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D3.6 · Compare nuclear fission and fusion

Learn to compare nuclear fission and fusion through clear examples and targeted practice.

Ontario Grade 11 Physics

Energy and Society

A Grade 11 comparison of two ways nuclei can change

This lesson compares nuclear fission and fusion, as required by D3.6. First, recall that an atom has a small centre called its nucleus. The nucleus contains protons and neutrons. A nuclear change alters a nucleus. This differs from a chemical change, which rearranges atoms without changing their nuclei. The physical system in each example is the nucleus or group of nuclei involved in the reaction. There is no positive spatial direction to choose: these comparisons concern changes in nuclear identity, not motion along an axis. A nuclear equation shows the nuclei and particles before and after a reaction. Its numbers help track the total counts of protons and neutrons.

What you will learn

Describe what happens during nuclear fission and nuclear fusion.
Compare the starting nuclei and products of each process.
Use balanced nuclear equations to compare examples.
Avoid common errors when identifying fission and fusion.

1. Prerequisite bridge: reading nuclear equations

A chemical element is identified by its number of protons. For example, every uranium nucleus has 92 protons. The number of neutrons can vary. Atoms of the same element with different neutron counts are called isotopes.

In nuclear notation, the lower number gives the proton count. The upper number gives the total number of protons and neutrons. A neutron is written as

\mathrm{^{1}_{0}n}

. Its lower number is zero because it has no protons.

A nuclear equation lists the starting nuclei on the left and the products on the right. A balanced equation has the same total upper numbers and the same total lower numbers on both sides. This checks that the nuclear bookkeeping is consistent. It does not, by itself, tell us how likely a reaction is or how much energy it releases.

\mathrm{^{A}_{Z}X}

The lower number identifies the proton count.
The upper number counts protons and neutrons together.
Check both totals across a nuclear equation.

2. Fission: one large nucleus splits

Nuclear fission is a process in which a large nucleus splits into smaller nuclei. In one example, a uranium-235 nucleus absorbs a neutron and then splits into two smaller nuclei. The reaction also produces three neutrons. Different fission events can produce different smaller nuclei, so this equation is one example, not the only possible outcome.

The fission equation makes the change visible. The reactants include one large nucleus and a neutron. The products include smaller nuclei and additional neutrons. To check the equation, add the upper numbers on each side and then add the lower numbers on each side.

In the equation, the upper-number total is 236 on each side. The lower-number total is 92 on each side. These totals are counts, not measurements with SI units.

\mathrm{^{1}_{0}n+{}^{235}_{92}U\rightarrow{}^{141}_{56}Ba+{}^{92}_{36}Kr+3{}^{1}_{0}n}

Fission begins with a large nucleus.
The large nucleus splits into smaller nuclei.
A fission equation can also show neutrons among the reactants or products.

3. Fusion: small nuclei join

Nuclear fusion is a process in which small nuclei join to form a larger nucleus. For example, a deuterium nucleus and a tritium nucleus can join to form a helium-4 nucleus and a neutron. Deuterium and tritium are hydrogen isotopes. Each has one proton, but they have different numbers of neutrons.

Fusion requires the small nuclei to come close enough to join. In stars, very high temperatures help make fusion possible. This is a comparison point; it does not mean every fusion reaction happens in the same setting.

In this example, the upper numbers add to 5 on each side. The lower numbers add to 2 on each side. The equation shows two small nuclei as reactants and a larger nucleus among the products.

\mathrm{^{2}_{1}H+{}^{3}_{1}H\rightarrow{}^{4}_{2}He+{}^{1}_{0}n}

Fusion begins with small nuclei.
The nuclei join to form a larger nucleus.
The example produces a neutron as well as a helium nucleus.

4. Compare the processes carefully

The key difference is the direction of the nuclear change: fission splits a large nucleus, while fusion joins small nuclei. Both are nuclear processes because the nuclei change. In both, a nuclear equation can be checked by comparing the total upper and lower numbers on each side.

The processes do not have identical starting conditions or products. The fission example starts with a neutron and a uranium nucleus. The fusion example starts with two hydrogen isotopes. Their products also differ: the fission example gives smaller nuclei and neutrons, while the fusion example gives a helium nucleus and a neutron.

Use the words split and join to classify a process. Do not classify it only by whether a neutron appears. Neutrons appear in both examples. Also, a balanced equation is a bookkeeping check, not a complete description of every detail of a reaction.

Fission: one large nucleus splits into smaller nuclei.
Fusion: small nuclei join to form a larger nucleus.
A neutron may appear in either kind of reaction.

Fission and fusion at a glance

Feature	Fission	Fusion
Starting nuclei	One large nucleus; a neutron may be involved	Small nuclei
Main change	A large nucleus splits	Small nuclei join
Products in lesson examples	Smaller nuclei and neutrons	A larger nucleus and a neutron
Classification clue	Look for splitting	Look for joining

Worked example

Classify a reaction by its starting nucleus

A reaction begins with a uranium-235 nucleus and a neutron. The uranium nucleus splits into two smaller nuclei. Is this fission or fusion?

Identify the change
The system is the uranium nucleus and the incoming neutron. The key clue is that one large nucleus splits into smaller nuclei.
Classify
A process in which a large nucleus splits is nuclear fission. The neutron is part of the starting situation, but the split is what identifies the process.

Answer: This is nuclear fission.

Check: The answer matches the definition: one large nucleus splits rather than small nuclei joining. No spatial direction or SI unit is needed because the question asks for a process classification, not a measured quantity.

Worked example

Classify a reaction by its products

Two small hydrogen-isotope nuclei combine. The products include a helium-4 nucleus and a neutron. Is this fission or fusion?

Compare the nuclei
The starting nuclei are small. They combine to make a larger nucleus among the products.
Classify
Joining small nuclei to form a larger nucleus is nuclear fusion. The neutron product does not change the classification.

Answer: This is nuclear fusion.

Check: The answer is reasonable because the defining change is joining, not splitting. No spatial direction or SI unit is needed for this classification.

Worked example

Check nuclear bookkeeping

Consider the fusion example in this lesson. Check whether the total upper and lower numbers match on both sides.

Add the upper numbers
On the reactant side, the upper numbers are 2 and 3. On the product side, they are 4 and 1. Both totals are 5.
$2+3=4+1=5$
Add the lower numbers
On the reactant side, the lower numbers are 1 and 1. On the product side, they are 2 and 0. Both totals are 2.
$1+1=2+0=2$

Answer: The equation is balanced by both totals.

Check: The upper-number totals match, and the lower-number totals match. These are counts, not measured quantities, so SI units and significant figures do not apply.

Common mistakes and how to avoid them

Calling any reaction that produces a neutron fission.

Correction: A neutron can appear in both fission and fusion. Identify whether the nuclei split or join.

Saying fusion is when a large nucleus breaks apart.

Correction: That describes fission. Fusion is the joining of small nuclei to form a larger nucleus.

Assuming one fission equation lists the only possible products.

Correction: Fission can produce different pairs of smaller nuclei. Treat a displayed equation as a specific example.

Checking only the element symbols in a nuclear equation.

Correction: Check the total upper numbers and total lower numbers on both sides.

Lesson summary

Fission is the splitting of a large nucleus into smaller nuclei.
Fusion is the joining of small nuclei to form a larger nucleus.
Neutrons can be involved in either process, so classify by the nuclear change.
A balanced nuclear equation has matching upper-number and lower-number totals on both sides.

Check your understanding

Question 1

Which description identifies nuclear fission?

Small nuclei join to form a larger nucleus.
A large nucleus splits into smaller nuclei.
An atom changes its arrangement of electrons only.
Two neutrons combine without any change to a nucleus.

Show answer and explanation

A large nucleus splits into smaller nuclei.

Fission is the splitting of a large nucleus. Joining small nuclei describes fusion.

Question 2

A reaction starts with two small nuclei and forms a larger nucleus. What is the process?

Fission
Fusion
A chemical change
The splitting of a large nucleus

Show answer and explanation

Fusion

The nuclei join, so this is fusion. A chemical change does not change the nuclei.

Question 3

What is the best way to distinguish fission from fusion when both examples include a neutron?

Count the neutrons only.
Check whether the process splits a large nucleus or joins small nuclei.
Look only at whether the products include an element symbol.
Assume both processes are fission.

Show answer and explanation

Check whether the process splits a large nucleus or joins small nuclei.

The defining difference is splitting versus joining. A neutron can appear in examples of both processes.

Key terms

Nucleus: The small centre of an atom, containing protons and neutrons.
Isotope: A form of an element with the same number of protons as other forms of that element but a different number of neutrons.
Nuclear fission: A process in which a large nucleus splits into smaller nuclei.
Nuclear fusion: A process in which small nuclei join to form a larger nucleus.

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About this lesson and its review

Published by DoAssignment. This reviewed lesson follows Ontario Grade 11 Physics (SPH3U), expectation D3.6. It is a study resource, not an official curriculum publication.

Before publication, content is checked for structure, mathematical or chemical notation, calculations, course boundaries, and readability. Errors can still occur, so corrections are welcomed.

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