The idea of oxidation numbers helps simplify the process of working out what has been reduced or oxidised in a reaction. These rules save us counting every single electron on every single atom, which could take forever!!
Rules
- The oxidation state of an uncombined element is 0
- The sum of all the oxidation states on a neutral compound is 0
- The sum of the oxidation states on an ion is equal to the charge on the ion
- The more electronegative element in a compound is given a negative oxidation state
Remember; fluorine is the most electronegative with oxygen the second - Some elements always have the same oxidation state in a compound:
| Element | Usual Oxidation state | exceptions |
|---|---|---|
| Group 2 Metals | always +2 | |
| Oxygen | Usually -2 | Except in Peroxides and F2O |
| Chlorine | Usually -1 | Except in compounds with F or O |
Reasons for exceptions
Hydrogen in the metal hydrides
Metal hydrides include compounds like sodium hydride, NaH. In this, the hydrogen is present as a hydride ion, H-. The oxidation state of a simple ion like hydride is equal to the charge on the ion – in this case, -1.
Alternatively, you can think of it that the sum of the oxidation states in a neutral compound is zero. Since Group 1 metals always have an oxidation state of +1 in their compounds, it follows that the hydrogen must have an oxidation state of -1 (+1 -1 = 0).
Oxygen in peroxides
Peroxides include hydrogen peroxide, H2O2. This is an electrically neutral compound and so the sum of the oxidation states of the hydrogen and oxygen must be zero.
Since each hydrogen has an oxidation state of +1, each oxygen must have an oxidation state of -1 to balance it.
Oxygen in F2O
The problem here is that oxygen isn’t the most electronegative element. The fluorine is more electronegative and has an oxidation state of -1. In this case, the oxygen has an oxidation state of +2.
Chlorine in compounds with fluorine or oxygen
There are so many different oxidation states that chlorine can have in these, that it is safer to simply remember that the chlorine doesn’t have an oxidation state of -1 in them, and work out its actual oxidation state when you need it. You will find an example of this below.
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