O₃ > H₂S
3PbO₂ > N₂O₃ > SO₃
4P₄O₆ > Cl₂O₇ > AlH₃
NTA Answer: Option 2(final)
NEET 2025
1+4, –4, and +6
2+1, –1, and +6
3+2, –2, and +6
4+1, –2, and +4
NTA Answer: Option 2(final)
NEET 2022
Identify the incorrect statement from the following
1Lithium is the strongest reducing agent among the alkali metals.
2Alkali metals react with water to form their hydroxides.
3The oxidation number of K in KO2 is +4.
4Ionisation enthalpy of alkali metals decreases from top to bottom in the group.
NTA Answer: Option 3(final)
NEET 2020
10 to −4
2+4 to +4
30 to +4
4−4 to +4
NTA Answer: Option 4(final)
All 21 past-paper questions from Redox Reactions and Electrochemistry →
Sources
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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