In both fission and fusion, the total mass of the products is slightly less than the total mass of the nuclei you started with. This missing mass is released as energy.
The mass lost by each nucleus is tiny, but a small amount of fuel contains an enormous number of nuclei. This is why 1 kg of nuclear fuel releases millions of times more energy than 1 kg of coal.
Fusion releases more energy per kilogram of fuel than fission because a larger share of the mass of the fuel is turned into energy.
This how it works covers How It Works: Why Mass Becomes Energy within Nuclear Fission & Fusion for GCSE Physics. Revise Nuclear Fission & Fusion in Atomic Structure for GCSE Physics with 13 exam-style questions and 25 flashcards. This topic appears less often, but it can still be a useful differentiator on mixed-topic papers. It is section 11 of 18 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Nuclear Fission & Fusion
What is nuclear fission?
Explain what is meant by a chain reaction in nuclear fission.