Geiger-Marsden experiment
The Geiger-Marsden experiment (also called the Gold foil experiment or the Rutherford experiment) was an experiment done by Hans Geiger and Ernest Marsden in 1909,
A beam of alpha particles, generated by the radioactive decay of radium, was directed normally onto a sheet of very thin gold foil. The gold foil was surrounded by a circular sheet of zinc sulfide (ZnS) which was used as an indicator. The ZnS sheet would light up when hit with alpha particles. Under the prevailing plum pudding model, the alpha particles should all have been deflected by, at most, a few degrees. However they observed that a very small percentage of particles were deflected through angles much larger than 90 degrees. From this observation Rutherford was able to observe and deduct the central structure of the atom:
It was quite the most incredible event that has ever happened to me in my life. It was almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you. On consideration, I realized that this scattering backward must be the result of a single collision, and when I made calculations I saw that it was impossible to get anything of that order of magnitude unless you took a system in which the greater part of the mass of the atom was concentrated in a minute nucleus. It was then that I had the idea of an atom with a minute massive centre, carrying a charge.
David C. Cassidy, Gerald James Holton, Gerald Holton, Floyd James Rutherford, (2002)
Understanding Physics
Harvard Project Physics
Published by Birkhäuser, p. 632
ISBN 0387987568, ISBN 9780387987569
Rutherford concluded that the atom contained a very physically-small (as compared with the size of the atom) charge which could repel the alpha particles if they came close enough. The electrons were so low in mass that they did not affect the alpha particles, although it was impossible for Rutherford to know that electrons existed as he had no physical way of viewing them. All he did know that there had to be some kind of negative charge within the atom's structure to neutralise the overall positive charge in the nucleus. What he was able to conclude was that most of the mass of the atom is located at the nucleus, a stated supported by his above quote "firing a 15-inch shell...hit you" These conclusions were subsequently developed into the Bohr model.
See also
 List of famous experiments
1909 in science
References
External Links
Images of Rutherford's 1911 paper
Description of the experiment, from the New Mexico Institute of Mining and Technology
Short biography of Ernest Rutherford
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