Cambridge IGCSE Chemistry · Syllabus 0620 · Chemical Reactions
Oxidation Number
What is Oxidation Number?
A number assigned to an element in a compound or ion that records how many electrons it has effectively gained or lost compared with the uncombined element. It is found using four rules, and a change in it during a reaction is what identifies oxidation and reduction on the Supplement route.
This definition is part of the Chemical Reactions chapter in Cambridge IGCSE Chemistry.
Common mistakes with Oxidation Number
- 20. “The Roman numeral is the charge on the ion.” Core 6.4.1 Why temptingFor simple ions such as \(\mathrm{Fe^{3+}}\) in iron(III) chloride, the two do coincide. RepairThe Roman numeral is the oxidation number of one element. In manganate(VII) manganese is \(+7\) while the whole \(\mathrm{MnO_4^{\,-}}\) ion carries only \(1-\). TransferState the oxidation number of chromium in dichromate(VI) and the charge on the ion. \(+6\) per chromium atom; the ion is \(2-\).
Examiner tips on Oxidation Number
- Read the numeral off the name, and read the charge off the formula. They are two different sources of information and they answer two different questions. If a question asks for the oxidation number of an element and the name contains a Roman numeral, the answer is already written down for you — add the sign and you are finished.
- Evidence first, name second. “Identify the oxidising agent” is asking for two things: which species it is, and how you know. Write the oxidation number change, or the electron transfer, and then name the agent. A bare name is an assertion; the evidence is what makes it an answer — and, since the inversion is so easy to get backwards, writing the evidence first is also the best way to catch your own error.
- If you have only one hour before the exam. Every candidate should read the seven danger zones and the nine comparison matrices. A Core candidate should then re-read Figure 6.2, the five factors, and the four colours in Section 6.3A. An Extended candidate should instead re-read the five-line specifications of the Haber and Contact processes and the four oxidation number rules. Those are the densest summaries of the topic for each route: the recall facts, the distinctions that are easiest to merge, and the errors that turn correct chemistry into a wrong answer.