Atomic structure

• Give two ways that an atom's electon arrangement can be changed

  1. Absorbing electromagnetic radiation
  2. Emitting electromagnetic radiation
• Explain how an atom's electron arrangement changes when it emits EM radiation

  • Electrons move closer to the nucleus
  • They move to a lower energy level
• Explain how an atoms electron arrangement changes when it absorbs EM radiation

  • Electrons move further away from the nucleus
  • They move to a higher energy level
• 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
• 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 protons
• Ways an atom's electron arrangement can change

  1. Absorbing electromagnetic radiation

  2. Emitting electromagnetic radiation
• How electron arrangement changes when atom absorbs EM radiation


  1. Electrons move further away from nucleus

  2. Electrons move to higher energy level
• How electron arrangement changes when atom emits EM radiation


  1. Electrons move closer to nucleus

  2. Electrons move to lower energy level
• Why atom has no overall charge

  Number of protons equals number of electrons, so charges cancel
• All forms of the same element have the same number of protons
• Atomic number


  The number of protons in an atom
• Mass number


  The total number of protons and neutrons in an atom
• Isotope


  An atom of an element with a different number of neutrons but the same number of protons
• How atoms turn into positive ions


  They lose one or more of their outer electrons, resulting in a positive charge
• Plum-pudding model


  A ball of positive charge with negatively charged electrons distributed evenly throughout it
• 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
• Currently accepted model of the atom

  The Bohr nuclear model
• James Chadwick's experiments proved the existence of neutrons
• Radioactive decay


  The process in which an unstable nucleus gives out radiation to become more stable
• Activity


  The rate of decay of a source of unstable nuclei
• Unit of radioactive activity


  Becquerel (Bq)
• Count-rate


  The number of radioactive decays per second for a radioactive source
• Detector used to measure count-rate

  • Geiger-Muller tube
• Types of nuclear radiation


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


  Two protons and two neutrons, same as a helium nucleus
• Range of alpha particle through air

  A few centimetres (normally in the range of 2-10cm)
• What stops beta radiation


  • A thin sheet of aluminium
  • Several metres of air
• What stops gamma radiation


  • Several centimetres of lead
  • A few metres of concrete
• Most ionising radiation


  Alpha radiation
• Least ionising radiation


  Gamma radiation
• Emission of gamma ray does not change mass or charge
• Nature of radioactive decay


  • Random
  • Which nuclei decays and when is determined only by chance
  • Impossible to predict which nuclei will decay and when
• Half-life


  • The time it takes for the number of unstable nuclei in a substance to halve
  • The time it takes for the count rate from a sample to fall to half its initial level
• Radioactive contamination


  The presence of unwanted radioactive nuclei on other materials
• Irradiation


  The process of exposing a material to nuclear radiation, but the material does not become radioactive
• Sources of background radiation


  • Rocks
  • Cosmic rays from space
  • Nuclear weapon testing
  • Nuclear accidents