Developing Models of the Atom

• JJ Thompson’s Plum Pudding Model:

  • At the end of the 19th Century, Physicist Thompson discovered the existence of electrons.
  • J J Thomson thought of the atom as being a positively charged mass embedded with small negatively charged electrons – a bit like a plum pudding.
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• Rutherford's Alpha Particle Scattering Experiment:

  • In 1909 a group of scientists were investigating the Plum Pudding model.
  • Physicist, Rutherford was instructing two of his students, Geiger and Marsden to carry out the experiment.
• Rutherford's Alpha Particle Scattering Experiment:

  • They were directing a beam of alpha particles (He2+ ions) at a thin gold foil.
  • They expected the alpha particles to travel through the gold foil, and maybe change direction a small amount.
• Rutherford's Alpha Particle Scattering Experiment:

  Instead, they discovered that :

  • Most of the alpha particles passed straight through the foil
  • Some of the alpha particles changed direction but continued through the foil
  • A few of the alpha particles bounced back off the gold foil
• When alpha particles are fired at thin gold foil, most of them go straight through, some are deflected and a very small number bounce straight back.
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• Alpha Scattering Findings and Conclusions Table:

  
• The Nuclear Model:

  • Rutherford proposed the nuclear model of the atom
  • In the nuclear model:
  • Nearly all of the mass of the atom is concentrated in the centre of the atom (in the nucleus)
  • The nucleus is positively charged
  • Negatively charged electrons orbit the nucleus at a distance
• Rutherford’s nuclear model replaced the Plum Pudding model:

  
• The Bohr Model of the Atom:

  • In 1913 the Physicist, Bohr, came up with an improved model of the atom
  • He used the nuclear model to create his model
• • In the Bohr model of the atom:
  • Electrons orbit the nucleus at different distances
  • The different orbit distances are called energy levels
  • Up to 2 electrons orbit in the first energy level
  • Up to 8 electrons can orbit in the second energy level
  • Up to 8 electrons can orbit in the third energy level
• Bohr model of the atom:

  
• The Bohr model became the accepted model because:

  • It was able to explain the findings from different experiments better than the nuclear model of the atom
  • It was able to explain the processes of absorption and emission of electromagnetic radiation
  • Theoretical calculations made using the Bohr model agreed with experimental results
• The understanding of the structure of an atom has changed over time:

  • The best model of an atom is the one that can explain the evidence of experiments best
  • As more evidence has been collected, the models have improved
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