---
title: "Energy Released in Fission and Fusion"
description: "Splitting a heavy nucleus and joining two light ones both move the products up the binding energy per nucleon curve, and the mass lost turns up as energy. The size of the release follows straight from"
canonical: https://lightmysky.com/learn/science/energy-released-in-fission-and-fusion-mt_VrqLAATXQY
source: https://lightmysky.com/learn/science/energy-released-in-fission-and-fusion-mt_VrqLAATXQY.md
retrieved: 2026-09-12
---

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# Energy Released in Fission and Fusion

Splitting a heavy nucleus and joining two light ones both move the products up the binding energy per nucleon curve, and the mass lost turns up as energy. The size of the release follows straight from E = mc squared.

Subject: Science · Area: Matter & Materials · Ages 17 to 18
Page: https://lightmysky.com/learn/science/energy-released-in-fission-and-fusion-mt_VrqLAATXQY

## Ready when they can

- Calculates the energy released from the mass difference across a stated reaction
- Explains from the curve why fission suits heavy nuclei and fusion suits light ones
- Says why fusion needs extreme temperature, in terms of electrostatic repulsion

## Lesson: Splitting and joining for energy

Fission splits a heavy nucleus into lighter fragments, while fusion joins light nuclei into a heavier one. Both moves shift the products up the binding energy curve. Weigh the starting nuclei and the products: the lost mass times c squared is the energy set free.

**Example.** Picture a uranium nucleus struck by a neutron. It wobbles, splits into two lighter fragments, and throws off extra neutrons. Add up both sides and the fragments weigh less in total. That missing mass appears as the fragments' motion plus radiation, through E equals m c squared.

Fusion demands extreme temperature for one reason: charge. Both nuclei are positive, so they repel each other electrically. Only insane speeds can slam them close enough for the short range attraction to grab hold. Stars manage it with crushing heat; labs must copy that violence.

**Tip.** Let the curve pick the technology. Fission suits heavy nuclei because splitting climbs toward the iron peak. Fusion suits light nuclei because joining climbs the same peak from below. Past iron in either direction slides downhill and would swallow energy instead.

**Recap.** Split heavy nuclei or join light ones, and the climb toward iron pays out the lost mass as energy.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Mass, Energy and the Unified Atomic Mass Unit](https://lightmysky.com/learn/science/mass-energy-and-the-unified-atomic-mass-unit-mt_GTfhALD90X)
- [Nuclear Fission and Fusion](https://lightmysky.com/learn/science/nuclear-fission-and-fusion-mt_jkO89KodSb)
- [Coulomb's Law and the Electric Field of a Point Charge](https://lightmysky.com/learn/science/coulombs-law-and-the-electric-field-of-a-point-charge-mt_p7FuzIsQch)
- [Mass Defect and Binding Energy per Nucleon](https://lightmysky.com/learn/science/mass-defect-and-binding-energy-per-nucleon-mt_UdYE8hgXyl)

## Opens up

- [Particles, Antiparticles and Annihilation](https://lightmysky.com/learn/science/particles-antiparticles-and-annihilation-mt_-Ud20HRNJz)
- [Fission Reactors and Fusion in the Laboratory](https://lightmysky.com/learn/science/fission-reactors-and-fusion-in-the-laboratory-mt_iTwgXI0iBy)
- [Hydrostatic Equilibrium and the Structure of a Star](https://lightmysky.com/learn/science/hydrostatic-equilibrium-and-the-structure-of-a-star-mt_RKeMW8iX56)