Alpha - Scattering Experiment

• Plum pudding model


  An atom is a ball of positive charge with negative electrons embedded in it
• Alpha scattering experiment


  1. Scientists took a piece of gold foil

  2. Fired tiny positive alpha particles at the gold foil

  3. Most alpha particles passed straight through the gold foil

  4. Some alpha particles were deflected

  5. Some alpha particles bounced straight back
• Most alpha particles went straight through the gold foil


  Atoms are mainly empty space
• Some alpha particles were deflected


  The center of the atom must have a positive charge
• Some alpha particles bounced straight back

  The center of the atom must contain a great deal of mass
• Nuclear model

  Most of the atom is empty space, with a tiny positive nucleus containing most of the mass, and negative electrons around the edge
• The plum pudding model was replaced by the nuclear model due to new experimental evidence from the alpha scattering experiment
• John Dalton's Atomic Theory (1807)

  • All matter is made up of atoms
  • Atoms are tiny, hard spheres
  • Atoms are indivisible
• J.J Thomson - Plum pudding model (1897)

  • Discovered electrons (sub-atomic particles)
  • Cathode ray experiment
  • Electrons are negatively charged --> Equal number of positive particles
  • Atoms are neutral
• Ernest Rutherford - Alpha scattering experiment

  • Thin gold foil (a few atoms thick)
  • Fired tiny alpha (positive) particles at the foil
  • Majority went through
  • 1/1000 alpha particles either bounced off or deflected. These particles hit the nucleus head-on, where nearly all of the mass is of the atom.
  • + concentrated on a very similar space
  • Atoms are not positively charges
• Nuclear model - Atoms were mainly empty space, the nucleus had majority of the mass

  Electrons orbit around
• Neils Bohr (1913)

  Found out that electrons orbit the nucleus at specific distances in energy levels
• James Chadwick (1933)

  Found neutrons (no charge)
• Isotopes - Different numbers of neutrons but same amount of protons
• Ions - Gain or lose electrons to become electrically charged
• Proton mass - 1

  Neutron mass - 1

  Electron mass - very small
• Proton charge - +1

  Neutron charge - +1

  Electron charge - -1