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Ernest Rutherford's Gold Foil Experiment

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Sue Youn Oh

on 27 September 2013

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Transcript of Ernest Rutherford's Gold Foil Experiment

Rutherford's Gold Foil Experiment
The Gold Foil Experiment
Ernest Rutherford
Scientific Background
So far, the most up-to-date and "accurate" model of the atom was J.J. Thompson's Plum Pudding Model.
Motivation for Experiment
Rutherford's main motivation for this experiment was simply his desire to discover or to learn more. He was academically ambitious; he wanted to study the effects of alpha particles on matter, but first, he had to find a way to count the individual particles. This led to the gold foil experiment.
Influence of Experiment
The nucleus of an atom was discovered! Through this experiment, Rutherford disproved J.J. Thompson's plum pudding model, and people could better understand the structure and function of atoms. Also, it motivated other scientists to study the atom more intricately.
Result Analysis
Sue Youn Oh
Experiment Set Up
Personal History
Setting of Experiment
Rutherford conducted his Gold Foil Experiment in 1911 at the University of Manchester, in the United Kingdom.
Ernest Rutherford was born at Spring Grove in Nelson, New Zealand on August 30, 1871, to James and Martha Rutherford. In 1887, he went to Nelson College on a scholarship, then to the University of New Zealand in 1889, also on a scholarship. From 1890 to 1894, Rutherford attended Canterbury College in Christchurch. He married Mary Georgina Newton in 1900.
John Dalton - Five Postulates, and Solid Sphere Model (1807): atom is a solid, uniform sphere
J.J. Thompson - Plum Pudding Model (1904): the atom was a large positively charged body that contained small, free-floating, negatively charged particles called electrons; the negative charge was equal to the positive charge, causing the atom to be neutral. Cathode Ray Tube Experiments: discovered electrons (1897)
Rutherford - discovered in 1898 two types of radioactive emissions: alpha, and beta (which were later (1908) identified as helium atoms stripped of their two electrons: He2+)
J. J. Thompson's Plum Pudding Model (left) and Cathode Ray Tube (below)
Rutherford hypothesized that every alpha particle would go directly through the foil, according to the plum pudding model.
His hypothesis was based on previous observation; J.J. Thompson's discovery of electrons, and the plum pudding model. Since nothing like his experiment had ever been done before, his hypothesis had to be theoretical.
This was the first time that this specific hypothesis was formulated.
Independent Variable- rate at which the alpha particles were shot out, and the location of the foil
Dependent Variable - number of electrons bounced back from the gold foil
This experiment did not have a control group.
In order to record the data, a zinc sulfide screen was placed behind the gold foil (though other foils were used as well) as a backdrop for the alpha (positive) particles to appear upon. A microscope was placed above this screen that would allow the experimenter (either Geiger or Marsden) to observe any contact between the alpha particles and the screen. Also, the experimenter that was making those observations had to sit in darkness for at least an hour before he could start observing, for better accuracy. (The whole experiment was done in complete darkness.)
Instruments used to collect data:
Zinc sulfide screen (to catch the fluorescence of alpha particles bouncing off)
Alpha particle source
Lead box
(Mainly) Gold foil
The diagram on the left shows the set up of the whole experiment, and the one on the right displays what is happening to the alpha particles on the atomic level. 1 in 20,000 alpha particles would deflect approximately 90 degrees or more from the parent beam.
If the rate at which the alpha particles were being fired was changed, or the location of the foil was different, the results would be the same (a few of the alpha particles would still be deflected), unless there was a greater amount of human error. In this experiment, the dependent variable would be the same for every independent variable.
Although Rutherford's original hypothesis, based on J.J. Thompson's observations, was right for most occasions, it was way off for some other trials, and was rejected. However, this led Rutherford to his proposal of the nucleus.
In 1911, Rutherford deduced that almost all of the mass of an atom is concentrated in a nucleus a thousand times smaller than the atom itself. He proposed the nuclear model of the model, replacing J.J. Thompson's plum pudding model.

There was a complete paradigm shift; the atom was no longer seen as a mass positively charged matter with a few scattered electrons; now the model of the atom was a small, dense, positively charged nucleus surrounded by mostly empty space, with electrons traveling around the nucleus.
The results of Rutherford's experiments are still valid, but now we know that the nucleus is also composed of neutrons, thanks to James Chadwick (1932), and not just protons. Also, Niels
Bohr came up with the idea of electron shells, which explains why chemicals react the way
they do, and where electrons would be in an atom.

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"The Gold Foil Experiment." The Gold Foil Experiment. University of
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