[Chemistry Tips] Charge vs oxidation number

Hi student,

Today's topic is about the two important yet confusing terms in chemistry - charge and oxidation number.

The oxidation number of an atom is defined as the number of valence electrons in the free atom form minus the number of valence electrons controlled by the same atom in a molecule. For an ionic molecule (electron transfer), the oxidation number of an atom equals the resulting charge. For a covalent molecule (shared electrons), the oxidation number does not equal the charge. Here is the rule: The sum of all the oxidation numbers in a molecule is equal to the charge on that molecule.

Here are general rules for oxidation number and total charge:
- A single element species has an oxidation number of zero (Na, H2, Cl2).
- The sum of oxidation numbers of all atoms in a neutral compound must equal zero.
- The sum of oxidation numbers of all atoms in an ionic compound must equal the molecule's ionic charge.
- Oxygen in a compound has the oxidation number of -2, except in peroxides, e.g. H2O2 (-1).
- Hydrogen in a compound has the oxidation number of +1, except in metal hydrides (-1).

That's all you need to know about their differences.

Until next time, student, keep it up!

Committed to Your Success
Wayne Huang, PhD
"The Rapid Learning Coach"
ChemTips@RapidLearningCenter.com

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