Oxidation numbers are bookkeeping tools that help chemists track how electrons are shared or transferred in substances. They are especially useful in redox reactions, where one substance loses electrons and another gains electrons. By assigning oxidation numbers, you can identify which atoms are oxidized and which are reduced.
This matters in batteries, corrosion, metal extraction, metabolism, and many other chemical processes.
An oxidation number is not always the same as a real ionic charge, but it follows consistent rules that make reactions easier to analyze. In the reaction Zn + Cu2+ -> Zn2+ + Cu, zinc changes from 0 to +2, showing that it loses electrons and is oxidized. Copper changes from +2 to 0, showing that it gains electrons and is reduced.
These changes let you balance redox reactions and connect chemical equations to electron flow.
Key Facts
- An element in its standard free form has oxidation number 0, such as Zn, Cu, O2, H2, and Cl2.
- A monatomic ion has an oxidation number equal to its charge, such as Na+ = +1 and S2- = -2.
- The sum of oxidation numbers in a neutral compound is 0.
- The sum of oxidation numbers in a polyatomic ion equals the ion charge.
- Oxygen is usually -2, except in peroxides where it is -1 and in compounds with fluorine where it can be positive.
- Oxidation is an increase in oxidation number, and reduction is a decrease in oxidation number.
Vocabulary
- Oxidation number
- A signed number assigned to an atom to track electron distribution in a compound or reaction.
- Oxidation
- A process in which an atom or ion loses electrons and its oxidation number increases.
- Reduction
- A process in which an atom or ion gains electrons and its oxidation number decreases.
- Redox reaction
- A chemical reaction in which oxidation and reduction happen together through electron transfer.
- Reducing agent
- A substance that causes another substance to be reduced while it is itself oxidized.
Common Mistakes to Avoid
- Assigning oxygen as -2 in every compound is wrong because oxygen is -1 in peroxides such as H2O2 and can be positive when bonded to fluorine.
- Forgetting that free elements have oxidation number 0 is wrong because atoms such as Zn, Cu, O2, and Cl2 are not ions when written alone.
- Treating oxidation number as always equal to the real charge is wrong because oxidation numbers are a formal counting method, especially in covalent compounds.
- Checking only one atom in a redox reaction is wrong because oxidation and reduction must occur together, so at least one oxidation number increases and another decreases.
Practice Questions
- 1 Find the oxidation number of sulfur in H2SO4. Use H = +1 and O = -2.
- 2 For the reaction Zn + Cu2+ -> Zn2+ + Cu, identify the oxidation number change for zinc and copper, and state how many electrons are transferred per zinc atom.
- 3 In the reaction 2 Mg + O2 -> 2 MgO, explain which element is oxidized, which element is reduced, and how the oxidation numbers show this.