THOMSONS ATOMIC MODEL

The atomic model in 1903. Thomson viewed the atom as a positively charged sphere emhedded with sufficient numbers of electrons to balance (neutralize) the total charge. 

Rutherford s atomic model solved problems inherent in Thomson s atomic model, but it also raised others. For example, an atomic nucleus composed entirely of positive charges should fly apart due to electrostatic forces of repulsion. Furthermore, Rutherford s nuclear atom could not adequately explain the total mass of an atom. The discovery of the neutron, in 1932, eventually helped to settle these questions. 

Students will demonstrate an understanding of the five basic atomic theories—the Dalton atom, the Thomson atom, the Rutherford atom, the Bohr atom, and the Schrodinger electron cloud model—and illustrate this understanding in a two-dimensional work of art. 

Dalton s atomic model does not include negatively charged (-) electrons and positively charged (+) protons. Thomson discovered the electron in 1897, while the discovery of the proton was made by Rutherford in 1919. We can summarize Thomson s ideas as follow ... 

One of the defects of Thomson s atomic model is that neutrons were not mentioned in any way. The arbitrary distribution of protons and electrons throughout the atom was also not true. 

Rutherford made important contributions to the explanation of atomic structure. He discovered the nucleus in 1911 and the proton in 1919. Prior to Rutherford, Thomson s atomic model was valid. His model stated that the atom was a sphere in which electrons and protons were moved arbitrarily. But there was an important question about how these protons and electrons were distributed. Was there any regularity or were they moving arbitrarily The answer to this question could not yet be seen. In order to get answers to these problems and to verify Thomson s atomic model, Rutherford proposed a model resulting from his a - particle experiment. 

What do the atomic models of Dalton, Thomson, Rutherford and Bohr look like ... 

Explain Thomson s atomic model. Which aspects of Thomson s model are deficient or false with respect to our modern understanding ... 

Rutherford described his new model of the atom during a lecture he gave in Cambridge in the fall of 1911. J. J. Thomson listened to the lecture, hut while the alpha-scattering data presented by Rutherford supported a nuclear model, Thomson did not. It may have heen that another physicist, Niels Bohr, also heard this... 

However, J. J. Thomson did not irrefutably establish the particulate nature of matter. It remained until 1909 for Jean Perrin to provide the definitive evidence for atoms, which he did by measuring the motion of microscopic pollen particles suspended in water. His detailed observations of this Brownian motion (named after the botanist Robert Brown) could be explained if it were assumed they were being buffeted about by moving atoms. His observations convinced the scientific community of the validity of the atomic model. Of course, they had been using the... 

J.J. Thomson s plum pudding atomic model proposed that negatively charged electrons were distributed throughout a uniform positive charge. 

Thinking Critically Compare and contrast Thomson s plum pudding atomic model with Rutherford s nuclear atomic model. 

You have learned about three different models of the atom Dalton s model, Thomson s model, and Rutherford s model. 

He pondered, in the midst of other work, for more than a year. He had a first quick intuition of what the experiment portended and then lost it. Even after he aimounced his spectacular conclusion he was reluctant to promote it. One reason for his reluctance might be that the discovery contradicted the atomic models J. J. Thomson and Lord Kelvin had postulated earlier. There were physical objections to his interpretation of Marsden s discovery that would require working out as well. 

It is important to appreciate that an electron oscillating in a sphere of positive electrification will execute a simple harmonic motion and emit radiation of definite frequency, giving a sharp spectrum line. Thomson investigated mathematically the stable distribution of electrons in rings in his atom model, and found that the results agreed with some experiments on floating magnets made by A. M. Mayer, which Thomson used to show in his lectures at the Royal Institution. 

Scientists such as Dalton, Thomson, and Rutherford proposed important models, which were ultimately challenged by later technology. What do we know to be false in Dalton s atomic model What was missing in Thomson s model of the atom What was Rutherford s experiment that led to the current model of the atom ... 

In the last 200 years, vast amounts of data have been accumulated to support atomic theory. When atoms were originally suggested by the early Greeks, no physical evidence existed to support their ideas. Early chemists did a variety of experiments, which culminated in Dalton s model of the atom. Because of the limitations of Dalton s model, modifications were proposed first by Thomson and then by Rutherford, which eventually led to our modern concept of the nuclear atom. These early models of the atom work reasonably well—in fact, we continue to use them to visualize a variety of chemical concepts. There remain questions that these models cannot answer, including an explanation of how atomic structure relates to the periodic table. In this chapter, we will present our modern model of the atom we will see how it varies from and improves upon the earlier atomic models. 

Although Thomson s atomic model would soon be discarded by Rutherford when he introduced his nuclear model of the atom, it did succeed in establishing two important concepts. One was that the electron held the key to chemical periodicity, and the other was the notion that the atoms of successive elements in the periodic table differ by the addition of a single electron. Both of these ideas were to become important aspects of Niels Bohr s atomic theory of periodicity, which would soon be published. 

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