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Rutherford's Gold Foil Experiment

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In 1909 Ernest Rutherford (1871-1937), New Zealand-born physicist and former student of J. J Thomson and his student Ernest Marsden (1889-1970), performed experiments in which positively charged radiation was directed toward a thin sheet of gold foil. Most passed straight through, but few alpha particles were deflected through large angles and some came almost straight back. The experimental set up is given below:

 Rutherford's Gold Foil Experiment
Rutherford's experiments with alpha particles led him to consider using them in experiments on the structure of the atom. By seeing the experiment, we can raise the questions what allowed most of the alpha particles to pass through the gold foil in a rather straight path? What caused a few alpha particles to be deflected? The conclusions of this gold foil experiment is described in next section. 

Conclusion

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The conclusions of Rutherford gold foil experiment are:

  • According to Rutherford's interpretation, the atom is mostly empty space and therefore offers little resistance to the alpha particle. Hence,most of the alpha particles pass through the foil on a rather straight path.

  • According to Rutherford's model of the atom, all of the positive charge and most of the mass of the atom must be concentrated in a very small volume at the center of the atom. He named this part of the atom, which contained most of the mass of the atom and all of the positive charge, the nucleus. When an alpha particle passing near the nucleus, because of the positive charge it gets deflected. If the alpha particle is exactly incident on the nucleus,it bounce back, since it does not have sufficient energy to penetrate the nucleus.

Atomic Model

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In 1906, Rutherford found that when a thin sheet of metal foil is bombarded with alpha particles, most of the particles penetrate the metal and suffer only small deflections from their original flight path. In 1909, at Rutherford's suggestion, Hans Geiger and Ernest Marsden performed an experiment to see if any alpha particles were deflected at a large angle on striking a gold foil. They discovered that some of the alpha particles actually were deflected by as much as 90° and a few by even larger angles. They concluded:

If the high velocity and mass of the alpha particle be taken in to account, it seems surprising that some of the alpha particles, as the experiment shows, can be turned within a layer of 6 ×10-5 cm of gold through an angle of 90° and even more. To produce a similar effect of a magnetic field, the enormous field of 109 absolute units would be required.

Rutherford quickly provided an explanation for this startling experimental result. He suggested that atoms consists of a positively charged nucleus surrounded by a system of electrons and that the atom is held together by electrostatic forces. The effective volume of the nucleus is extremely small compared with the effective volume of the atom, and almost all of the mass of the atom is concentrated in the nucleus. Rutherford reasoned that most of the alpha particles passed through the metal foil because the metal atom is mainly empty space and that occasionally a particle passed close to the positively charged nucleus, thereby being severely deflected because of the strong coulombic repulsive force.

Atomic Theory

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After conducting the gold foil experiment, Rutherford formulated the atomic theory. His theory is considered as the fundamental theory and further development is done by Bohr in this theory.

Main postulates are:
  • Most of the space inside an atom is empty
  • All the mass is concentrated at the center termed as nucleus
  • Nucleus has positive charge and very small compared to the size of an atom
  • The negatively charged particles(electrons) are revolve around the nucleus in different circular orbits