Thomson's atomic model
The Thomson atomic model is a model of the atom , to which the atom of evenly distributed, positively charged mass is located, in which the negatively charged electrons move. This model was developed by Joseph John Thomson in 1903 . Due to the assumed arrangement of the electrons in the mass, comparable to raisins in a cake, it is also known as the plum pudding or raisin cake model. In the ground state, the electrons are distributed in such a way that their potential energy is minimal. When excited, they begin to vibrate.
Emergence
In 1897 Joseph John Thomson was able to prove that cathode rays consist of charged particles, the electrons. A greatly improved vacuum enabled him to determine the charge-to- mass ratio . Thomson assumed that the electrons were already present in the atoms of the cathode , and in 1903 he set up the first model of the atom that assigned an internal structure to the atoms.
construction
In Thomson's original model, the electrons alone were responsible for the mass of the respective atom. So there had to be more than 1,800 electrons in a hydrogen atom. On the other hand, he assumed the equally large amount of positive charge also present to be massless. The positive charge filled the volume of the atom and was - except for electrons - impenetrable. Although Thomson knew that an atom constructed in this way could not be stable, he was able to quantitatively explain the normal Zeeman effect .
Problems and refinement
Experiments with X-rays showed Thomson from 1906 onwards that the number of electrons had to be significantly lower than he had predicted. Their number in an atom could only be roughly equal to the mass number of the atom.
The one electron in the hydrogen atom , when excited, emits harmonic oscillations through the center of the atom and thereby emits light . In contrast to the experimental observation, however, this only allows one spectral line .
The Rutherford scattering experiment (1909), carried out by Hans Geiger , Ernest Marsden and Ernest Rutherford , showed that the positive charge is united in an atomic nucleus and makes up the majority of the atomic mass. With the help of this knowledge, the Thomson model was refined in 1911 to become the Rutherford atomic model .
literature
- Joseph John Thomson: On the Structure of the Atom: an Investigation of the Stability and Periods of Oscillation of a number of Corpuscles arranged at equal intervals around the Circumference of a Circle; with Application of the Results to the Theory of Atomic Structure . In: Philosophical Magazine Series 6 . 7, No. 39, March 1904, pp. 237-265. Retrieved August 30, 2017.
- Hans Geiger, Ernest Marsden: On a Diffuse Reflection of the α-Particles Archived from the original on April 24, 2008. In: Proceedings of the Royal Society . 82A, 1909, pp. 495-500. Retrieved September 30, 2008.
- Ernest Rutherford: The Scattering of α and β Particles by Matter and the Structure of the Atom Archived from the original on March 17, 2004. In: Philosophical Magazine (Series 6) . May 21, 1911, pp. 669-688. Retrieved September 30, 2008.
- Jörn Bleck-Neuhaus: Elementary Particles . From the atoms to the standard model to the Higgs boson. 2nd, revised edition. Springer, 2013, ISBN 978-3-642-32578-6 , ISSN 0937-7433 , p. 16 , doi : 10.1007 / 978-3-642-32579-3 .