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Gold Foil Experiment
Earnest Rutheford, 1905 photo in his lab.

Rutherford's gold foil experiment was a major milestone in the determination of the size and proportions of the different constituents of the atom. It was successful in determining that the majority of the mass of an atom lies within the nucleus, just as the majority of the mass of a planetary system lies within the stellar system of that planetary system.

Rutherford measured the amount and frequency of deflection of alpha particles as they were shot through a very thin gold foil. This helped to determine that the nucleus of the atom is contained to a very small portion of the atom, at it's center. The deflection of the alpha particles was incorrectly reasoned to be due to the like charges of the alpha particle and the nucleus of the gold atoms. This incorrect reasoning was no fault of Rutherford, however, as by that time in history, the concept of electric charges within the atom had already been incorrectly established as the reason for attraction and repulsion of the atomic constituents.