This cheat sheet explains how electrons move in redox reactions and how to remember the direction of that movement with LEO says GER. Students need this reference because oxidation and reduction are often confused, especially when charges and reaction roles change at the same time. It helps connect electron loss, electron gain, oxidation numbers, and reaction agents in one clear memory aid.
The main idea is that oxidation is loss of electrons and reduction is gain of electrons. LEO means Loss of Electrons is Oxidation, and GER means Gain of Electrons is Reduction. A substance that loses electrons is oxidized and acts as the reducing agent, while a substance that gains electrons is reduced and acts as the oxidizing agent.
Key Facts
- LEO says GER means and .
- Oxidation is shown by electron loss, such as .
- Reduction is shown by electron gain, such as .
- The substance oxidized loses electrons and causes another substance to be reduced, so it is the reducing agent.
- The substance reduced gains electrons and causes another substance to be oxidized, so it is the oxidizing agent.
- An increase in oxidation number means oxidation, such as .
- A decrease in oxidation number means reduction, such as .
- In a balanced redox reaction, the number of electrons lost must equal the number of electrons gained.
Vocabulary
- Oxidation
- Oxidation is the loss of electrons by a substance, often shown by an increase in oxidation number.
- Reduction
- Reduction is the gain of electrons by a substance, often shown by a decrease in oxidation number.
- Electron
- An electron is a negatively charged particle written as in chemical half-reactions.
- Reducing agent
- A reducing agent donates electrons, becomes oxidized, and causes another substance to be reduced.
- Oxidizing agent
- An oxidizing agent accepts electrons, becomes reduced, and causes another substance to be oxidized.
- Oxidation number
- An oxidation number is a bookkeeping charge used to track electron transfer in a redox reaction.
Common Mistakes to Avoid
- Confusing the agent with the substance changed is a common mistake, because the reducing agent is the substance oxidized and the oxidizing agent is the substance reduced.
- Saying oxidation always means gaining oxygen is incomplete, because the modern definition of oxidation is electron loss: .
- Forgetting charge balance in half-reactions is wrong because electrons must be added to make total charge equal on both sides.
- Treating electron gain as a positive charge increase is incorrect, because gaining electrons usually makes a species more negative or lowers its oxidation number.
- Balancing atoms but not electrons in a redox equation is incomplete, because total electrons lost must equal total electrons gained.
Practice Questions
- 1 In the half-reaction , is magnesium oxidized or reduced, and how many electrons are transferred?
- 2 In the half-reaction , is silver ion oxidized or reduced, and what happens to its oxidation number?
- 3 For the reaction , identify the reducing agent and the oxidizing agent.
- 4 Explain why a substance that gains electrons is called reduced even though it may seem like it is gaining something.
Understanding Electron transfer in redox reactions (LEO says GER) Memory Aid
Electron transfer changes the electrical charge of particles. Atoms contain positive protons in the nucleus and negative electrons outside it. When a neutral metal atom gives away electrons, it has more protons than electrons left, so it becomes a positive ion.
When a positive ion receives electrons, its positive charge becomes smaller or may disappear. This is why charges are useful clues in redox work. A species becoming more positive has undergone oxidation.
A species becoming less positive has undergone reduction. For compounds, chemists track oxidation numbers because electrons are often shared in bonds rather than sitting on one atom as free particles.
Redox reactions must conserve charge as well as atoms. Electrons cannot simply appear in one half of a reaction and vanish in the other. One useful balancing method begins by separating the overall reaction into two half reactions.
Write the change for the particle that loses electrons, then write the change for the particle that gains them. Multiply one or both half reactions until each side involves the same number of electrons. Only then can the half reactions be added together, with the electrons cancelled.
Students often balance atoms first and stop too soon. Checking total charge on both sides is essential. Equal atom counts do not prove that a redox equation is balanced.
The names of the agents can feel backwards because they describe what each substance does to its reaction partner. A reducing agent supplies electrons. By supplying them, it makes the other substance undergo reduction.
The reducing agent is therefore used up or changed by oxidation. An oxidizing agent removes electrons from another substance. It makes the other substance undergo oxidation, while it receives electrons itself.
A good way to avoid guessing is to identify the electron donor and electron receiver first. Assign the agent names only after that step. This approach works even when the reaction contains several ions or complex formulas.
Redox chemistry appears far beyond textbook equations. A battery produces electric current because electrons move through a wire from one reaction site to another. Rust forms when iron is oxidized over time, usually in the presence of water and oxygen.
Bleach works because it can oxidize colored molecules into different substances that no longer absorb light in the same way. Living cells use controlled electron transfer during respiration to release energy from food. When studying, pay attention to the difference between charge and oxidation number.
They are related, but an oxidation number is a bookkeeping value assigned by rules. Practice with simple ionic equations first, state which particle loses and gains electrons, then use oxidation numbers to handle harder reactions.