Oxidation number and redox
Oxidation number and redox
- The oxidation number shows how many electrons an atom has lost or gained vs the free element.
- A redox reaction moves electrons from one species to another.
- One mnemonic ties it all together: OIL RIG.
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.
Practice
The oxidation number of an uncombined element (like O₂ or Fe) is:
A free, uncombined element always has an oxidation number of 0.
Practice
The usual oxidation number of oxygen in a compound is:
Oxygen is normally −2, except −1 in peroxides.
Practice
In KMnO₄, K is +1 and each O is −2. What is the oxidation number of Mn? (give the number, e.g. 7)
+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).
Practice
Using OIL RIG, oxidation is:
Oxidation Is Loss of electrons; the oxidation number rises. Reduction Is Gain (number falls).
You've got it
Key idea
- 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 Is Gain (number ↓)
- oxidation and reduction always happen together (electron transfer)