🎓 Prepared by students from Boğaziçi University

What Are Oxidation Numbers?

Oxidation numbers are assigned values that represent the number of electrons lost or gained by an atom in a compound. They are a bookkeeping tool to track electron transfer in redox reactions and help identify which atom is oxidized and which is reduced.

Short answer

Oxidation numbers are numbers assigned to elements in compounds showing how many electrons are lost or gained. Key rules: uncombined elements = 0, oxygen = −2 (usually), hydrogen = +1 (in compounds), alkali metals = +1, alkaline earth metals = +2, and the sum in a neutral compound = 0.

Common Oxidation Number Rules

Always True

•Uncombined elements: 0
•Alkali metals (Na, K): +1
•Alkaline earth metals (Ca, Mg): +2
•Oxygen in compounds: −2
•Hydrogen in compounds: +1
•Fluorine: −1 (always)

Exceptions & Neutral Sum

•Oxygen in peroxides: −1
•Oxygen in OF₂: +2
•Hydrogen in metal hydrides: −1
•Polyatomic ions: sum = charge
•Neutral compounds: sum = 0
•Example: in H₂O, H=+1, O=−2

Step-by-step worked examples

Assign oxidation numbers in H₂O.

H is +1 (hydrogen in compound).
O is −2 (oxygen in compound).
Check: 2(+1) + (−2) = 0 ✓ (neutral compound).

Assign oxidation numbers in KMnO₄.

K is +1 (alkali metal).
O is −2 (oxygen in compound).
Mn: +1 + Mn + 4(−2) = 0 → Mn = +7.

Assign oxidation numbers in the ion NO₃⁻.

O is −2 (oxygen in compound).
Sum = charge: N + 3(−2) = −1 → N = +5.

Flashcards

Quick quiz

Q1.Oxidation number of N in NO₂?

Correct answer: C. O = −2. N + 2(−2) = 0 → N = +4.

Q2.Oxidation number of S in SO₄²⁻?

Correct answer: A. O = −2. S + 4(−2) = −2 → S = +6.

Q3.Oxidation number of Cr in K₂Cr₂O₇?

Correct answer: C. K = +1, O = −2. 2(+1) + 2(Cr) + 7(−2) = 0 → Cr = +6.

Q4.Oxidation number of H in LiH?

Correct answer: B. LiH is a metal hydride. In metal hydrides, H = −1.

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Common mistakes

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Oxidation number = formal charge. — Correct: Oxidation number is a bookkeeping tool for electron tracking; it differs from formal charge.

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Oxygen is always −2. — Correct: Oxygen is −2 usually, but −1 in peroxides and +2 in OF₂.

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Hydrogen is always +1. — Correct: Hydrogen is +1 in most compounds, but −1 in metal hydrides (NaH, CaH₂).

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Oxidation numbers must be whole numbers. — Correct: Oxidation numbers can be fractional (e.g., +2.5 for Fe₃O₄).

FAQ

What are oxidation numbers used for?

To track electron transfer in redox reactions and identify which atoms are oxidized and reduced.

What is the oxidation number of O in peroxides?

−1, not −2. Example: in H₂O₂, O = −1.

Why do oxidation numbers matter?

They help identify redox reactions and determine the oxidizing and reducing agents.

Can oxidation numbers be negative?

Yes — negative numbers indicate electron gain. Example: N in N³⁻ = −3.