Metallic bond

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A metallic bond or metal bond is the chemical bond that is present in metals and alloys . This is characterized by the appearance of freely movable (delocalized) electrons in the metal lattice, which are responsible, among other things, for the macroscopic properties of electrical conductivity , metallic luster and ductility ( forgeability or deformability ). It is caused by the electrostatic attraction between metal ions and free electrons .

The aforementioned metallic properties arise only through this bond; They do not have individual atoms of these elements. Since gloss and ductility also occur in non-metallic materials, the necessary condition for the presence of a metallic conductor in a material (metal, alloy or intermetallic phase ) is the negative temperature coefficient of electrical conductivity (poorer conductivity when the temperature increases).

Emergence

Metal grid: positive atomic cores surrounded by freely movable electrons

The formation of the metallic bond can be illustrated according to the simple electron gas model as follows: outer electrons ( valence electrons ) of the metals, which are located on the outermost shell, are only weakly bound and can therefore be easily separated from the atom. A lattice of (periodically arranged) positively charged metal ions, the so-called atomic cores , which each carry the core charge , is therefore formed in the metal . The external electrons released are no longer assigned to a single atom and can move almost freely within the lattice. One speaks of an electron gas or an electron gas cloud. The electron gas can be described as Fermigas (after the Italian physicist Enrico Fermi ). As a result of the electrostatic attraction between the atomic bodies and the electron gas, an undirected bond is obtained between the atomic bodies and the electrons. The delocalized electrons bring about the good electrical conductivity and high thermal conductivity of the metals, which decreases with increasing temperature. The reason for this is the lattice vibrations ( phonons ) that increase with temperature, on which the charge carriers scatter more and more with increasing temperature. However, the simple electron gas model cannot make any quantitative statements about electrical conductivity, for example.

The quantum mechanical band model comes from molecular orbital theory and is accordingly more complex. However, it provides reliable quantitative information on electrical conductivity, thermal conductivity, the photoelectric effect and many other measurable phenomena.

In both models, the positively charged atomic cores form a so-called metal lattice (metal crystal), in which they are periodically arranged analogous to the ion lattice . Different types of grids are possible. A face-centered cubic lattice, also known as the cubic closest packing, occurs with the alkali and alkaline earth metals, aluminum as well as the platinum metals and the coin metals copper , silver and gold . The high ductility (deformability) of the metals is characteristic of this grid . The alternative hexagonal face-centered lattice, also called hexagonal closest packing of spheres, may occur. a. the rather brittle metals magnesium , titanium , cobalt and zinc . For transition metals, due to the directional bond between the d orbitals, the body-centered cubic lattice is also stable; the most important representatives of these very brittle and hard metals are tungsten , chromium and iron (pure cast iron is brittle). Other types of grids are rather rare with unalloyed metals.

Occur

A pure metallic bond occurs with metallic elements ( metals ) and alloys (mixtures of metals). Different types of metallic solids have mixed forms of chemical bonds. Some salts show e.g. B. a transition form between ionic and metallic bonds, because they have a metallic sheen, but are colored, or conduct electricity well only in one direction ( one-dimensional metals ). However, it has also been possible to produce plastics ( polymers that are normally characterized by covalent bonds ) with metallic properties. Here, delocalized electrons were "built into" the substance through the targeted insertion of double bonds. At very high pressure even the normally gaseous substance and electrical insulator hydrogen (H 2 ) can assume metallic properties. It is assumed that this metallic hydrogen occurs, for example, in the “gas giant planets” Jupiter and Saturn.

This makes it clear that the terms covalent bond, ionic bond and metallic bond are easier to understand, but molecular orbital theory can provide better descriptions and predictions for intermediate areas .

Individual evidence

  1. ↑ Hans P. Latscha, Helmut A. Klein: Inorganic Chemistry Chemistry Basics I . Springer-Verlag, 2013, ISBN 978-3-662-05762-9 , pp. 98 ( limited preview in Google Book search).
  2. ^ A b D. F. Shriver, PW Atkins, CH Langford: Inorganic Chemistry . Ed .: J. Heck, W. Kaim, M. Weidenbruch. 2nd Edition. WILEY-VCH, Weinheim 1997, ISBN 3-527-29250-0 , pp. 85-94 .
  3. ↑ a b c E. Riedel, C. Janiak: Inorganic Chemistry . 8th edition. Walter de Gruyter, Berlin / New York 2011, ISBN 978-3-11-022566-2 , pp. 177-187 .
  4. ^ AF Holleman, E. and N. Wiberg: Textbook of Inorganic Chemistry . 102nd edition. Verlag Walter de Gruyter, Berlin / New York 2007, ISBN 978-3-11-017770-1 , p. 113-120 .
  5. ^ AF Holleman, E. and N. Wiberg: Textbook of Inorganic Chemistry . 102nd edition. Walter de Gruyter, Berlin / New York 2007, p. 143-147 .