Models

• Early Models of the Atom

  • Greek and Indian philosophers were the first to try and describe the idea of everything being made up of smaller parts
  • The Greek philosopher, Democritus, thought that although objects could be cut into smaller pieces, the smallest possible piece would be indivisible (it could not be cut any further)
  • The Greek word for ‘indivisible’ is atomos
  • Therefore, atoms were initially thought to be tiny spheres that could not be divided before the discovery of the electron
  • Later models described the atom as small solid spheres
• JJ Thompson’s Plum Pudding Model

  • At the end of the 19th Century, Physicist Joseph Jon Thompson discovered the existence of electrons
  • This new evidence meant a better model of the atom was required
  • Thompson proposed the Plum Pudding model
  • The atom was thought to consist of negatively charged electrons (the ‘plums’) in a positively charged ‘dough’ or ‘pudding’
• PLUM PUDDING MODEL:

  A) POSITIVELY

  B) NEGATIVELY

  
• 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

  • It was known that electrons were much smaller than atoms, so it made sense that they should be embedded within the larger atom
  • Since electrons have a negative charge, it was reasoned that the rest of the atom would be positive, making the atom neutral overall
• Rutherford's Alpha Particle Scattering Experiment

  • In 1909 a group of scientists were investigating the Plum Pudding model
  • Physicist, Ernest Rutherford was instructing two of his students, Hans Geiger and Ernest Marsden to carry out the 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:

  • 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
  • The bouncing back could not be explained by the Plum Pudding model, so a new model had to be created
• RUTHERFORD

  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

  • Ernest Rutherford made different conclusions from the findings of the experiment
• RUTHERFORDS CONCLUSIONS:

  A) Most

  B) empty space

  C) few

  D) deflected

  E) positive

  F) rebounded

  G) heavy nucleus

  
• 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 nuclear model could explain experimental observations better than the Plum Pudding model
• NUCLEAR MODEL:

  A) Plum pudding

  B) Nuclear

  C) Mass

  D) Nucleus

  E) electrons

  F) orbiting

  
• The Bohr Model of the Atom

  • In 1913 the Danish Physicist, Niels 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'S ATOMIC MODEL:

  
• Successes of the Bohr Model

  • 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 Changing Models of the Atom

  • 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