Oxidation number and redox
Introduced| English | Meaning |
|---|---|
| oxidation number/ˌɒksɪˈdeɪʃn ˈnʌmbə/ | The formal charge an atom would have if bonding electrons were assigned according to agreed electronegativity rules. |
| redox/rɪˈdɒks/ | A process in which oxidation and reduction occur together through changes in oxidation state. |
| oxidation/ˌɒksɪˈdeɪʃn/ | A chemical change involving a loss of electrons or an increase in oxidation state. |
| electron transfer/ɪˈlektrɒn ˈtrænsfɜː/ | The movement of one or more electrons from one species to another, as in many redox reactions. |
| reduction/rɪˈdʌkʃn/ | A chemical change involving a gain of electrons or a decrease in oxidation state. |
Tracking the electrons
- The oxidation number 氧化数 shows how many electrons an atom has lost or gained vs the free element.
- A redox 氧化还原 reaction moves electrons from one species to another.
- One mnemonic ties it all together: OIL RIG.
A substance that loses electrons in a reaction is being ______.
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OIL: Oxidation Is Loss of electrons.
The rules
- an uncombined element is $0$.
- a simple ion equals its charge ($\text{Mg}^{2+}$ is $+2$).
- Group 1 is $+1$, Group 2 is $+2$; H is $+1$ (but $-1$ in metal hydrides); O is $-2$ (but $-1$ in peroxides); F is $-1$.
- the numbers add to 0 in a compound, or to the charge in an ion.
Balancing redox: total rise in oxidation 氧化 number equals total fall
Disproportionation: the same element is both oxidised and reduced
Redox is electron transfer
Step through it: the metal loses (is oxidised) and the non-metal gains (is reduced) — oxidation number rises for one, falls for the other.
The oxidation number of an uncombined element (like O₂ or Fe) is:
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A free, uncombined element always has an oxidation number of 0.
The usual oxidation number of oxygen in a compound is:
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Oxygen is normally −2, except −1 in peroxides.
In KMnO₄, K is +1 and each O is −2. What is the oxidation number of Mn? (give the number, e.g. 7)
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+1 (K) + Mn + 4×(−2) = 0, so Mn = +7.
OIL RIG
- Oxidation Is Loss of electrons → oxidation number goes up.
- Reduction Is Gain of electrons → oxidation number goes down.
- They always happen together — electrons lost by one are gained by another (electron transfer 电子转移).
A battery uses redox reactions to push electrons round a circuit
Using OIL RIG, oxidation is:
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Oxidation Is Loss of electrons; the oxidation number rises. Reduction Is Gain (number falls).
Oxidation-number rules
- Uncombined elements are 0; simple ions equal their charge; oxygen is usually −2 and hydrogen +1.
- The oxidation numbers in a neutral compound add up to 0.
Rusting is the oxidation of iron
You've got it
- oxidation number = electrons lost/gained vs the free element (element = 0, ion = charge)
- key values: H $+1$, O $-2$, F $-1$; sum to 0 (compound) or charge (ion)
- OIL RIG: Oxidation Is Loss (number ↑), Reduction 还原 Is Gain (number ↓)
- oxidation and reduction always happen together (electron transfer)