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lecture_16 - 86 The Atomic Nucleus The electrolysis...

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86 The Atomic Nucleus The electrolysis experiments carried out by Michael Faraday demonstrated that there is an exact relationship between the masses of metals deposited at the cathode in an electrolysis experiment and the current flowing through the electrolysis cell. These experiments suggested that the carrier of electrical charge has a discrete value associated with that negative charge. Others who contributed to understanding the nature of negative charge: J. J. Thomson (1897) used “cathode rays” to determine the charge/mass ratio of the electron R. A. Millikan (1906) , in his “oil drop” experiment, measured the charge independently At the beginning of the 20 th century, it was recognized that matter is electrically neutral , and therefore, the amount of negative charge must be balanced by the amount of positive charge in an atom. The “Plum Pudding” model of the atom suggested that both kinds of charge were essentially uniformly mixed throughout the entire volume of an atom. Ernest Rutherford , at the Cavendish Laboratory in England, carried out a very important experiment in 1910 designed to probe the nature of positive charge in atoms. He did not expect the experiment to produce an important result, so he assigned one of his “average” students to the problem. The experiments used a collimated source of alpha particles (doubly-charged helium nuclei) impinging on a very thin gold foil. The first sketch shows how the experiment was done. The expectation was that because positive charge was uniformly distributed in atoms, the alpha particle would pass through without significant deflection. Trajectories A and B showed the expected behavior. The real surprise came from trajectories like “C”, in which a strong backward deflection was observed.
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87 It was possible to analyze the data to determine the actual “size” of the center of positive charge in the nucleus. Although the dimensions of an atom were known (from density measurements on AgCl, for
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lecture_16 - 86 The Atomic Nucleus The electrolysis...

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