CIE IGCSE Physics

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 notesUnstable 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

 

Exam Question: Alpha Emission

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 notesA 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

 

Exam Question: Beta Emission

Author: Jenna

Jenna studied at Cardiff University before training to become a science teacher at the University of Bath specialising in Biology (although she loves teaching all three sciences at GCSE level!). Teaching is her passion, and with 10 years experience teaching across a wide range of specifications – from GCSE and A Level Biology in the UK to IGCSE and IB Biology internationally – she knows what is required to pass those Biology exams.
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