Development of the model of the atom

• 500 BC - Democritus : He proposed that everything is made up of tiny solid spheres called atoms, which are the smallest possible unit
• 1897 - J.J. Thomson : he said that an atom consists of a ball of positive charge with negative electrons embedded in it. This was called the Plum Pudding model
• 1911 - Rutherford : he did an alpha particle experiment, where they were fired at a thin sheet of gold foil, where most passed through while some deflected. This meant that most of the mass was in the nucleus and it was positively charged, while the negative electrons surrounded the nucleus in a cloud.
• 1913 - Bohr : he suggested that electrons orbit the nucleus in shells
• 1932 - Chadwick : he discovered neutral particles in the nucleus, called neutrons
• Order of discovery of the subatomic particles is :

  1. Electrons (J.J. Thompson)
  2. Protons (Ernest Rutherford)
  3. Nucleus (Ernest Rutherford)
  4. Neutron (James Chadwick)
• Plum pudding model:

  A) Sphere of positive charge

  B) Negative electrons embedded

  
• Nuclear model - Rutherford

  A) Electron energy levels

  B) Nucleus

  C) Proton

  D) Neutron

  E) Electrons

  
• Bohr atom model

  A) Electron energy shells

  B) Electron

  C) Proton