Describe the processes of nuclear fission and nuclear fusion as the splitting or joining of nuclei, to include the nuclide equation and qualitative description of mass and energy changes without values

Published by Patrick Mutisya · 14 days ago

IGCSE Physics 0625 – The Nucleus

5.1.2 The Nucleus

Objective

Describe the processes of nuclear fission and nuclear fusion as the splitting or joining of nuclei, to include the nuclide equation and a qualitative description of mass and energy changes (no numerical values).

Nuclear Fission

Fission is the process in which a heavy nucleus absorbs a neutron and breaks into two (or more) lighter nuclei, together with the release of additional neutrons and a large amount of energy.

  • The incident neutron makes the heavy nucleus unstable.

  • The nucleus splits into two fragments of roughly equal mass.

  • Two or three neutrons are usually emitted, which can induce further fission events (chain reaction).

  • The total mass of the products is slightly less than the mass of the original nucleus plus the incident neutron.

  • The missing mass is converted into energy according to \$E = \Delta m\,c^{2}\$, producing a large release of energy.

Typical nuclide equation (without numerical values):

\$\$

\,^{235}{92}\text{U} + \,^{1}{0}\text{n} \;\rightarrow\; \,^{141}{56}\text{Ba} + \,^{92}{36}\text{Kr} + 3\,^{1}_{0}\text{n} + \text{energy}

\$\$

Suggested diagram: A heavy nucleus (e.g., uranium‑235) absorbs a neutron and splits into two lighter nuclei plus additional neutrons.

Nuclear Fusion

Fusion is the process in which two light nuclei combine to form a heavier nucleus, accompanied by the release of energy and often the emission of a particle such as a neutron or a positron.

  • The nuclei must overcome their electrostatic repulsion, usually requiring very high temperature and pressure.

  • When they come close enough, the strong nuclear force binds them together.

  • The resulting nucleus has a slightly lower total mass than the sum of the original nuclei.

  • The loss of mass is released as energy according to \$E = \Delta m\,c^{2}\$, providing a huge energy output.

Typical nuclide equation (without numerical values):

\$\$

\,^{2}{1}\text{H} + \,^{3}{1}\text{H} \;\rightarrow\; \,^{4}{2}\text{He} + \,^{1}{0}\text{n} + \text{energy}

\$\$

Suggested diagram: Two light nuclei (deuterium and tritium) fuse to form a helium nucleus and a neutron, releasing energy.

Comparison of Fission and Fusion

AspectNuclear FissionNuclear Fusion
Typical reactantsHeavy nucleus (e.g., \$^{235}_{92}\text{U}\$) + neutronTwo light nuclei (e.g., \$^{2}{1}\text{H}\$ and \$^{3}{1}\text{H}\$)
Typical productsTwo lighter nuclei + 2–3 neutrons + energyOne heavier nucleus + a particle (often a neutron) + energy
Mass changeMass of products < mass of reactants (mass defect)Mass of products < mass of reactants (mass defect)
Energy releaseLarge, but less per nucleon than fusionEven larger per nucleon; the most energetic natural process
Conditions requiredCan occur at relatively low temperatures; chain reaction can be sustainedRequires extremely high temperature and pressure to overcome Coulomb barrier
Common applicationsPower generation in nuclear reactors, atomic bombsStars (including the Sun), experimental fusion reactors (e.g., tokamaks)

Key Points to Remember

  1. Both fission and fusion involve a conversion of a small amount of mass into a large amount of energy.

  2. In fission, a heavy nucleus splits; in fusion, light nuclei join.

  3. The energy released in fusion per nucleon is greater than that released in fission.

  4. Understanding the nuclide equations helps to identify the change in atomic number (protons) and mass number (nucleons).

  5. Control of the chain reaction is essential for safe use of fission energy, while achieving the required conditions for fusion remains a major scientific challenge.