Atomic structure

• Democritus


  Greek philosopher in the 5th Century BC who thought all matter was made up of identical lumps called "atomos"
• Democritus' idea about matter being made up of "atomos" did not progress further until the 1800s
• Rutherford


  Replaced the Plum Pudding Model with the Nuclear Model
• Dalton's atomic theory


  1. Matter is made up of tiny spheres ("atoms") that can't be broken up

  2. Each element is made up of a different type of "atom"
• Thomson's atomic model


  Atoms are spheres of positive charge with tiny negative electrons stuck in them like fruit in a plum pudding (the plum pudding model)
• Rutherford's alpha scattering experiment


  1. Firing a beam of alpha particles at a thin gold sheet

  2. Most particles went straight through, but some were deflected more than expected, and a few were deflected back the way they had come
• Rutherford's findings from the alpha scattering experiment


  • Most of the mass of the atom must be concentrated at the centre in a tiny nucleus
  • The nucleus must also have a positive charge, since it repelled the positive alpha particles
  • Most of an atom is just empty space
• Nuclear model of the atom

  A positively charged nucleus surrounded by a cloud of negative electrons
• Bohr's model


  Electrons orbiting the nucleus do so at certain distances, and his theoretical calculations agreed with experimental data
• Development of the current model of the atom

  1. Evidence from further experiments changed the model to have a nucleus made up of a group of particles (protons) which all had the same positive charge that added up to the overall charge of the nucleus

  2. In 1932, James Chadwick proved the existence of the neutron, which explained the imbalance between the atomic and mass numbers
• Current model of the atom

  • The nucleus is tiny but it makes up most of the mass of the atom
  • The nucleus contains protons (positively charged) and neutrons (neutral)
  • The rest of the atom is mostly empty space
  • Electrons (negatively charged) whizz round the outside of the nucleus really fast and give the atom its overall size
  • The number of protons = the number of electrons, as protons and electrons have an equal but opposite charge and atoms have no overall charge
  • Electrons in energy levels can move within (or sometimes leave) the atom
• The current model of the atom may change in the future, just like the plum pudding model did
• Isotopes


  Different forms of the same element, with the same number of protons but a different number of neutrons
• Ionisation


  The process of knocking electrons off atoms, creating positive ions
• Types of ionising radiation

  • Alpha
  • Beta
  • Gamma
• Alpha particles


  • Helium nuclei
  • Don't penetrate very far into materials and are stopped quickly
  • Strongly ionising
• Beta particles


  • Fast-moving electrons
  • Moderately ionising
  • Penetrate moderately far into materials
• Gamma rays


  • Electromagnetic radiation with a short wavelength
  • Penetrate far into materials without being stopped
  • Weakly ionising
• Nuclear equations


  1. Show radioactive decay

  2. The total mass and atomic numbers must be equal on both sides
• Alpha decay


  Decreases the charge and mass of the nucleus
• Beta decay


  Increases the charge of the nucleus
• Gamma rays don't change the charge or mass of the nucleus
• Radioactivity


  A totally random process where radioactive substances give out radiation from the nuclei of their atoms
• Activity


  The rate at which a radioactive source decays, measured in becquerels (Bq)
• Half-life


  The time taken for the number of radioactive nuclei in an isotope to halve, or for the activity to fall to half of its initial value
• Calculating final activity after two half-lives

  • Initial activity 640 Bq
  • After 1 half-life: 640 ÷ 2 = 320 Bq
  • After 2 half-lives: 320 ÷ 2 = 160 Bq
  • Final activity as a percentage of initial: (160 ÷ 640) x 100 = 25%
• Graphing half-life


  • The half-life is found from the graph by finding the time interval on the bottom axis corresponding to a halving of the activity on the vertical axis
• Irradiation


  Objects near a radioactive source are exposed to it, but this does not make them radioactive
• Contamination


  Unwanted radioactive atoms getting onto or into an object
• Dangers of different types of radiation

  • Outside the body, beta and gamma sources are the most dangerous
  • Inside the body, alpha sources are the most dangerous
  • Beta sources are less damaging inside the body
  • Gamma sources are the least dangerous inside the body
• Research about how different types of radiation affect the body is important for improving the safe use of radioactive sources