Worked example 1
Balance this fission event: n + ²³⁵U → ¹⁴¹Ba + ⁹²Kr + ? neutrons
Try it first: Add the mass numbers on each side and see what is missing.
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Nuclear ChemistryFission and FusionContent level: Core 16 min
Nuclear Fission and Fusion
What you'll be able to do: Distinguish fission from fusion, explain why each releases energy, and describe how a chain reaction is controlled in a reactor.
Best after: Types of Nuclear Decay and Balancing Nuclear Equations
Introduction
Fission splits a heavy nucleus and fusion joins light ones, yet both release enormous energy. The reason is the same in each case, and it is worth getting straight before the numbers appear.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Define nuclear fission and nuclear fusion and give an example of each
- Explain why both fission and fusion release energy
- Describe a chain reaction, critical mass, moderators and control rods
- Explain why fusion requires extremely high temperatures
- Compare the fuel, conditions and products of fission and fusion
Lesson
Fission: splitting a heavy nucleus
A slow neutron is absorbed by a ²³⁵U nucleus, which becomes unstable and splits into two mid-sized fragments plus two or three fresh neutrons and a large amount of energy, on the order of 200 MeV per event. The fragments are not fixed; fission produces a distribution of products.
¹₀n + ²³⁵₉₂U → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n + energy
Chain reactions and critical mass
Because each fission releases more neutrons than it consumes, the reaction can sustain itself. That only happens if enough of those neutrons hit other fissile nuclei rather than escaping the sample, which is why a minimum critical mass is required. Below it the reaction fizzles out.
²³⁵U is fissile with slow neutrons; the far more abundant ²³⁸U is not, which is why reactor fuel must be enriched.
Inside a reactor
The moderator, often water or graphite, slows fast neutrons to thermal speeds so ²³⁵U absorbs them efficiently. Control rods made of neutron absorbers such as boron or cadmium are lowered into the core to soak up surplus neutrons and hold the reaction exactly self-sustaining. Raising them speeds the reaction up; lowering them shuts it down.
Fusion: joining light nuclei
In the Sun, four hydrogen-1 nuclei are converted into one ⁴He nucleus plus two positrons (β⁺), releasing energy. Fusion gives more energy per gram of fuel than fission and produces no long-lived radioactive waste, but two positive nuclei repel each other strongly, so temperatures of tens of millions of kelvin are needed to give them enough kinetic energy to touch.
4 ¹₁H → ⁴₂He + 2 ⁰₊₁e + energy
Why both release energy
Binding energy per nucleon rises steeply from hydrogen up to about ⁵⁶Fe, then falls slowly. Splitting something heavier than iron moves the products up the curve; fusing something lighter than iron also moves the products up the curve. Either way the products are more tightly bound, and the difference appears as released energy.
⁵⁶Fe sits at the peak, so it can neither fission nor fuse to release energy. It is the end of the line.
Key ideas
Definition
Fission
Splitting a heavy nucleus into two mid-sized nuclei plus neutrons and energy.
Definition
Fusion
Combining light nuclei into a heavier nucleus with the release of energy.
Definition
Critical mass
The minimum mass of fissile material needed to sustain a chain reaction.
Rule
Moderator vs control rod
A moderator slows neutrons down; a control rod absorbs them.
Key concept
Common cause
Both processes release energy because the products have a higher binding energy per nucleon than the reactants.
Equation
Representative fission
¹₀n + ²³⁵₉₂U → ¹⁴¹₅₆Ba + ⁹²₃₆Kr + 3 ¹₀n
- n = neutron, which both starts and continues the chain
Equation
Solar fusion (net)
4 ¹₁H → ⁴₂He + 2 ⁰₊₁e + energy
- e(+1) = positron
Equation
Mass-energy equivalence
E = mc²
- m = mass converted to energy
- c = speed of light
Worked examples
Worked example 2
Explain, in terms of binding energy, why fusing two hydrogen nuclei releases energy but fusing two ⁵⁶Fe nuclei would not.
0 of 3 steps revealed.
Common mistakes
Mixing up the moderator and the control rods.
Why it's wrong: They do opposite jobs: the moderator slows neutrons so they are more likely to cause fission, while control rods remove neutrons from the core.
Check instead: Remember moderator = moderate the speed; control rod = control the number.
Saying fusion is easy because the nuclei are small.
Why it's wrong: Both nuclei are positively charged and repel each other strongly at short range, so enormous kinetic energy is needed.
Check instead: Tie the temperature requirement to electrostatic repulsion, not to nuclear size.
Claiming any nucleus splitting releases energy.
Why it's wrong: Only nuclei heavier than iron-56 release energy on splitting; splitting a light nucleus would consume energy.
Check instead: Check which side of the binding energy peak the reactant sits on.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Nuclear fission splits a heavy nucleus, typically uranium-235 or plutonium-239, into two mid-sized fragments plus two or three neutrons. Those neutrons can trigger further fissions, giving a self-sustaining chain reaction once a critical mass is present. In a reactor a moderator slows the neutrons so they are absorbed efficiently, and control rods absorb surplus neutrons to hold the reaction steady. Nuclear fusion joins light nuclei, as in the proton-proton chain that powers the Sun, but it requires temperatures of millions of kelvin to overcome the electrostatic repulsion between the positive nuclei. Both processes release energy because the products have a higher binding energy per nucleon than the reactants.
- Fission splits a heavy nucleus; fusion joins light nuclei
- Fission of ²³⁵U releases 2-3 neutrons, allowing a chain reaction above the critical mass
- Moderators slow neutrons; control rods absorb them
- Fusion needs millions of kelvin to overcome repulsion between positive nuclei
- Both release energy because the products are more tightly bound per nucleon
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.