Oxidation vs Reduction
Quick Answer
Oxidation is loss of electrons. Reduction is gain of electrons.
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What Each One Really Means
The old textbooks used to define oxidation as 'gain of oxygen' and reduction as 'loss of oxygen'. That was true for the reactions they were looking at, but it is not the full picture. The modern definition is about electrons, and once you understand that, everything else falls into place. Oxidation is loss of electrons. Reduction is gain of electrons. Now here is where students get confused. They think 'reduction' means the substance becomes smaller or less. Not at all. Reduction means the oxidation number goes down, because the substance gained electrons. It is about charge, not size. The mnemonic OIL RIG has saved many of my students. Oxidation Is Loss, Reduction Is Gain. Some people prefer LEO the lion says GER. Loss of Electrons is Oxidation, Gain of Electrons is Reduction. Either one works. The most important insight is that oxidation and reduction always happen together. You cannot have one without the other. If one species loses electrons, another must gain them. This is why we call it a redox reaction, a combination of reduction and oxidation. When you are balancing equations, follow the electrons. Look for the element that changed oxidation number. One went up, that is oxidation. One went down, that is reduction. If nothing changed, there is no redox.
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Side-by-Side Comparison
| Aspect | Oxidation | Reduction |
|---|---|---|
| Electrons | Lost | Gained |
| Oxidation number | Increases | Decreases |
| Position in a redox reaction | The species being oxidised is the reducing agent | The species being reduced is the oxidising agent |
| Classic example | Zn → Zn²⁺ + 2e⁻ | Cu²⁺ + 2e⁻ → Cu |
| Mnemonic | Oil (Oxidation Is Loss) | Rig (Reduction Is Gain) |
Memory Device
OIL RIG. That is all. Do not overthink it.