Revision Notes

5.2.3 Radioactive Decay

Radioactive Decay: Basics

  • Some isotopes are unstable – usually because of their large size or because the number of protons and neutrons within them are out of balance
  • As a result, these isotopes will decay – emitting little chunks (radiation) in order to reduce their size or bring them back into balance

radioactive-decay, IGCSE & GCSE Physics revision notes

Unstable nuclei decay by emitting high energy articles – radiation

  • When an isotope emits radiation, the constitution of its nucleus (the number of protons and neutron) changes
  • As a result, the isotope will change into a different element


Extended Only

Alpha Emission

  • 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


Extended Only

Beta Emission

  • A beta particle is a high energy electron emitted from the nucleus
  • It is emitted when a neutron in the nucleus suddenly changes into a proton – an electron is created in order to balance the positive charge of the proton
    (Note: The electron is created at the moment of decay – it is not present in the neutron beforehand)
  • When a beta particle is emitted from a nucleus:
    • The number of protons in the nucleus increases by 1:
      The proton (atomic) number increases by 1
    • The total number of particles in the nucleus remains the same
      The nucleon (mass) number doesn’t change


beta-emission, IGCSE & GCSE Physics revision notes

A beta particle is emitted when a neutron turns into a proton, emitting an electron


  • 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


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