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Contributions to Atomic Theory

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Shen Yueyue

on 17 August 2017

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Transcript of Contributions to Atomic Theory

1800
1900
2000
500 B.C.
Contributions to Atomic Theory
J. J. Thomson's Experiment (1897)
In one of Thomson's experiments with cathode ray tubes, he discovered that cathode rays could not travel around solid objects placed in the path of the electric charge but could be focused like a beam of light.

The tubes are made of glass and contain wires at both ends that can be electrified. The wire allows an electric charge to pass from one end of the tube to the other, as if an electric circuit allowing charge to flow.
J. J. Thomson's Contribution (1897)
Thomson discovered the electron. He demonstrated that cathode rays were negatively charged. He also studied positively charged particles in neon gas. Thomson realized that the accepted model of an atom did not account for negatively or positively charged particles. Thus, he proposed a model of the atom called the "Plum Pudding Model." The raisins in the pudding represented the negative electrons and the dough contained the positive charge. Thomson's model of the atom explained some of the electrical properties of the atom due to the electrons, but failed to recognize the positive charges in the atom as particles.
Millikan's Experiment (1909)
Millikan puts a charge on a tiny drop of oil, and measures how strong an applied electric field has to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must have. By varying the charge on different drops, he noticed that the charge was always a multiple of -1.6 x 10^-19 C, the charge on a single electron. This meant that it was the electrons carrying this unit charge.
Millikan's Contribution (1911)
The experiment in 1909 determined the size of the charge on an electron, which is -1.6 x 10^-19 C. He also determined that there was a smallest 'unit' charge, or that charge is 'quantized'. He received the Nobel Prize for his work.
Rutherford's Experiment (1911)
Rutherford observed the scattering of alpha-particles (helium nuclei) from a thin gold foil. As he had assumed a relatively uniform mass distribution throughout the gold foil, he was surprised to observe that most of the alpha-particles passed through the foil essentially undeflected, as if through an empty space. Occasionally, an alpha-particle would be scattered strongly, as if it had collided with a dense concentration of mass. He, thus, concluded that most of the mass of the gold atoms was concentrated in a tiny, dense kernel that he called the nucleus.
Rutherford's Contribution (1911)
Rutherford estimated the "diameter'' of an atom to be approximately 10^-8 cm and that of the nucleus to be approximately 10^-13 cm. He proposed a model of the atom as consisting of a small, dense nucleus surrounded by enough electrons to yield an overall charge-neutral aggregate. That is, if the nuclear charge is +Ze, then the atom must possess Z electrons. The model later introduced by Bohr, and even the current quantum mechanical picture of the atom, were built upon Rutherford's original model.
Bibliography
(n.d.). Retrieved August 15, 2017, from http://study.com/academy/lesson/cathode-ray-experiment-summary-explanation.html
(n.d.). Retrieved August 15, 2017, from http://www.abcte.org/files/previews/chemistry/s1_p5.html
(n.d.). Retrieved August 16, 2017, from http://ffden-2.phys.uaf.edu/212_fall2003.web.dir/Ryan_McAllister/Slide3.htm
(n.d.). Retrieved August 16, 2017, from https://www.nyu.edu/classes/tuckerman/adv.chem/lectures/lecture_3/node2.html
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