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⚗️ Oxidation Numbers: A Practical Guide

Learn to assign oxidation numbers, solve unknowns, and use changes in the numbers to identify redox reactions.

4
lessons
~15 min
to learn
🔬 Science
subject
Adults
level
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What you’ll learn

  1. AssignRecognize what oxidation numbers represent and assign the two basic starting values.Oxidation numbers are formal bookkeeping values; elemental substances are 0 and monatomic ions equal their charges.
  2. RulesApply common oxidation-number rules without overlooking their important exceptions.Oxygen, hydrogen, and halogens follow useful defaults, but their partners and molecular context matter.
  3. SolveCalculate an unknown oxidation number from a formula and its total charge.Subscripts multiply per-atom values, while coefficients only multiply the amount of substance.
  4. InterpretUse oxidation-number changes to identify redox and check your arithmetic.An increase is oxidation, a decrease is reduction, and a final audit exposes sign and counting errors.

Questions this course answers

What is the oxidation number of each oxygen atom in elemental O2?

An element in its uncombined elemental form is assigned 0.

What does an increase in oxidation number indicate?

Oxidation is defined as an increase in oxidation number.

What must the oxidation numbers in a polyatomic ion add up to?

The sum equals the charge of the species.

In 2FeCl3, what does the coefficient 2 change?

Coefficients multiply the amount of substance, not each atom's oxidation number.

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