Planetary atoms

Third, multimedia can corrupt. Because it can generate such impressive images, those images can deflect students from the truth. We have only to think of the compelling imagery of the Bohr planetary atom to appreciate just how deeply a false pictorial model can embed itself in our consciousness. One fear could be that multimedia could spawn a plague of Bohr models. 

Bohr s planetary atomic model proved to be a tremendous success. By utilizing Planck s quantum hypothesis, Bohr s model solved the mystery of atomic spectra. Despite its successes, though, Bohr s model was limited because it did not explain why energy levels in an atom are quantized. Bohr himself was quick to point out that his model was to be interpreted only as a crude beginning, and the picture of electrons whirling about the nucleus like planets about the sun was not to be taken literally (a warning to which popularizers of science paid no heed). 

Synthesis of the three observations led to Bohr s proposal of a planetary atom consisting of a heavy small stationary heavy nucleus and a number of orbiting electrons. Each electron, like a planet, had its own stable orbit centred at the atomic nucleus. The simplest atom, that of hydrogen, with atomic number 1 could therefore be described as a single electron orbiting a proton at a fixed, relatively large, distance. The mechanical requirement to stabilize the orbit is a balance between electrostatic and mechanical forces, expressed in simple electrostatic units, and particle momentum p = mv, as ... 

Burgers, A. and Wintgen, D. (1994). Inhibited autoionization of planetary atom states, J. Phys. B27, L131-L135. 

Richter, K. and Wintgen, D. (1990a). Stable planetary atom configurations, Phys. Rev. Lett. 65, 1965. 

Rutherford s planetary model of the atom assumes that an atom of atomic number Z comprises a dense, central nucleus of positive charge +Ze surrounded by a total of Z electrons moving around the nucleus. The attractive forces between each electron and the nucleus, and the repulsive forces between the electrons, are described by Coulomb s law. We first discuss Coulomb s law in general terms, and then apply it to the planetary atom. 

Let s apply these insights to the planetary atom. Associated with each electron (of charge —e) and the nnclens (of charge +Ze) there is potential energy ... 

I he atom is the most fundamental concept in the science of chemistry. A chem- I ical reaction occurs by regrouping a set of atoms initially found in those molecules called reactants to form those molecules called products. Atoms are neither created nor destroyed in chemical reactions. Chemical bonds between atoms in the reactants are broken, and new bonds are formed between atoms in the products. We have traced the concept of the atom from the suppositions of the Greek philosophers to the physics experiments of Thomson and Rutherford and we have arrived at the planetary model of the atom. We have used the Coulomb force and potential energy laws describing the interactions among the nucleus and the electrons in the planetary atom to account for the gain and loss of electrons by atoms,... 

Quantum mechanics explains the physical stability of the planetary atom, predicts its allowed energy levels, and defines the wave functions (also called atomic orbitals), which determine the probability density for finding the electrons at particular locations in the atom. 

The dualistic theory of chemical combination proposed by Davy and Berzelius, although it is not as simply and widely applicable as they had hoped, explains quite successfully in a qualitative way the formation of chemical compounds by atomic species from opposite sides of the periodic table. At the turn of the century, even before Ernest Rutherford developed the picture of the planetary atom, J. J. Thomson had suggested that the electrons are arranged in groups or layers in an atom, and that the number of electrons in the outermost layer largely determines the chemical properties of the species. 

Fig. 5.27 Double excited planetary atoms (a) level diagram for two-step excitation of two electrons in the Ba atom with subsequent autoionization to Ba (b) experimental arrangement [574]...

In 1911, following some experiments by his students Hans Geiger and Ernest Marsden, Rutherford revived the notion of a planetary atom in which electrons were believed to circulate around a central nucleus. As discussed in chapter 7, Jean Perrin and, in a somewhat different version, Hantaro Nagaoka were the first to propose such atomic modek. But the nuclear atom had since been echpsed by the work ofThomson, which had suggested that the electrons were embedded in the main body of the atom. 

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