Oxidation number

An oxidation number is a signed value assigned to an atom that shows how many electrons it has effectively gained or lost. An atom in an uncombined element has an oxidation number of zero. Malay: nombor pengoksidaan · Chinese: 氧化数.

The oxidation number is a bookkeeping tool that lets you decide, without watching the electrons directly, whether a species has been oxidised or reduced. A few rules cover most cases: an uncombined element is 0; a simple ion equals its charge; hydrogen is usually +1 and oxygen usually -2; and the oxidation numbers in a neutral compound add up to zero, while in an ion they add up to the ion’s charge. Using these, you can work out the oxidation number of any element in a formula.

Example. In MnO₄⁻, oxygen is -2, so four oxygens give -8; the whole ion has charge -1, so manganese must be +7. When manganate(VII) acts as an oxidising agent it becomes Mn²⁺, a fall from +7 to +2, confirming that manganese is reduced.

Confusion to avoid. Do not confuse oxidation number with the actual charge on an atom or with valency. Oxidation number carries a sign and is written before the number (for example +2), whereas ionic charge is written after (for example 2+); they match for simple ions but not within covalent species. Always assign oxygen and hydrogen first, then solve for the unknown element. Watch too for the exceptions: oxygen is -1 in peroxides and hydrogen is -1 in metal hydrides, so a careless rule can give the wrong answer if you do not check the compound.

Tracking the rise or fall of an oxidation number is how you identify oxidation, reduction and the oxidising and reducing agents throughout this chapter, and calculating one from a formula, then using it to justify which species is oxidised, is a frequent short-answer task in SPM Chemistry.