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What Is a Nuclear Reaction?
A nuclear reaction is a process that changes the nucleus of an atom — its number of protons, neutrons, or both. Unlike chemical reactions (which only rearrange electrons), nuclear reactions release or absorb millions of times more energy.
Every nuclear reaction must obey two conservation laws:
| Conservation law | Meaning |
|---|---|
| Mass number (A) | The total number of protons + neutrons is the same on both sides |
| Atomic number (Z) | The total number of protons is the same on both sides |
Types of Radiation
Alpha Radiation (\(\alpha\))
An alpha particle is a helium-4 nucleus: two protons and two neutrons bound together. It is emitted by heavy unstable nuclei.
\[\alpha = ^{4}_{2}\text{He}\]
- Charge: +2
- Penetrating power: very low — stopped by a sheet of paper or a few centimetres of air
- Ionising power: very high — dangerous if inhaled or ingested
When a nucleus emits an alpha particle its mass number decreases by 4 and its atomic number decreases by 2:
\[^{A}_{Z}\text{X} \rightarrow ^{A-4}_{Z-2}\text{Y} + ^{4}_{2}\text{He}\]
Example: alpha decay of uranium-238
\[^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + ^{4}_{2}\text{He}\]
Check: \(A\) left \(= 238\), \(A\) right \(= 234 + 4 = 238\) ✓ \(Z\) left \(= 92\), \(Z\) right \(= 90 + 2 = 92\) ✓
Neutron Radiation (\(\text{n}\))
A neutron is a neutral particle found in the nucleus. Free neutrons are released in fission reactions and some bombardment reactions.
\[\text{neutron} = ^{1}_{0}\text{n}\]
- Charge: 0 (neutral)
- Penetrating power: very high — penetrates most materials
- Effect on nuclei: neutrons can be absorbed by a nucleus, making it unstable and triggering further reactions
Radioactive Decay: Alpha Decay
Radioactive decay is a spontaneous nuclear reaction in which an unstable nucleus emits radiation to become more stable. In alpha decay, the nucleus loses an alpha particle.
Why do nuclei decay?
Very large nuclei (Z > 83, bismuth) have too many protons packed together. Protons repel each other electrically, so the nucleus is unstable. Emitting an alpha particle reduces both the size and the proton count, moving toward stability.
Decay Chain
A single nucleus may undergo several consecutive alpha decays. For example, uranium-238 decays through a series of steps — including multiple alpha emissions — eventually becoming stable lead-206.
| Step | Reaction |
|---|---|
| 1 | \(^{238}_{92}\text{U} \rightarrow ^{234}_{90}\text{Th} + ^{4}_{2}\text{He}\) |
| 2 | \(^{226}_{88}\text{Ra} \rightarrow ^{222}_{86}\text{Rn} + ^{4}_{2}\text{He}\) |
| 3 | \(^{210}_{84}\text{Po} \rightarrow ^{206}_{82}\text{Pb} + ^{4}_{2}\text{He}\) |
Nuclear Fission
Fission is the splitting of a large nucleus into two smaller nuclei, releasing a large amount of energy and several neutrons.
\[^{235}_{92}\text{U} + ^{1}_{0}\text{n} \rightarrow ^{141}_{56}\text{Ba} + ^{92}_{36}\text{Kr} + 3\ ^{1}_{0}\text{n}\]
- The reaction is triggered by a neutron hitting the uranium nucleus
- The two daughter nuclei (Ba and Kr) are called fission fragments
- 3 neutrons are released — each can trigger another fission event
Chain Reaction
If the 3 released neutrons each trigger another fission, the number of reactions grows exponentially. This is called a chain reaction.
- Controlled chain reaction (nuclear power plant): the extra neutrons are absorbed by control rods so that exactly one neutron triggers the next fission → steady, controlled energy release
- Uncontrolled chain reaction (nuclear weapon): all neutrons are free to trigger more fissions → explosive release of energy
Energy Released
The energy released per fission event is enormous compared to chemical reactions. The source is a tiny mass defect: the products have slightly less mass than the reactants, and that missing mass is converted to energy according to Einstein's equation:
\[E = mc^2\]
Nuclear Fusion
Fusion is the joining of two light nuclei into a heavier nucleus. It also releases a large amount of energy and is the reaction that powers the Sun and all stars.
\[^{2}_{1}\text{H} + ^{3}_{1}\text{H} \rightarrow ^{4}_{2}\text{He} + ^{1}_{0}\text{n}\]
- Deuterium (\(^{2}_{1}\text{H}\)) and tritium (\(^{3}_{1}\text{H}\)) are isotopes of hydrogen
- The reaction produces a helium-4 nucleus (alpha particle) and a neutron
- Fusion releases even more energy per unit mass than fission
Why is fusion so hard to achieve on Earth?
Nuclei are positively charged and repel each other strongly. To fuse, they must be brought close enough for the strong nuclear force to take over. This requires temperatures above 100 million °C — hotter than the centre of the Sun. Scientists are working on fusion reactors (e.g. ITER) to harness this energy.
Comparison: Fission vs. Fusion
| Fission | Fusion | |
|---|---|---|
| Process | Splitting a heavy nucleus | Joining two light nuclei |
| Fuel | Uranium-235, Plutonium-239 | Deuterium, Tritium |
| Radiation produced | Neutrons + fission fragments | Alpha particle + neutron |
| Energy per reaction | Very high | Even higher |
| Current use | Nuclear power plants | Stars; research reactors (not yet practical) |
| Radioactive waste | Yes, long-lived | Very little |
Balancing Nuclear Equations
To find an unknown in a nuclear equation, apply both conservation laws:
Step 1 – Mass number (A): add all \(A\) values on the left; the right side must equal the same total.
Step 2 – Atomic number (Z): add all \(Z\) values on the left; the right side must equal the same total.
Worked Example: missing mass number
\[^{226}_{88}\text{Ra} \rightarrow ^{?}_{86}\text{Rn} + ^{4}_{2}\text{He}\]
Mass number: \(226 = ? + 4 \Rightarrow ? = 222\)
Atomic number: \(88 = 86 + 2\) ✓
The missing nucleus is \(^{222}_{86}\text{Rn}\) (radon-222).
Worked Example: missing coefficient
\[^{235}_{92}\text{U} + ^{1}_{0}\text{n} \rightarrow ^{141}_{56}\text{Ba} + ^{92}_{36}\text{Kr} + ?\ ^{1}_{0}\text{n}\]
Mass number: \(235 + 1 = 141 + 92 + ? \cdot 1 \Rightarrow 236 = 233 + ? \Rightarrow ? = 3\)
Atomic number: \(92 + 0 = 56 + 36 + 0\) ✓