History of the Atom

• 400BC - Democritus proposes that all things are made out of atomos
• 1803 - John Dalton creates the principles of atomic theory.
• 1897 - JJ Thompson discovers the electron by measuring the curve of charged particles passing through charged plates.

  He proposes the plum Pudding model.

  
• 1909 - Earnest Rutherford leads a team that fires charged alpha particles at gold foil. About one in 7000 are reflected back. This means two things:

  • Most of the atom is empty space, with most of the matter in a small dense nucleus.

  • This nucleus is positively charged, so it can reflect the positively charged particles fired at it.

  He proposes a new model of the atom.

  
• 1913 - Niels Bohr revises the current atomic model. He proposes that electrons exist in energy shells rather than orbiting the nucleus. In addition these electrons can move between energy shells.

  
• 1932 - The nucleus has been identified as the positive subatomic particle in the nucleus, however a large amount of mass in each atom is unaccounted for. James Chadwick discovers this in the form of the neutron.