Oxidation Numbers

Oxidation Numbers are a very convenient way to determine what has been oxidised and what has been reduced, particularly if you don't have half-equations (for example, if you have an overall equation).

There are some simple rules for assigning oxidation numbers:

1. All elements = 0   e.g. Fe, O₂
2. Monatomic ions = their charge   e.g. Fe²⁺ = +2, Fe³⁺ = +3
3. Oxygen (in a compound or ion) = -2, except in hydrogen peroxide (H₂O₂) = -1
4. Hydrogen (in a compound or ion) = +1, except in a metal hydride = -1 e.g. LiH
5. The sum of all oxidation numbers in a compound = 0
6. The sum of all oxidation numbers in a polyatomic ion = its charge

Here is the video from 2013, which covers it in a slightly different way: