Revision Notes

5.1.3 Nuclear Reactions

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Nuclear Fission

  • Usually large unstable nuclei break up gradually by the process of radioactive decay, but a small number (including Uranium-235, a naturally occurring isotope of Uranium) can break up in one big go – a process known as nuclear fission
  • In order to undergo nuclear fission, a nucleus usually requires some energy which can be given by hitting the nucleus with a neutron
    (Neutrons are used because they are chargeless and so are not repelled by the positive charge of the nucleus)


nuclear-fission, IGCSE & GCSE Physics revision notes

Nuclear fission: A Uranium-235 nucleus is struck by a neutron, breaking it into two smaller daughter nuclei and 2 or 3 neutrons


  • When this happens, the original nucleus breaks apart into two smaller ‘daughter’ nuclei, along with two or three neutrons
  • These fission products carry away the energy released in the form of kinetic energy


Extended Only

Nuclear Fusion

  • Nuclear fusion involves taking small nuclei (such as hydrogen) and colliding them together at high speed to form larger nuclei


nuclear fusion, IGCSE & GCSE Physics revision notes

Fusion is the process in which small nuclei, such as hydrogen, are fused together to form larger nuclei


  • This process also releases energy


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Nuclear Equations

  • Nuclear reactions, such as fission and fusion, can be represented using nuclear equations (which are similar to chemical equations in Chemistry)
    For example:

Fission equation 1


  • The above equation represents a fission reaction in which a Uranium nucleus is hit with a neutron and splits into two smaller nuclei – a Strontium nucleus and a Xenon nucleus, releasing two neutrons in the process
  • In the above reaction:
    The sum of top (nucleon) numbers on the left-hand side equals the sum of top number on the right-hand side:
235 + 1 = 236 = 90 + 144 + 2 × 1

The same is true for the lower (proton) numbers:

92 + 0 = 92 = 38 + 54 + 2 × 0
  • By balancing equations in this way, you can determine, for example, the number of neutrons emitted by a process like this


Fission equation 2, IGCSE & GCSE Physics revision notes

  • In the above example, balancing the numbers on the top shows that 3 neutrons must be released in the reaction (i.e. N = 3)

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