Atomic Structure

• Atom radius


  1x10⁻¹⁰ metres
• Subatomic constituents of an atom

  • Proton
  • Neutron
  • Electron
• Most of the mass of an atom is concentrated in the nucleus
• Proportion of atom radius that is the nucleus radius

  1/10,000
• Arrangement of protons, neutrons and electrons in an atom

  1. Protons and neutrons in the nucleus

  2. Electrons in discrete energy levels around the nucleus
• Charge of the nucleus

  Positive charge, due to the protons
• Ways an atom's electron arrangement can change

  1. Absorbing electromagnetic radiation

  2. Emitting electromagnetic radiation
• Electron arrangement changes when absorbing EM radiation

  Electrons move further away from nucleus, to higher energy level
• Radioactive decay


  The process in which an unstable nucleus gives out radiation to become more stable
• Electron arrangement changes when emitting EM radiation

  Electrons move closer to nucleus, to lower energy level
• Unstable nuclei


  • Undergo decay to become more stable
  • As they release radiation their stability increases
• Reason atoms have no overall charge

  Number of protons equals number of electrons, so charges cancel
• Activity


  The rate of decay of a source of unstable nuclei
• All forms of the same element have the same number of protons
• Unit of radioactive activity

  Becquerel (Bq)
• Number of protons in an atom

  Atomic Number
• Count-rate


  The number of radioactive decays per second for a radioactive source
• Mass number of an atom


  Total number of protons and neutrons
• Detector used to measure count-rate

  • Geiger-Muller tube
• Isotope


  Atom of an element with a different number of neutrons, but the same number of protons
• Types of nuclear radiation

  • Alpha particles
  • Beta particles
  • Gamma rays
  • Neutrons
• Alpha particle


  Two protons and two neutrons, same as a helium nucleus
• How atoms turn into positive ions


  Lose one or more outer electrons, resulting in a positive charge
• Range of alpha particle through air

  A few centimetres (normally in the range of 2-10cm)
• New experimental evidence that doesn't agree with existing theory can lead to a scientific model being changed or replaced
• Plum-pudding model


  Ball of positive charge with electrons distributed evenly throughout
• Prior to the discovery of the electron, the atom was believed to be indivisible
• Experiment that led to discarding the plum-pudding model

  Rutherford's alpha-scattering experiment
• What stops beta radiation

  • A thin sheet of aluminium
  • Several metres of air
• Currently accepted model of the atom

  Bohr nuclear model
• What stops gamma radiation

  • Several centimetres of lead
  • A few metres of concrete
• Sources of background radiation

  • Rocks
  • Cosmic rays from space
  • Nuclear weapon testing
  • Nuclear accidents
• Most ionising radiation


  Alpha radiation
• How to deal with background radiation in calculations

  Subtract the background count from any readings before calculations (half life etc.) are attempted
• James Chadwick's experiments proved the existence of neutrons
• Unit used to measure radiation dosage

  Sieverts (Sv)
• Least ionising radiation


  Gamma radiation
• Relationship between sieverts and millisieverts

  1000 millisieverts = 1 sievert
• Emission of gamma ray does not change mass or charge
• Reasons why radiation dosage may differ for different people

  • Some occupations involve working with radiation
  • Background radiation differs with location due to things such as the locality of nuclear power stations or radiation related testing