Direct comparison
Oxidation vs reduction
Oxidation is the loss of electrons; reduction is the gain of electrons. The two always happen together in a redox reaction.
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Side-by-side comparison
| Dimension | Oxidation | Reduction |
|---|---|---|
| Electrons (modern definition) | Electrons are lost. | Electrons are gained. |
| Oxygen (older definition) | Oxygen is gained. | Oxygen is lost. |
| Hydrogen (older definition) | Hydrogen is lost. | Hydrogen is gained. |
| Oxidation state | Increases (becomes more positive). | Decreases (becomes more negative). |
| Memory aid | OIL — Oxidation Is Loss of electrons. | RIG — Reduction Is Gain of electrons. |
| Role in the reaction | The substance acts as the reducing agent. | The substance acts as the oxidising agent. |
| Occurs alone? | No — always paired with a reduction. | No — always paired with an oxidation. |
| Example | Iron loses electrons as it rusts. | Oxygen gains electrons as iron rusts. |
| At an electrode | Happens at the anode. | Happens at the cathode. |
They are two halves of one process
Oxidation and reduction never happen in isolation: electrons lost by one substance must be gained by another, so the two always occur together in what is called a redox (reduction–oxidation) reaction. The substance that is oxidised gives up electrons and so reduces its partner — it is the reducing agent. The substance that is reduced takes electrons and so oxidises its partner — it is the oxidising agent. This pairing is why the modern electron-based definition is more general than the old oxygen-based one, which only described reactions involving oxygen.
Common questions
FAQ
What does OIL RIG mean?+
OIL RIG is a memory aid for the modern definitions. OIL stands for Oxidation Is Loss, and RIG stands for Reduction Is Gain — both referring to electrons. So oxidation is the loss of electrons and reduction is the gain of electrons. It is the quickest way to keep the two straight, since the names can feel counter-intuitive.
Why does reduction mean gaining electrons, not losing them?+
The name predates the electron definition. "Reduction" originally described a metal ore being reduced to the pure metal by removing oxygen, which lowered its mass and its oxidation state. We later learned this involves gaining electrons. So reduction lowers the oxidation state — it becomes more negative — which corresponds to gaining electrons.
Can oxidation happen without oxygen?+
Yes. Under the modern definition, oxidation is the loss of electrons, whether or not oxygen is involved. When sodium reacts with chlorine, sodium is oxidised by losing an electron, with no oxygen present. The older oxygen-based definition was a special case; the electron definition covers far more reactions.
