Thomson model Atomic structure

It was apparent to Thomson that if atoms in the metal electrode contained negative particles (electrons), they must also contain positive charges because atoms are electrically neutral. Thomson proposed a model for the atom in which positive and negative particles were embedded in some sort of matrix. The model became known as the plum pudding model because it resembled plums embedded in a pudding. Somehow, an equal number of positive and negative particles were held in this material. Of course we now know that this is an incorrect view of the atom, but the model did account for several features of atomic structure. 

In 1910 Rutherford wrote to a friend, I think I can devise an atom much superior to J.J. s, for the explanation of and stoppage of alpha and beta particles, and at the same time I think it will fit in extraordinary well with the experimental numbers. Rather than devise a model of the atom based on theoretical ideas as Thomson had done, Rutherford intended to probe atomic structure by bombarding atoms with particles ejected from radioactive atoms. Rutherford felt that experimental physics was the only real physics and that by performing experiments he could gain greater insight into atomic structure than Thomson had been able to get using only theory. 

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. 

J. J. Thomson (1856-1940) performed extensive work in electrochemistry, atomic structure, and valency and his interest in chemistry led him to the construction of a series of models of the chemical atom.92... 

Ernest Rutherford studied atomic structure in 1910-1911 by firing a beam of alpha particles at thin layers of gold leaf. According to Thomson s model, the path of an alpha particle should be deflected only slightly if it struck an atom, but Rutherford observed some alpha particles bouncing almost backwards, suggesting that nearly all the mass of an atom is contained in a small positively charged nucleus. 

Thomson s "Plum Pudding" Model of Atomic Structure... 

Soon after Thomson developed his model, tremendous insight into atomic structure was provided by one of Thomson s former students, Ernest Rutherford (1871-1937), who was the outstanding experimental physicist of his time. 

Several major discoveries at the turn of the 20 century ied to our current model of atomic structure. Cathode rays were shown to consist of negative particles (electrons) that exist in ail matter. J. J. Thomson measured their mass/charge ratio and con-ciuded that they are much smalier and iighter than atoms. Robert Miliikan determined the charge of the electron, which he combined with other data to calculate its mass. Ernest Rutherford proposed that atoms consist of a tiny, massive, positive nucleus surrounded by electrons. 

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. 

In 1904 ].] Thomson (Nobel Prize in Physics in 1906) surveyed the experimental evidence suggesting that atoms consist of charged particles. He described the atom as a uniform (always the same) sphere of positive electricity and mass, in which negative electrons are embedded. This model was known as the plum pudding model of atomic structure (Figure 2.5). 

Thomson s model could explain the electrical neutrality of the atom, but could not be reconciled with the Rutherford scattering of alpha particles from thin metal foils. Rutherford s nuclear model was another paradigm shift ( scientific revolution ) in thinking about atomic structure and Thomson s model was abandoned. 

Atomic structure, (a) In 1895, P. Lenard detected electrons outside a thin glass tube in which they were produced, and concluded that the atoms in the glass must have a very open structure. Explain these results in terms of the Rutherford atom, (b) Describe the fundamental difiEierences among the Dalton, Thomson, Rudierfbrd, and Schrodinger models of the atom. 

Models are often used to describe the structure of atoms, the configuration of their subatomic particles, and their interactive behavior. Of the several models for atomic structure, including those of J.J. Thomson in 1907 and Lord Rutherford in 1911, the Bohr model, proposed in 1913, is used here to illustrate atomic principles.Two assumptions made by Bohr are critical to using his model (1) Stationary energy states exist such that an atom in one of these states is stable and the atom in this state is populated for a finite period of time and (2) the emission or absorption of radiation from an atom is exactly equal to the difference between two of the discreet energy states. 

One example that is often used in chemistry classrooms may illustrate this. In the core of learning about the nature of science is learning about scientific models. Among other characteristics it is important to understand that models in science are developed by scientists, these models are never fully true or false, and can be changed or replaced in the light of new evidence. Different historical models of atomic structure are a good example to reflect about the nature of models in chemistry education. Models of Democritus, Dalton, Thomson, Rutherford and Bohr can be compared in the chemistry classroom, e.g. in a drama play (see Chapter 7). Students can start reflecting about the predictive potential and limitations of the different models. But students can also learn about the time in which the models were developed and about the scientists behind them. Other examples are different models of oxidation and reduction or acid-base chemistry. 

The first steps toward the understanding of the nature of the chemical bond could not be taken until the composition and structure of atoms had been elucidated. The model of the atom that emerged from the early work of Thomson, Rutherford, Moseley, and Bohr was of... 

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