M8-S6: Rutherford's Atomic Model and The Geiger-Marsden Experiment

  • investigate, assess and model the experimental evidence supporting the nuclear model of the atom, including:

– the Geiger-Marsden experiment
– Rutherford’s atomic model
– Chadwick’s discovery of the neutron (ACSPH026)

 

Rutherford’s Model of the Atom

  • Rutherford founded the basis of the nuclear model of the atom.
    1. He proposed that an atom consists of a positive centre (nucleus) and is surrounded by smaller and negatively charged electrons.
    2. He suggested that an atom consists of mostly empty space. 

Illustration of Rutherford's atomic model. Positive nucleus in the centre surrounded by mostly empty space and small negatively charged electrons.

 

Rutherford’s Gold Foil Experiment (Geiger-Marsden Experiment)

  • In order to come up with this model, Rutherford, Geiger and Marsden conducted a series of experiments in which positively charged alpha particles (helium nuclei) were fired at a thin sheet of gold foil which was surrounded by a ring-shaped fluorescent screen for detecting the alpha particles.

Rutherford's gold foil experiment was actually performed by Geiger and Marsden. Students are recommended to learn how to draw the experimental set-up as it will deepen understanding and quality of exam responses. 

  • Most of the projected alpha particles passed through the gold foil and only a select few were deflected and reflected back. The angle of deflection or reflection was measured by the position they were detected on the fluorescent screen.
  • Large proportion of alpha particles passing through indicates that the gold atoms consist of mostly empty space.
  • Deflection and reflection of alpha particles are caused by positively charged nuclei. The trajectory of alpha particles is unlikely to be hindered by electrons as they are comparatively very small in size.
    • Geiger and Marsden confirmed that each scintillation (a flash of light on a scintillation / fluorescent screen) was produced by an alpha particle, and that all of the alpha particles produced a scintillation.
  • Most of the particles passed through (indicating the largely empty atom) but there were a few which were deflected or reflected back (indicative of the small positive nucleus).

J.J. Thomson's 'plum pudding' atomic model in which he describes electrons being scattered within a positively charged cloud. This model was modified and improved by Rutherford.

  • Significance of this experiment – Not only did the experimental result gather more information regarding the content and characteristics of atoms, the scientists showed that the Thomson’s model of the atom is flawed. Thomson proposed that an atom only consists of a sea of electrons scattered within a positive ‘cloud’. Had his model been correct, all the alpha particles should have passed through the gold foil with minimal scattering.
  • Historical context of this experiment (not required for HSC Physics, for interest only) - Ernest Rutherford was a honoured profession in Manchester when he was visited by Hans Geiger. Rutherford was impressed by Geiger’s intellect, so he invited Geiger to stay. Along with Geiger was Ernest Marsden, who was an undergraduate student under the supervision of Geiger.

Chadwick’s Discovery of the Neutron

  • Rutherford’s atomic model did not show what was inside the positive nucleus. Besides protons, he realised that there must be other particles because the mass number of an atom was always found to exceed its atomic number (number of protons).
  • Context – Beryllium was found to release a stream of radiation when bombarded with alpha particles. These were in fact neutrons but were initially thought to be gamma radiation because they were unaffected by magnetic fields. However, photoelectric effect related experiments showed that this stream of ‘radiation’ could not be radiation because no electrons could be emitted.
  • Bothe and Becker fired alpha particles at beryllium and subsequently observed the emission of a form of ‘radiation’. This radiation couldn’t be detected in a cloud chamber and didn’t appear to be a particle (no charge).
  • Joliot and Curie (husband and wife) later carried out the same experiment with a slight adjustment; letting the radiation fall on a paraffin block (proton-rich material) and finding that protons were being knocked off (detected by Geiger counter), concluding that the unknown ‘radiation’ possessed momentum therefore it was not gamma radiation.

Illustration of Chadwick's experiment (discovery of neutrons). Students are recommended to learn how to draw diagrams similar to this one for learning and exam purposes. 

 

  • This led Chadwick to his own experiments and conclusions through the application of the laws of conservation of energy and momentum as well as conservation of atomic mass and number.

  • He found that the energy required to eject the protons couldn’t have been provided by electromagnetic radiation. He rationalised that the speed at which protons were hitting the detector would have momentum that is much greater than gamma radiation. Therefore, this would have violated the law of conservation of momentum and energy.
  • He realised then that a neutral particle would be capable of striking a proton and imparting its momentum onto it, this wouldn’t violate the law of conservation of momentum.
  • Finally, in observing the conservation of atomic mass and numbers, he was able to deduce that the missing particle had a mass of 1 amu and thus through the application of the conservation laws he was able to discover the neutron .

Practice Question 1

Describe Rutherford's gold-foil experiment (3 marks)

Practice Question 2

Explain the experimental evidence supporting Rutherford's gold-foil experiment. (2 marks)

Practice Question 3

Explain why the neutron was initially thought to be radiation and describe how Chadwick managed to verify its nature and existence. (4 marks)

 

Previous section: The Electron (Cathode Ray, J.J. Thomson and R. Millikan's experiments)

Next section: Bohr's Model of the Atom