I am having trouble understanding the Rutherford experiment. Please provide a de
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Question
I am having trouble understanding the Rutherford experiment. Please provide a description of the state of knowledge about atomic structure before the experiment (including the famous "plum pudding model"); describe the purpose of the experiment; describe how the experiment worked (in particular, explain why alpha particles were used as the bombarding particle, how they were obtained and how they were detected, and also explain why the target chosen was gold foil); describe the result that world be expected if the plum pudding model were correct; describe the actual results and clearly explain what these results implied about the structure of the atom; describe the logic behind the method that experimental physicists conduct experiments in which they accelerate particles to high speed and then allow them to collide with other particles in so-called "scattering experiments," and explain why we give credit to Rutherford for basically discovering this technique.I think that this is a really interesting experiment, and I want to understand as much as I can about it. Thanks!
Explanation / Answer
Solution in such a manner that each color indicates the soultion asked for
plum pudding model
Before the Rutherford experiment : the famous "plum pudding model"
Atomic Structure at that time ...
The plum pudding model of the atom by J. J. Thomson was proposed in 1904 before the discovery of the atomic nucleus in order to add the electron to the atomic model. In this model, the atom is composed of electrons (which Thomson still called "corpuscles") surrounded by a soup of positive charge to balance the electrons' negative charges, like negatively-charged "plums" surrounded by positively-charged "pudding". The electrons (as we know them today) were thought to be positioned throughout the atom, but with many structures possible for positioning multiple electrons, particularly rotating rings of electrons . Instead of a soup, the atom was also sometimes said to have had a "cloud" of positive charge.
knowledge about atomic structure before the experiment
This was done using the Cathode Ray Tube experiment (CRT) which yielded same results with all the metals which meant that every metal has the same basic elements (here electrons) which was against the Dalton model which suggested that each metal had a different composition.
So with the CRT expt. brought plum pudding model to light and hence Dalton's model of atom rejected.
purpose of the Rutherford experiment
Rutherford asked his assistants to do this experiment to verify the plum pudding model proposed by JJ thompson.
Alpha particles were used because they were the only available positive charge that could be produced at that time and the concept of like charges repel each other was known .
Alpha particles were produced from the polonium sample and detected on a Zinc Sulphide flourescent screen.
Target chosen was a gold foil because
2. The use of gold had nothing to do with its reactivity, any more than the experiment had any connection to chemistry. Rutherford wanted to fire alpha particles through the foil. Alpha particles are not very penetrating; a sheet of notepaper will stop most of them. To achieve a reasonable percentage of penetration the foil needed to be really thin.
Gold is highly malleable; gold leaf can be made in thicknesses around 100 nm or 4 micro-inch. It was the best material for the job.
description of Rutherford Experiment
The initial discovery was made by Hans Geiger and Ernest Marsden in 1909 when they performed the gold foil experiment under the direction of Rutherford, in which they fired a beam of alpha particles (helium nuclei) at layers of gold leaf only a few atoms thick. At the time of the experiment, the atom was thought to be analogous to a plum pudding (as proposed by J.J. Thomson), with the negative charges (the plums) found throughout a positive sphere (the pudding).
result that would be expected if the plum pudding model were correct
If the plum-pudding model were correct, the positive “pudding”, being more spread out than in the current model of a concentrated nucleus, would not be able to exert such large coulombic forces, and the alpha particles should only be deflected by small angles as they pass through.
Actaual results of the experiment
However, the intriguing results showed that around 1 in 8000 alpha particles were deflected by very large angles (over 90°), while the rest passed straight through with little or no deflection.
what the results implied
From this, Rutherford concluded that the majority of the mass was concentrated in a minute, positively charged region (the nucleus) surrounded by electrons. When a (positive) alpha particle approached sufficiently close to the nucleus, it was repelled strongly enough to rebound at high angles. The small size of the nucleus explained the small number of alpha particles that were repelled in this way.
The logic behind using this method by Experimental physicts is that scattering allows us to know information about atomic and molecular forces.
Since the idea was taken from Rutherford's sacttering experiment ; he is given the credit
Hope this clears Your doubt on
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