atomic structure

• Plum Pudding Model


  Model of the atom that was replaced by the Nuclear Model John dalton
• J.J. Thomson discovered particles called electrons that could be moved from atoms


  Nearly 100 years later
• Plum Pudding Model


  Thomson suggested atoms were spheres of positive charge with tiny negative electrons stuck in them like the fruit in a plum pudding
• Nucleus


  The scientists realised that most of the mass of the atom was concentrated at the centre in a tiny nucleus, which must also have a positive charge, since it repelled the positive alpha particles
• Nuclear Model of the Atom


  This was the first nuclear model of the atom
• Bohr


  Refined Rutherford's Nuclear Model of the Atom
• Bohr's model of the atom

  • Electrons orbiting the nucleus do so at certain distances called energy levels
  • His theoretical calculations agreed with experimental data
• Evidence from further experiments

  1. Changed the model to have a nucleus made up of a group of particles

  2. All had the same positive charge that added up to the overall charge of the nucleus
• Our Current Model of the Atom

  • The nucleus is tiny but it makes up most of the mass of the atom
  • It contains protons which are positively charged and neutrons which are neutral
  • The rest of the atom is mostly empty space
  • Negative electrons whizz round the outside of the nucleus really fast and give the atom its overall size
• The number of protons equals the number of electrons in atoms, as protons and electrons have an equal but opposite charge and atoms have no overall charge
• isotope 


  Different forms of the same element
• Types of ionising radiation

  • Alpha
  • Beta
  • Gamma
• Alpha radiation


  1. Alpha particle emitted from the nucleus

  2. Alpha particle has 2 neutrons and 2 protons like a helium nucleus
• Alpha radiation


  • Penetrates very little into materials
  • Absorbed by a sheet of paper
  • Strongly ionising
• Applications of alpha radiation


  • Used in smoke detectors to ionise air particles and detect smoke
• Beta particles


  High-speed electrons released by the nucleus
• Beta radiation


  • Moderately ionising
  • Penetrates moderately far into materials
  • Absorbed by a sheet of aluminium
  • Emitted when a neutron turns into a proton
• Uses of beta radiation

  • Used to test the thickness of metal sheets
• Gamma rays


  Electromagnetic waves with a short wavelength released by the nucleus
• Gamma radiation


  • Penetrates far into materials without being stopped
  • Weakly ionising as they tend to pass through rather than collide with atoms
  • Can be absorbed by thick sheets of lead or metres of concrete
• Alpha particles are more ionising than beta particles
• half life 


  the time take for the number of radioactive nuclei in an isotope to halve
• nuclear fission 


  splitting a large unstable nucleus
• nuclear fusion 


  joining small nuclei
• contamination 


  Radioactive particles getting into objects