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, at specific distances from the nucleus
• 1932 - Chadwick : he discovered neutral particles in the nucleus, called neutrons
• Nucleus radius : 1x10^-14
• Atom radius: 1x10^-10
• Electrons occupy energy levels, staring with the lowest energy level (closest to the nucleus)
• The first energy level can only have 2 electrons, while the rest have 8 electrons each.
• Electron structure of an element can be represented as numbers, this is called an electron configuration. For example 2,8,1 for sodium.
• People involved in the development of the atom in order:

  • John Dalton - (idea of everything made of atoms).
  • J.J Thomson - (plum pudding, electron & proton).
  • Rutherford - (alpha particle experiment, mostly empty space).
  • Bohr - (electron energy levels).
  • James Chadwick - (neutrons).