Rutherford model

The Rutherford model of the atom is an atom model, which was set up in 1911 by Ernest Rutherford. It forms the basis for today's picture of the atom, by introducing the atomic nucleus, which unites to be extremely small, positively charged sphere in the center of the atom almost its entire mass. In order for the Rutherford atomic model overcame the slightly older Thomson's atomic model and served in turn as the starting point of the Bohr atomic model of 1913, with the energy levels of the atomic shell could be successfully described for the first time.

Formation

Before the rutherford scattering experiment between (1909 ) by Hans Geiger, Ernest Marsden and Ernest Rutherford was known that atoms contain negatively charged electrons and the same amount of positive charge. An attempt to explain the structure of atoms introduced the Thomson model of the atom dar. Accordingly, there is the atom of uniformly distributed positive charges and electrons that move in it. The Rutherford scattering experiment showed that the positive charge and most of the mass are combined in an atomic nucleus and thus refuted this model of the atom.

The attempt

Rutherford's employee Hans Geiger and Ernest Marsden conducted an experiment with alpha particles. First, it was observed that light emitted from bismuth -214 alpha - particles were at least partially reflected back from a platinum plate as a reflector. It was then popular idea that can not be alpha particles of matter distract backwards. Was therefore unexpected that a few particles have been reflected by the metal layer.

In a more sophisticated test setup, the distribution of the deflection angle (ranging from 0 ° and 180 °) in vacuo, with thin films of various metals, including gold, examined. Again, it was found that about one was thrown back from 8000 alpha particles, as if it had encountered a massive center inside atoms. This massive center inside the atom described Rutherford as the nucleus.

" It was almost as incredible as if you shoot with a 15 -inch shell at a piece of tissue paper and the shell comes back and hits himself. "

The model

Using the Rutherford scattering formula named after him ran from the experiments from values ​​for the size and charge of the positive charge distribution of gold and other elements. He came to the conclusion that the enormous electric field strength that is required for the measured strong deflection of alpha particles can only be explained by assuming that the positive charge of the atom is concentrated entirely in a compact atomic nucleus whose radius around a factor of about 3000 is smaller than the atomic radius.

To explain the electrical neutrality of atoms Rutherford assumed that the nucleus is surrounded by electrons, the total number of electrons per atom corresponds exactly to the atomic number. However, no information could be derived from the experiments on the spatial distribution of the electrons, because the electrons do not contribute due to their small mass to any detectable extent to deflect the alpha rays. Contrary to often -to-find illustrations in textbooks and other secondary sources Rutherford did not develop its own model of the electronic structure of atoms, he only quoted in February 1911 at a lecture to the Manchester Literary and Philosophical Society, which was based on an in May, published in the Philosophical Magazine articles, Nagaoka's " planetary model " to check the plausibility of his estimate of the atomic number of gold.

Problems

A problem exists in the Declaration of emission and absorption of energy quanta: With the model of Rutherford no explanation for the so-called spectral lines of various gases can be given. It is therefore superseded by the Bohr atomic model, which assigns the electrons of different energy levels.

Another problem that could not be solved by the Bohr atomic model, is that the model does not explain why not overthrow the electrons in the core, although circular and thus accelerated charge radiates energy according to Maxwell constantly. Furthermore, could emit a slow stürzendes in the core electron photon with any frequency, what is impossible but because of the quantized energy levels after.