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Chemical Principles 8th Edition

Chemical Principles (8th Edition)

Book Edition8th Edition
Author(s)DeCoste, Zumdahl
ISBN9781305581982
PublisherCengage Learning
SubjectChemistry
Chapter 2, Section 2.5, Critical Thinking, Exercise 01
Page 28

You have learned about three different models of the atom: Dalton's model, Thomson's model, and Rutherford's model. What if Dalton was correct? What would Rutherford have expected from his experiments with gold foil? What if Thomson was correct? What would Rutherford have expected from his experiments with gold foil?

Explanation

Thomson was a brilliant man, and though he believed, certainly, in atoms before he did his experiments showing the existence of the negatively charged corpuscles we now call electrons, his ideas of the atomic structure were somewhat vague before his multiple experiments on the cathode rays. He believed that atoms were made out of some sort of aethereal vortices. He believed in the general idea that chemists had, that atoms of different types were built out of atoms of one basic type - the hypothesis had been raised already that this atom was hydrogen. But as to the structure of hydrogen itself, he had the idea it resembled a nebula in space.

 

Thomson's model was correct at the time because it explained all that scientists then knew about the atom. The Japanese physicist Hantaro Nagaoka rejected Thomson's model. In 1911, Rutherford showed that Thomson's model was "wrong": the distribution of positive and negative particles was not uniform. The gold-foil experiment showed that the atom consists of a small, massive, positively charged nucleus with the negatively charged electrons being at a great distance from the center. Thomson's plum pudding model of the atom had negatively charged electrons embedded within a positively charged "soup." Rutherford's gold foil experiment showed that the atom is mostly empty space with a tiny, dense, positively charged nucleus.

Answer

What if Dalton was correct? What would Rutherford have expected from his experiments with gold foil? 

Alpha particles were quite massive and positively charged, and the plum pudding model assumed a uniform distribution of positive charge with the odd negative electron distributed therein. If that had been correct, the massive positive alpha particles should have simply passed straight through without much, if any, deflection. There would not have been enough concentrated mass to cause the alpha particles to bounce backward.

 

What if Thomson was correct? What would Rutherford have expected from his experiments with gold foil?

If Thomson's Plum Pudding model were correct, the alpha particles in the gold foil experiment would pass through the atomic structure of the foil unimpeded. Rutherford reasoned that if Thomson's model was correct then the mass of the atom was spread out throughout the atom. As expected, most alpha particles went right through the gold foil but to his amazement, a few alpha particles rebounded almost directly backward.

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