Rutherford's Gold Foil Experiment

History -

Before Rutherford, the most widely accepted theory about the distribution of charges and masses on any atom was the Plum pudding model. The plum pudding model explained that 'an atom is a large positively charged body that contained small,free-floating, negatively charged particles called electrons and the atom is neutral since the negative charge of the electrons cancels out with the positive charge of the protons'.

How did the plum pudding fail for later research?
Ernest Rutherford did similar experiments in which he sent alpha beams or alpha particles over a wide variety of different foils. E.g.: Aluminum, Iron, Lead, Gold etc.
However, Rutherford noticed that alpha particles would not behave in similar manner, as is expected according to the plum pudding model, when directed at a gold foil.

This led to further research and is popularly called the 'Gold Foil Experiment' or the 'Geiger-Marsden Experiment'.

Experiment -

A Zinc Sulphide screen (ZnS) was placed as a screen so that alpha particles emitted from the source would be seen. Then a microscope was kept directly above the screen to observe the collisions of alpha particles with the screen and where these collisions took place.

Observations-

According to the plum pudding model, all the alpha particles or beams should travel without getting deflected. However, there were 2 or 3 small deflections after colliding with the gold foil.
But the most interesting observation is that some ( about 1 in 20,000 particles) would completely reflect back at the source or close to the source.

This can be shown in a diagram as follows:

(Photo credit : jahschem.wikispaces.com)

Conclusion:
This experiment led to two important conclusions on the research on the atom:

Since most alpha beams traveled unreflected, most of the volume of an atom is composed of empty space
Since some of the alpha particles reflected back and since alpha particles are positive, the mass in the middle must be positively charged (as repulsion occurs between like charges).

These information can be summarized as follows:

(Photo credit: student-sc.blogspot.com)

(Photo credit: www.teachastronomy.com)

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