P4

• 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
• Radioactive decay


  The process in which an unstable nucleus gives out radiation to become more stable
• How electron arrangement changes when atom absorbs EM radiation


  1. Electrons move further away from nucleus

  2. Electrons move to higher energy level
• Activity


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


  Becquerel (Bq)
• How electron arrangement changes when atom emits EM radiation


  1. Electrons move closer to nucleus

  2. Electrons move to lower energy level
• Count-rate


  The number of radioactive decays per second for a radioactive source
• Reason atoms have no overall charge

  Number of protons equals number of electrons, so charges cancel
• Detector used to measure count-rate

  • Geiger-Muller tube
• Types of nuclear radiation

  • Alpha particles
  • Beta particles
  • Gamma rays
  • Neutrons
• All forms of the same element have the same number of protons
• Sources of background radiation

  • Rocks
  • Cosmic rays from space
  • Nuclear weapon testing
  • Nuclear accidents
• Number of protons in an atom

  Atomic Number
• Alpha particle


  Two protons and two neutrons, the same as a helium nucleus
• How to deal with background radiation in calculations


  Subtract the background count from any readings before calculations (half life etc.) are attempted
• Mass number of an atom


  Total number of protons and neutrons
• Range of alpha particle through air

  A few centimetres (normally in the range of 2-10cm)
• Unit used to measure radiation dosage


  Sieverts (Sv)
• Isotope


  Atom of an element with a different number of neutrons but the same number of protons
• What stops beta radiation

  A thin sheet of aluminium or several metres of air
• Relationship between sieverts and millisieverts

  1000 millisieverts = 1 sievert
• How atoms turn into positive ions


  Lose one or more outer electrons, resulting in a positive charge
• What stops gamma radiation


  Several centimetres of lead or a few metres of concrete
• New experimental evidence that doesn't agree with existing theory can lead to a scientific model being changed or replaced
• 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
• Most ionising radiation


  Alpha radiation
• Plum-pudding model


  Ball of positive charge with electrons distributed evenly throughout
• Nuclear fission


  The splitting of large, unstable nuclei to form smaller more stable nuclei (+the emission of spare neutrons)
• Factor that determines how dangerous a particular radioactive isotope is

  The half-life of the isotope
• Least ionising radiation


  Gamma radiation
• Prior to discovery of electron, atom was believed to be indivisible
• Fissionable isotope


  • Uranium - 235
• Isotopes with long half-lives

  • They remain radioactive for much longer periods of time
  • They must be stored in specific ways to avoid humans and the environment from being exposed to radiation for too long
• What usually needs to happen to induce fission

  1. The unstable nuclei must absorb a neutron

  2. Spontaneous fission (where no neutron absorption occurs) is rare
• Emission of a gamma ray does not change mass or charge