Developing the Model of the Atom

• Atomos


  Identical lumps that matter was made up of, according to Democritus
• Democritus thought that all matter was made up of tiny spheres ("atoms") that couldn't be broken up

  5th Century BC
• Dalton's atomic theory


  • Matter is made up of tiny spheres ("atoms") that couldn't be broken up
  • Each element is made up of a different type of "atom"
• Thomson's plum pudding model

  • Atoms are spheres of positive charge with tiny negative electrons stuck in them like the fruit in a plum pudding
• Rutherford's alpha scattering experiment


  1. Firing a beam of alpha particles at thin gold foil

  2. Most particles passed straight through or were slightly deflected

  3. Some were deflected more than expected, and a few were deflected back
• Rutherford's nuclear model


  • Most of the mass of the atom is concentrated at the centre in a tiny nucleus
  • The nucleus has a positive charge, which repelled the positive alpha particles
  • Most of an atom is just empty space
• Bohr's model of the atom

  • Electrons orbiting the nucleus do so at certain distances called energy levels
  • The nucleus is made up of a group of particles (protons) with the same positive charge
• Chadwick proved the existence of the neutron in 1932
• Current model of the atom

  • The nucleus is tiny but makes up most of the mass of the atom
  • The nucleus contains protons and neutrons
  • The rest of the atom is mostly empty space
  • Negative electrons whizz round the outside of the nucleus really fast
• In atoms, the number of protons equals the number of electrons, as protons and electrons have an equal but opposite charge and atoms have no overall charge
• If electrons gain energy

  They release EM radiation and move to a lower energy level that is closer to the nucleus
• If one or more outer electrons leave the atom

  The atom becomes a positively charged ion