GCSE · Physics · AQA · Spec 8463
Nuclear fusion (physics only)
Two light nuclei join and become one, and radiation comes out. So where does the energy it carries come from?
One fusion event
Two separate nuclei, each small and light. These are nuclei, the tiny centres of atoms, not whole atoms.
Drag along the track, or press Play, to run the event from start to finish.
Predict, then check
Now think about mass. You have just watched two light nuclei become one heavier nucleus while radiation left.
Compare the mass of the new, heavier nucleus with the total mass of the two light nuclei that joined to make it. What do you predict?
Put the event together
Before the nuclei join, or after?
Take one fusion event from start to finish. Sort each description: is it true before the two nuclei join, or after?
Still to sort
Before the nuclei join (0)
The two light nuclei, still apart.
Where the line is: Before the event, all of the mass is still in the two light nuclei and nothing has been given out.
After the nuclei join (0)
The new nucleus, and whatever was given out.
Where the line is: After the event, one heavier nucleus is left and radiation has carried energy away.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Two light nuclei join to form one heavier nucleus, and some of their mass may be converted into the energy of radiation.
What you need to know
- Nuclear fusion is the joining of two light nuclei to form a heavier nucleus.
- Fusion happens to nuclei, not to whole atoms, and nothing bonds as it would in a chemical reaction.
- In fusion, some of the mass may be converted into the energy of radiation.
- When mass is converted in this way, the heavier nucleus has less mass than the two light nuclei had together.
The big picture
Nuclear fusion is the joining of two light nuclei to form a heavier nucleus. It happens to nuclei, the tiny centres of atoms, rather than to whole atoms bonding as they do in a chemical reaction, and it is a joining, not a splitting. In this process some of the mass may be converted into the energy of radiation. When that happens, the new nucleus has less mass than the two light nuclei had together: the missing mass has become the energy the radiation carries away.
Key points
Worked example
Problem
A student writes: 'In nuclear fusion, two atoms bond together to make a bigger atom, and the new atom has exactly the same mass as the two atoms it was made from.' Find the two mistakes and write a corrected description.
⚠ Watch out
Reading 'heavier nucleus' as 'more mass than the two light nuclei put together'. The new nucleus is heavier than each light nucleus on its own, but when some mass is converted into the energy of radiation it has less mass than the pair combined.
Memory hook
FUSE means join. Two light nuclei fuse into one heavier one, and any mass that goes missing has left as the energy of radiation.
Check yourself
Cover the page. Say, in order, what goes into one fusion event, what comes out of it, and what the energy carried away by the radiation was converted from.
Flashcards
(5)Define nuclear fusion in one sentence.
Does fusion happen to whole atoms or to nuclei?
In fusion, what may some of the mass be converted into?
When radiation is given out in fusion, how does the new nucleus's mass compare with the two light nuclei together?
How does the new nucleus compare with EACH of the light nuclei that joined to make it?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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How this lesson was checked. This AQA GCSE Physics (specification 8463)lesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 29 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.