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Gold Foil Experiment

Who Did the Gold Foil Experiment?

The gold foil experiment was a pathbreaking work done by scientists Hans Geiger and Ernest Marsden under the supervision of Nobel laureate physicist Ernest Rutherford that led to the discovery of the proper structure of an atom. Also known as the Geiger-Marsden experiment, it was performed at the Physical Laboratories of the University of Manchester between the period 1908 and 1913.

Gold Foil Experiment

Gold Foil Experiment

History

The prevalent atomic theory at the time of the research was the plum pudding model that was developed by Lord Kelvin and further improved by J.J. Thomson. According to the theory, an atom was a positively charged sphere with the electrons embedded in it like plums in a Christmas pudding.

The Plum Pudding Model

The Plum Pudding Model

With neutrons and protons yet to be discovered, the theory was derived following the classical Newtonian Physics. However, in the absence of experimental proof, the theory lacked a proper acceptance by the scientific community.

What is the Gold Foil Experiment

Description

The scientists bombarded a thin gold foil of thickness approximately 8.6 x 10-6 cm with a beam of alpha particles in vacuum. They used gold since it is highly malleable, producing sheets that can be only a few atoms thick, thereby ensuring smooth passage of the alpha particles. A circular screen coated with zinc sulphide surrounded the foil. Since the positively charged alpha particles possess mass and move very fast, it was hypothesized that they would penetrate the thin gold foil and land themselves on the screen, producing fluorescence in the part they struck.

In line with the plum pudding model, since the positive charge of atoms was evenly distributed and too small as compared to that of the alpha particles, the deflection of the particulate matter, if any, was predicted to be less than a small fraction of a degree.

Observation

Though most of the alpha particles behaved as expected, there was a noticeable fraction of particles that got scattered by angles greater than 90 degrees. In fact, there were about 1 in every 2000 particles that got scattered by a full 180 degree, that is, they simply retraced their path after hitting the gold foil.

Conclusion

The unexpected outcome could have only one explanation – a highly concentrated positive charge at the center of an atom that caused an electrostatic repulsion of the particles strong enough to bounce them back to their source. The particles that got deflected by huge angles passed close to the said concentrated mass. Most of the particles passed undeviated as there was no obstruction to their path, proving that the majority of an atom is empty.

In addition to the above, Rutherford drew the conclusion that since the dense alpha particles could be deflected by the central core, it shows that almost the entire mass of the atom is concentrated there. Rutherford named it the “nucleus” after performing the experiment in various gases. He also used materials other than gold for the foil, though the gold foil version gained the most popularity.

He further went on to reject the plum pudding model and developed a new atomic structure called the planetary model in which a vastly empty atom holds a tiny nucleus at the center surrounded by a cloud of electrons. His version holds good even today.

Rutherford’s Atomic Model

Rutherford’s Atomic Model

Rutherford’s Gold Foil Experiment Animation

Rutherford, with the aid of this experiment, contributed immensely to the atomic theory by proposing his nuclear atomic model. The discovery of nucleus paved the way for further research, unraveling the hitherto unknown fundamental particles.

Article was last reviewed on Wednesday, December 6, 2017

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