Relativistic effect

from Wikipedia, the free encyclopedia

As a relativistic effect in is physics a phenomenon called then, if it is not already in the classical physics , but only by the theory of relativity can be adequately described.

In a more specific sense, the term is used in physical chemistry for the properties of heavy elements that can only be explained through the application of relativistic quantum mechanics . This article is limited to this.

Relativistic effects in the atom

With higher atomic number rises electrostatic attractive force of the nucleus on the electrons. The electrons of the inner shells reach speeds close to the speed of light (e.g. in the Oganesson up to 86% of the speed of light). As a result, the non-relativistic formula for the kinetic energy is no longer valid. The application of the special theory of relativity leads to a contraction of the s orbitals (and some p orbitals). As a result, the electrons shield the nuclear charge better and the energy levels of the other orbitals are raised.

A prime example of this is the striking color difference between silver and gold . But the liquid aggregate state of mercury can also be explained by the relativistic effect.

One consequence of the relativistic effect is that the assignment of artificial chemical elements (with a high atomic number) to the groups of the periodic table becomes uncertain. For example, it was discussed whether Copernicium possesses noble gas properties.

The relativistic effect also explains the “inert electron pair effect ” ( inert pair effect ), i. H. it explains why the outermost electron pair in the valence s orbital is apparently inert .

Theoretical consideration

Mathematically, one has to replace the non-relativistic Hamilton operator with a relativistic one. With atoms, this works relatively well with the Dirac equation instead of the Schrödinger equation . For the lighter elements, terms such as the Breit correction for the electron-electron interaction and the quantum electrodynamics (QED) of vacuum polarization and vacuum fluctuation predominate . From about the ordinal number 50, the latter term no longer plays a role, since the vacuum polarization and the vacuum fluctuation assume almost the same values. Within a group of the periodic table, the term for relativistic effects increases and in the 6th period reaches a size that can no longer be neglected. Therefore, they must be taken into account for elements from cesium (ordinal number 55).

With the elements of the 5th period of the periodic table, the lanthanide contraction plays a crucial role in describing the behavior. According to this, however, the s and d energy levels of silver and gold should be about the same. However, a contraction of the 6s and an expansion of the 5d level is observed in gold. In the case of Copernicium (atomic number 112) this effect is even more pronounced; possibly the difference in level between the 6d and 7p electrons is so great that Copernicium has the character of a noble gas.

Further examples

In the non-relativistic case, the 5d and 6s energy levels of silver and gold would be similar. However, due to the relativistic effect, the 6s levels are contracted and the 5d levels are expanded. An energy difference is created which corresponds to the wavelength of blue light (blue light is absorbed , what remains is the familiar golden yellow color). At the same time, the bond lengths in gold compounds are shortened (by approx. 20 pm for the gold dimer ). In the case of the element Roentgenium , this effect is probably even more pronounced.

The tendency of heavy elements to form oxides does not follow the expected properties. Thus, PbO most stable oxygen compound of the lead , whereas silicon , germanium and tin stable Dioxide the form Me O 2 form. According to theoretical estimates, a large part of the electrical voltage applied to the poles of the lead-acid battery is also due to relativity. Likewise, no stable bismuth (V) oxide is known, but phosphorus , arsenic and antimony are known.

literature

  • Pekka Pyykkö : Relativistic theory of atoms and molecules. A bibliography 1916-1985, Lecture Notes in Chemistry, No. 41, 389 p. Springer-Verlag, Berlin-Heidelberg-New York (1986). ISBN 3-540-17167-3 .
  • Pekka Pyykkö: Relativistic Effects in Chemistry: More Common Than You Thought . In: Annual Review of Physical Chemistry, Vol. 63: 45-64. doi : 10.1146 / annurev-physchem-032511-143755

Web links

Individual evidence

  1. Cracks in the Periodic Table, Eric Scerri Cracks in the Periodic Table, Eric Scerri , Spectrum of Science Issue 8/14, page 78 ff.