Atomic structure and radiation

• Positively charged nucleus (which contains neutrons and protons) surrounded by negatively charged electrons
• Subatomic Particle


  • Proton
  • Neutron
  • Electron
• Subatomic Particle Properties


  • Proton: Relative Mass 1, Relative Charge +1
  • Neutron: Relative Mass 1, Relative Charge 0
  • Electron: Relative Mass 0 (0.0005), Relative Charge -1
• Typical radius of an atom: 1 × 10−10 metres
• The radius of the nucleus is 10,000 times smaller than the atom
• Most (nearly all) the mass of the atom is concentrated at the nucleus
• Electron Arrangement: Electrons lie at different distances from the nucleus (different energy levels). The electron arrangements may change with the interaction with EM radiation
• All atoms of the same element have the same number of protons
• Neutral atoms have the same number of electrons and protons
• Isotopes are atoms of the same element, but with different masses, which have the same number of protons but different number of neutrons
• Example of Isotopes: Carbon-12, Carbon-13, and Carbon-14
• Atoms and EM Radiation: When electrons change orbit (move closer or further from the nucleus), the atom can absorb or emit EM radiation
• If an electron gains enough energy, it can leave the atom to form a positive ion
• Dalton said everything was made of tiny spheres (atoms) that could not be divided


  1800
• JJ Thomson discovered the electron


  1897
• Rutherford realised most of the atom was empty space


  1911
• Rutherford Model


  1913
• Gold Foil Experiment: Most 𝛼 particles went straight through, indicating most of the atom is empty space. Some were deflected, indicating a charged nucleus, and a few were deflected by >90°, indicating the nucleus contained most of the mass
• Bohr produced the final model of the atom

  1913
• Positive nucleus at the centre of the atom, and negative electrons existing in a cloud around the nucleus
• Positive charge of nucleus could be subdivided into smaller particles, each with the same amount of charge – the proton
• 20 years after the ‘nucleus’ was an accepted scientific idea, James Chadwick provided evidence to prove neutrons existed
• Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable. This is a random process called radioactive decay
• Activity


  The rate at which a source of unstable nuclei decays
• Sample with high activity has a fast rate of decay
• Activity is measured in Becquerel, Bq
• Count-rate


  The number of decays recorded by a detector per second
• Forms of decay


  • Alpha α (a helium nucleus)
  • Beta Minus β (electron)
  • Gamma γ (radiation)
  • Neutrons
• Nuclear equations are used to represent radioactive decay
• If Rutherford was right, the electrons in the cloud close to the nucleus would get attracted, and cause the atom to collapse. So now the electrons exist in fixed ‘orbitals’
• The emission of the different types of nuclear radiation may cause a change in the mass and /or the charge of the nucleus
• Alpha decay causes both the mass and charge of the nucleus to decrease
• Beta decay does not cause the mass of the nucleus to change but does cause the charge of the nucleus to increase
• Gamma decay does not cause the mass or charge to change
• Half Life


  The time taken for half the nuclei in a sample to decay or the time taken for the activity or count rate of a sample to decay by half
• The half-life is a constant that enables the activity of a very large number of nuclei to be predicted during the decay
• If 80 atoms fall to 20 over 10 mins, the half-life is 5 mins
• A short half-life means the source presents less of a risk, as it does not remain strongly radioactive, presenting less of a long-term risk
• A long half-life means the source remains weakly radioactive for a long period of time
• Half-life is 5 minutes