Rutherford's model of the atom

Introduction

  • Ernest Rutherford (1871-1937) was a scientist who worked with J.J. Thomson (1856-1940) at the Cavendish Laboratory at Cambridge University in England.
  • At the time, the most advanced model of the atom was J.J. Thomson's Plum Pudding model.
    • This was a model proposed after Thomson's discovery of the electron.
    • This model of the atom was one where the atom consisted of a "soup" of positive charge with electrons floating around inside it
  • Later in 1898, Rutherford went to work with Becquerel, a scientist researching radioactivity.
    • Becquerel was the scientist that first discovered radioactivity
    • Becquerel was the one who performed the first alpha scattering experiment as he studied the passage of alpha particles through magnetic fields.
    • FUN FACT: Rutherford didn't get along well with Becquerel, since he proved one of Becquerel's theories to be wrong.

The Geiger-Marsden experiment

  • In 1907, Rutherford gained two scientists: Hans Geiger and Ernest Marsden as his assistants.
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  • These two then conducted what was known as the Geiger-Marsden experiment:
    • The following apparatus was placed within a vacuum.
    • An alpha particle source (they used Polonium) was placed in a lead box with a slit.
    • This was aimed at an extremely thin piece of metal foil (they used Gold).
    • Around the entire system was a rotate-able outer box, which contained a scintillation viewer (they used a microscope).
      • A scintillation is when a flash of light appears as a particle hits a surface. (e.g. Cathode Rays)
    • This was then repeated with increasing thickness of gold foil.
  • The results of this experiment was that:
    • The scattering increased with increased thickness (but remained constant after a while).
      • This showed that the alpha particles were interacting with the gold atoms (as predicted, since it is a positive helium ion.)
    • Most of the alpha particles went straight through the foil, or were deflected very slightly.
      • This challenged the Plum-Pudding model, since if the charges were randomly floating about, the electrostatic force should have scattered the particles in all directions
      • Few of the particles were reflected back with an angle above 90 degrees.
  • Through these results, Rutherford proposed that the positive charge in an atom was concentrated in a region about 10000 times smaller than the radius of the atom.

Rutherford's model of the atom

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  • Rutherford's model of the atom is the basic structure of most atoms seen today.
  • Its properties were:
    • The atom consisted of mostly empty space.
    • There is a very small, dense nucleus in which all of the positive charge and most of the mass is contained.
    • Electrons resided in the empty space and orbited the nucleus in circular orbits
      • It should be noted that Rutherford's model did not state that electrons could only reside in fixed distances from the nucleus
    • The overall charge of the atom was neutral.
      • Also to be noted is that Rutherford did not know of the existence of Neutrons.
  • Whilst this was a dramatic improvement to Thomson's model, it had many limitations. The biggest of which was the stability of electron orbits
    • For an electron to travel in a circular orbit, it should be accelerating
    • Under Maxwell's EM theory, the electron should emit EM radiation and lose energy.
    • This would mean that electrons would eventually spiral into the nucleus and collapse.
    • As this was not the case, there was something wrong with this model.
  • Yet, Rutherford could not explain why this had not happened.