Oxidation Number Calculator Chemistry • PCM Notes
Supported ions and compounds: Enter formula with charge if ionic (e.g., SO4 with charge -2, MnO4 with charge -1). For neutral molecules, charge is 0.

Understanding Oxidation Numbers

Quick Answer: The oxidation number (or oxidation state) is the hypothetical charge an atom would have if all bonds were completely ionic. It helps track electron transfer in redox reactions.

Key Rules for Assigning Oxidation Numbers

Rule 1: Free elements have oxidation number = 0. Rule 2: For monatomic ions, it equals the ion charge. Rule 3: Oxygen is usually −2 (except in peroxides where it is −1). Rule 4: Hydrogen is +1 with nonmetals, −1 with metals. Rule 5: The sum of oxidation numbers equals the overall charge of the species.

Solved Examples

KMnO₄
K = +1, O₄ = 4(−2) = −8
Sum = 0 ⇒ +1 + Mn + (−8) = 0
Mn = +7
Cr₂O₇²⁻
O₇ = 7(−2) = −14
2Cr + (−14) = −2
2Cr = +12
Cr = +6
H₂SO₄
H₂ = 2(+1) = +2, O₄ = −8
(+2) + S + (−8) = 0
S = +6

FAQs

What is the difference between oxidation number and valency?

Valency is the combining capacity of an element (always positive), while oxidation number can be positive, negative, or zero and indicates the electron shift in a compound.

Why is the oxidation number of oxygen -1 in peroxides?

In peroxides (like H₂O₂), oxygen atoms are bonded to each other (O—O bond). Each oxygen shares equally with itself, resulting in an oxidation state of −1 instead of the usual −2.

Can an element have fractional oxidation numbers?

Yes, in some cases like Fe₃O₄ where iron has an average oxidation number of +8/3. This represents a mixture of Fe(+2) and Fe(+3) ions.

How do I determine the oxidizing and reducing agent?

The substance that gets reduced (gains electrons, oxidation number decreases) is the oxidizing agent. The substance that gets oxidized (loses electrons, oxidation number increases) is the reducing agent.