1.1.3 History of the Atom

• Atomic theory:

  • Everything is made up of tiny particles that can’t be broken any further.
  • These particles are separated by empty space.
  • Derived by Democritus in around 500 BC.
• Solid sphere theory:

  • Derived by John Dalton in the 1800s.
  • Describes atoms as small spheres.
  • Different types of spheres may make up different elements.
• Plum Pudding Model:

  • Derived by JJ Thompson in 1897.
  • Atoms were proven to not be solid spheres.
  • Atoms must contain some form of negatively charged particle.
  • Proposed that atoms were a general ball of positive charge with discrete electrons inside.
• Nuclear Model:

  • Derived by Ernest Rutherford in 1909.
  • Took positively charged alpha particles and fired them at a really thin sheet of gold.
  • If positive charge was ‘spread out’, the particles should have passed through.
  • Particles were deflected to the side, and sometimes even backwards, proving JJ Thompson wrong.
  • He proposed instead there was some sort of compact nucleus which contained all the positive charge, leaving the negative charge to exist in some sort of cloud around it.
  • The atom would have automatically collapsed if this was the case.
• Electron Shell Theory:

  • Derived by Niels Bohr in 1913.
  • He suggested that the electrons orbited the nucleus like planets in shells.
• Proton Theory:

  • Derived by Ernest Rutherford.
  • Small discreet particles were discovered inside the nucleus.
• Neutron Theory:

  • Derived by James Chadwick.
  • Suggested there were neutral particles in the nucleus too.