What Are Oxidation and Reduction Rules?
Oxidation states are numbers assigned to atoms to track electron transfer in redox reactions. The rules for assigning them are consistent and help you identify which atoms are oxidized or reduced.
Oxidation states follow a hierarchy of rules: free elements are 0; alkali metals are +1; oxygen is usually −2; hydrogen is usually +1; the sum equals the charge. Use these to predict redox behaviour.
Step-by-step worked examples
Assign oxidation states to Cr in K₂Cr₂O₇.
K: +1 (alkali metal, 2 atoms × +1 = +2) O: −2 (6 atoms × −2 = −12) Cr: unknown (2 atoms × x) Sum: +2 + 2x + (−12) = 0 2x = +10 → x = +6 Cr in K₂Cr₂O₇ is +6.
What is N's oxidation state in NH₄⁺?
H: +1 (4 atoms × +1 = +4) N: unknown Sum = +1 (charge) x + 4 = +1 x = −3 N in NH₄⁺ is −3.
In H₂O₂, what is O's oxidation state?
This is a peroxide—exception to −2 rule. H: +1 (2 atoms × +1 = +2) O: unknown (2 atoms × x) Sum = 0 2 + 2x = 0 x = −1 O in H₂O₂ is −1 (peroxide).
Flashcards
Quick quiz
Q1.What is S's oxidation state in H₂SO₄?
Q2.In KMnO₄, Mn's oxidation state is…
Q3.Oxidation state of O in K₂O₂?
Q4.In NH₃, N's oxidation state is…
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Common mistakes
Oxidation state is the same as charge. — Correct: Oxidation state tracks electrons; charge is net imbalance.
O is always −2. — Correct: O is −2 usually, but −1 in peroxides.
Oxidation state must be a whole number. — Correct: It usually is, but fractional states exist in some compounds.
You can't assign states to elements in their standard form. — Correct: Yes you can—free elements have oxidation state 0.
FAQ
Why do we use oxidation states?
To track electron transfer in redox reactions and predict reactivity.
Is oxidation state the same as charge?
No—oxidation state assumes ionic bonding; charge is actual net imbalance.
Can oxidation states be fractional?
Rarely, but yes—in some mixed-valence compounds.
How do you know if an atom is oxidized?
Its oxidation state increases; it's losing electrons.