Oxidation number
High School
Definition
A value assigned to an atom that shows how many electrons it has apparently gained or lost in a compound. Oxidation numbers help track electron transfer in redox reactions and are used to balance redox equations.
Worked examples
In \(\)H\(_2\)O\(\), hydrogen is +1 and oxygen is -2.
Hydrogen almost always has +1, oxygen almost always -2 in compounds.
In \(\)NaCl\(\), sodium is +1 and chlorine is -1.
Group 1 metals are always +1; chlorine here takes -1 to balance the compound to zero.
In \(\)Fe\(_2\)O\(_3\), each iron is +3 and each oxygen is -2.
Total oxidation numbers in a neutral compound must sum to zero: \(2(+3) + 3(-2) = 0\).
Common mistakes
- Oxidation number is the same as charge → Oxidation number is a bookkeeping tool; it may differ from actual ionic charge in covalent compounds Oxidation numbers assign electrons as if bonds were ionic, even when they're covalent.
- Oxygen is always -2 → Oxygen is -2 except in peroxides (-1) and when bonded to fluorine (+2) Peroxides like \(\)H\(_2\)O\(_2\) have oxygen at -1, not -2.
- Forgetting that the sum of oxidation numbers in a polyatomic ion equals the ion's charge → In \(\)SO\(_4^{2-}\), sulfur is +6 and four oxygens are -2 each, totaling -2 The oxidation numbers must add up to the overall charge of the ion, not zero.
Where you'll use it next
You'll use oxidation numbers to balance redox equations in chemistry, identify oxidizing and reducing agents, and analyze electron transfer in electrochemistry and biochemistry.
Found in 1 StudyPug lesson
Mastering Oxidation Numbers: Your Gateway to Redox Reactions
11th Grade11thChemistry
Dive into the world of oxidation numbers! Learn to assign charges, track electron movement, and conquer redox reactions. Our clear, step-by-step approach makes complex concepts simple.
See also
Redox reactionsElectron configurationElectronegativityBalancing chemical equationsIonic bondingChemical Equations
Reviewed by Pat Cheng, M.Ed. — StudyPug Curriculum Lead · Last updated June 6, 2026