The History of the Atom

• Atoms


  Smallest units of matter that make up all substances
• Ideas about what atoms look like

  • Have changed over time
• Plum pudding model


  Atom as a ball of positive charge with electrons gathered in this ball
• Experiments that showed the plum pudding model was wrong


  1. Fired positively charged alpha particles at a thin sheet of gold

  2. Expected most particles to go straight through

  3. But some particles changed direction more than expected, even went backwards

  4. This meant the plum pudding model was not right
• Rutherford's nuclear model


  Tiny positively charged nucleus at the centre of the atom, most of the mass is in the nucleus, surrounded by a 'cloud' of negative electrons, most of the atom is empty space
• Bohr's nuclear model


  Electrons orbit the nucleus in shells (levels), each shell is a fixed distance from the nucleus
• Bohr's theory was supported by many experiments
• Experiments later showed that Bohr's theory was correct
• Protons and neutrons


  • Smaller particles that make up the nucleus
• This happened about 20 years after scientists agreed that atoms have nuclei
• Electron shell rules


  • Electrons always move in shells
  • Inner shells are filled up first
  • Only certain number of electrons allowed in each shell
  • Atoms are more stable when they have full electron shells
  • In most atoms, the outer shell is not full
• How to work out electronic structure


  1. Follow the electron shell rules

  2. Example: Sodium has 11 protons, so it has 11 electrons

  3. First shell can take 2 electrons, second shell can take 8 electrons, third shell takes the remaining 2 electrons
• Electronic structure of some elements

  • Hydrogen: 1 electron
  • Helium: 2 electrons
  • Carbon: 6 electrons