Metallic bonding is the electromagnetic interaction between
delocalized electrons, called
conduction electrons and gathered in an "electron sea", and the metallic nuclei within
metals. Understood as the sharing of "free" electrons among a
lattice of positively-charged ions (
cations), metallic bonding is sometimes compared with that of molten salts; however, this simplistic view holds true for very few metals. In a more
quantum-mechanical view, the conduction electrons divide their density equally over all atoms that function as neutral (non-charged) entities. Metallic bonding accounts for many
physical properties of metals, such as
strength,
malleability,
ductility,
thermal and
electrical conductivity,
opacity, and
luster.
[1][2][3][4] Although the term
metallic bond is often used in contrast to the term
covalent bond, it is more preferable to use the term
metallic bonding, because this type of bonding is
collective in nature and a single "metallic bond" does not exist. (
Note that covalent metal-metal bonds are well known for many metals and one example is the mercurous ion Hg2+2, metallic bonding is a different type of bonding found in bulk metals.)
The nature of metals has fascinated humankind for many centuries, because these materials provided people with tools of unsurpassed properties both in war and in peace. The reason for their properties and the nature of the bonding that keeps them together remained a mystery for centuries, even though great progress was made in their preparation and processing.
As chemistry developed into a science it became clear that metals formed the large majority of the periodic table of the elements and great progress was made in the description of the salts that can be formed in reactions with acids. With the advent of electrochemistry it became clear that metals generally go into solution as positively charged ions and the oxidation reactions of the metals became well understood in the electrochemical series. A picture emerged of metals as positive ions held together by an ocean of negative electrons.
With the advent of quantum mechanics this picture was given more formal interpretation in the form of the free electron model and its further extension, the nearly-free electron model. In both of these models the electrons are seen as a gas traveling through the lattice of the solid with an energy that is essentially isotropic in that it depends on the square of the magnitude, not the direction of the momentum vector k. In three-dimensional k-space, the set of points of the highest filled levels (the Fermi surface) should therefore be a sphere. In the nearly-free correction of the model, box-like Brillouin zones are added to k-space by the periodic potential experienced from the (ionic) lattice.
