Oxidation and reduction: who loses electrons and who gains them
HESI A2 topic: Oxidation and reduction
- OIL RIG: oxidation is loss of electrons, reduction is gain.
- Oxidation raises the oxidation number; reduction lowers it.
- Oxidation numbers add up to 0, or to the ion's charge.
- The oxidizing agent gets reduced; the reducing agent gets oxidized.
- Oxidation does not need oxygen: sodium is oxidized by chlorine.
Key facts
- Oxidation is loss of electrons (oxidation number goes up); reduction is gain of electrons (oxidation number goes down).
- An atom in a pure element, such as Na, O₂ or Cl₂, has an oxidation number of 0.
- A monatomic ion's oxidation number equals its charge; oxygen is usually −2, hydrogen is usually +1 with nonmetals, and fluorine is always −1.
Show all 7 key factsShow fewer
- Oxidation numbers in a compound add up to 0; in a polyatomic ion they add up to the ion's charge (S is +6 in SO₄²⁻).
- The reducing agent is the species that is oxidized; the oxidizing agent is the species that is reduced.
- Rust forms when iron is oxidized in the presence of oxygen and water; oxygen is reduced.
- In cellular respiration glucose is oxidized, NAD⁺ is reduced to NADH, and oxygen is the final electron acceptor, forming water.
Quick check1 question · 2 more at the end
In an oxidation-reduction reaction, what happens to the substance that is oxidized?
Correct answer: D.
Oxidation is the loss of electrons, and reduction is the gain of electrons (OIL RIG: Oxidation Is Loss, Reduction Is Gain). Losing negative electrons raises the oxidation number. Because electrons are transferred, every oxidation is paired with a reduction.
A Gaining electrons is reduction, the opposite process.
B Redox reactions change electrons, not the nucleus; changes in neutrons belong to nuclear reactions.
C Gaining a proton (H⁺) is part of an acid-base reaction, where H⁺ is transferred; oxidation and reduction are about electrons.
D Correct: oxidation is loss of electrons, which raises the oxidation number.
Full lesson
Each part starts with its main idea. Open a part when you want the details.
The short answer: Oxidation is the loss of electrons, which raises an atom's oxidation number; reduction is the gain of electrons, which lowers it (OIL RIG). Every redox reaction has both. The substance that is oxidized is the reducing agent, and the substance that is reduced is the oxidizing agent. Rusting, burning and cellular respiration are all redox reactions.
1 What are oxidation and reduction?Oxidation means losing electrons, and reduction means gaining them. 1 min
Oxidation means losing electrons, and reduction means gaining them. Two classic memory aids say the same thing:
- OIL RIG: Oxidation Is Loss, Reduction Is Gain.
- LEO says GER: Lose Electrons Oxidation, Gain Electrons Reduction.
The two always happen together. Electrons cannot vanish, so whatever one substance loses, another gains. That is why these are called redox (reduction-oxidation) reactions.
2 How do you assign oxidation numbers?An oxidation number is a bookkeeping charge that lets you track electrons. 1 min
An oxidation number is a bookkeeping charge that lets you track electrons. Oxidation raises it, and reduction lowers it. Use these rules in order:
- An atom in a pure element is 0 (Na, Fe, O₂, Cl₂).
- A single-atom ion equals its charge (Na⁺ is +1, Cl⁻ is −1).
- Fluorine is always −1. Oxygen is usually −2 (−1 in peroxides). Hydrogen is +1 with nonmetals and −1 with metals.
- The numbers add up to the overall charge: 0 for a neutral compound, or the ion's charge.
Worked example: sulfur in sulfate, SO₄²⁻. Four oxygens give 4 × (−2) = −8. The total must be −2, so S + (−8) = −2, and sulfur is +6.
3 Which substance is the oxidizing agent?The agents are named for what they do to the other substance. 1 min
The agents are named for what they do to the other substance. Look at 2Na + Cl₂ → 2NaCl:
| Species | Oxidation number | Electrons | Process | Role |
|---|---|---|---|---|
| Na | 0 → +1 | Loses | Oxidized | Reducing agent |
| Cl | 0 → −1 | Gains | Reduced | Oxidizing agent |
Sodium gives its electron to chlorine, so sodium causes chlorine's reduction and is the reducing agent. Chlorine takes the electron, so it causes sodium's oxidation and is the oxidizing agent. The rule to memorize: the oxidizing agent gets reduced, and the reducing agent gets oxidized.
4 What do everyday redox reactions look like?Many familiar reactions are redox reactions, even when no oxygen is involved. 1 min
Many familiar reactions are redox reactions, even when no oxygen is involved.
- Single replacement: in Zn + 2HCl → ZnCl₂ + H₂, zinc goes from 0 to +2 (oxidized) and hydrogen from +1 to 0 (reduced).
- Rusting: iron exposed to oxygen and water is oxidized, and oxygen is reduced. Rust is a hydrated iron(III) oxide. Unlike the blue-green patina on copper, rust does not form a protective layer, so the iron keeps corroding.
- Combustion: when a fuel burns, the carbon in the fuel is oxidized and oxygen is reduced. Ethylene burns as C₂H₄ + 3O₂ → 2CO₂ + 2H₂O.
5 How does redox power your cells?Cellular respiration is a slow, controlled redox process. 1 min
Cellular respiration is a slow, controlled redox process. Glucose is oxidized, and its energy is released in small steps that the cell uses to make ATP.
Electrons removed from food molecules are carried by NAD⁺. When NAD⁺ picks up electrons (with hydrogen), it becomes NADH, its reduced form. A handy rule: the form with the extra "H" is usually the reduced one. At the end of the chain, oxygen is the final electron acceptor: it is reduced and forms water.
Common mistakes
The traps students fall into most. Read these before you practice.
Watch out: The oxidizing agent is reduced, not oxidized; it causes oxidation in the other substance by taking its electrons.
Watch out: Reduction lowers the oxidation number even though the atom gains something; electrons are negative.
Watch out: Oxidation does not require oxygen; sodium is oxidized when it reacts with chlorine.
Watch out: Oxidation and reduction never happen alone; electrons lost by one species are gained by another.
How to remember it
Memory hook: "OIL RIG, and agents do the opposite." Oxidation is loss and reduction is gain; the oxidizing agent itself is reduced, and the reducing agent itself is oxidized.
Self-check:
- Find the oxidation number of carbon in CO₂.
- In Zn + 2HCl → ZnCl₂ + H₂, name the oxidizing agent and the reducing agent.
- Which molecule is the final electron acceptor in cellular respiration, and what does it become?
Show answers
- +4.
- Two oxygens: 2 × (−2) = −4.
- CO₂ is neutral, so the total is 0: C + (−4) = 0.
- Carbon is +4.
- HCl is the oxidizing agent; Zn is the reducing agent.
- Zinc goes from 0 to +2. It loses electrons, so it is oxidized.
- Hydrogen goes from +1 to 0. It gains electrons, so it is reduced.
- The oxidizing agent gets reduced (the H in HCl). The reducing agent gets oxidized (Zn).
- Oxygen, which becomes water.
- At the end of the chain, oxygen takes the electrons.
- It is reduced and forms water.