How and why the atomic model has changed over time

• 1800

  Dalton said everything was made of tiny spheres (atoms) that could not be divided
• 1897

  JJ Thomson discovered the electron

  The Plum Pudding Model was formed:

  • overall charge of an atom is neutral, so the negative electrons were dispersed through the positive 'pudding' to cancel out the charges

  
• 1911

  Rutherford realised most of the atom was empty space
• Gold Foil Experiment - 1911

  • most alpha particles went straight through
  • so most of the atom is empty space
  • some alpha particles were slightly deflected
  • so nucleus must be charged
  • few alpha particles deflected by >90 degrees
  • so nucleus contained most of the mass
• 1913

  Rutherford model:

  • positive nucleus at the centre of the atom, and negative electrons existing in a cloud around the nucleus

  
• 1913

  Bohr produced the final model of the atom:

  • if Rutherford was right, the electrons in the cloud close to the nucleus would get attracted, and cause the atom to collapse.
  • so now the electrons exist in fixed orbits

  
• Later on?

  • positive charge of the nucleus could be subdivided into smaller particles, each with the same amount of charge - the proton
  • 20 years after the nucleus was an accepted scientific idea, James Chadwick provided evidence to prove neutrons existed