atomic structure

• Geiger and Marsden experiment

  1. Directed a beam of alpha particles at a thin gold foil

  2. Most alpha particles travelled straight through, but a small fraction bounced back
• Conclusions from Geiger and Marsden experiment

  • The alpha particles are repelled by a very small, positively charged nucleus, which contains most of the mass of the atom
  • The nucleus must be small because only a small fraction of alpha particles bounce back
  • The nucleus is positive because its strong electric field repels the positively charged alpha particles
  • The nucleus must be massive, because a small nucleus would be knocked forwards by the alpha particle
• Bohr model of the atom

  • Electrons move round the nucleus in circular orbits
  • Electrons can change their orbit by absorbing or emitting electromagnetic radiation
• Later experiments showed that the nucleus could be divided further into protons and neutrons, and James Chadwick discovered neutrons in 1932