Edexcel International A Level Physics

Revision Notes

5.15 Nuclear Decay Equations

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Nuclear Decay Equations

Alpha Decay

  • An alpha particle consists of 2 protons and 2 neutrons

    (It is emitted from large unstable nuclei)
  • When an alpha particle is emitted from a nucleus:
    • The nucleus loses 2 protons:

      The proton (atomic) number decreases by 2
    • The nucleus loses 4 particles (nucleons) in total:

      The nucleon (mass) number decreases by 4

  • Equation for alpha emission:

Alpha equation

  • Nuclear equations, just like chemical equations, balance:
    • The sum of the upper (mass) numbers on the left of each equation should equal the sum on the right
    • The sum of the lower (atomic) numbers should also balance

Beta Emission

  • Equation for beta emission:

Beta equation

  • Note that the beta particle is given an atomic number of -1 in the above examples

    This is because the atomic number is being used to measure charge in this case:

    Protons, being positive particles, have positive atomic numbers

    Electrons, being negative, have a negative number

Gamma Decay

  • During gamma decay, a gamma ray is emitted from an unstable nucleus
  • The process that makes the nucleus less energetic but does not change its structure


Gamma decay does not affect the mass number or the atomic number of the radioactive nucleus, but it does reduce the energy of the nucleus

  • The gamma ray that is emitted has a lot of energy, but no mass or charge
  • Here is an example of Uranium-238 undergoing gamma decay
    • Notice that the mass number and atomic number of the unstable nuclei remains the same during the decay

Gamma decay equation


An example of Barium decay through the release of a gamma ray

Worked example

WE - Alpha decay question image(1)_3, downloadable AS & A Level Physics revision notes


Alpha decay worked example (2), downloadable AS & A Level Physics revision notes

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