Discovery of the atom (Edexcel GCSE Physics): Revision Notes

Discovery of the atom

Developing the Model of the Atom

Historical Development

• Greeks:
  • Atoms are tiny spheres which cannot be divided.
• Thomson's Plum Pudding Model:
  • An atom is a ball of positive charge with negative electrons embedded in it.

1909 - Rutherford's Alpha Scattering Experiment

• Fired a beam of alpha particles (positively charged) at a thin sheet of gold foil.
  • Most particles went straight through → Atom is mostly empty space.
  • Some were deflected → The Centre of the atom must have a positive charge, repelling particles that come close to it.
  • Some bounced straight back → Mass is concentrated in the centre.
  • This research led to the development of the first nuclear model.

Bohr's Model

• Electrons orbit the nucleus at specific distances.
  • Orbits are called energy levels or shells.

Updated Nuclear Model

• Positive Nucleus: Contains protons and neutrons.
• Electron Energy Levels: Electrons occupy energy levels or shells around the nucleus.

James Chadwick

• Proved the existence of the neutron which explained the imbalance between atomic and mass numbers.

Current Model of the Atom

• Nucleus: Contains protons and neutrons.
• Electrons: Occupy energy levels or shells around the nucleus.

Summary of models

Jonny Dalton

1803

J.J Thomson

1897

E. Rutherford

1911

Believed matter was made from spheres

All matter is made from atoms which are indestructible & cant be broken

Plum pudding theory

Discovered atoms can be broken up as discovered electrons can be removed from atoms. Suggested atoms were spheres or positive charge with tiny electrons stuck in them

Atomic model

Discovered the model of the atom