Metal bond

At the nodes of the crystal lattice positive metal ions are located. Conduction electrons originating from metal atoms during the formation of ions move between them randomly, like gas molecules. These electrons play the role of "cement", holding together the positive ions; otherwise, the lattice would decay due to the repulsive forces between the ions. At the same time, electrons are also trapped by ions within the crystal lattice and cannot leave it. The communication forces are not localized and not directed. In metals, in most cases, high coordination numbers appear (for example, 12 or 8).

Thus, alkali metals crystallize in a cubic body-centered lattice, and each positively charged alkali metal ion has eight nearest neighbors in the crystal - positively charged alkali metal ions (Fig. 1). The Coulomb repulsion of like-charged particles (ions) is compensated by the electrostatic attraction of electrons to the bonds in the form of a distorted flattened octahedron - a square bipyramid whose height and edges of the base are equal to the lattice constant a _{w of an} alkali metal crystal (Fig. 2).

Binding electrons become common to a system of six positive alkali metal ions and keep the latter from Coulomb repulsion.

The value of the constant translational lattice a _{w of an} alkali metal crystal significantly exceeds the length of the covalent bond of the alkali metal molecule , therefore it is generally accepted that the electrons in the metal are in a free state

The mathematical construction associated with the properties of free electrons in a metal is usually identified with the “ Fermi surface ”, which should be considered as a geometrical place in the k- space where the electrons are located, providing the main property of the metal to conduct electric current ^[3] . Thus, the electric current in metals is the movement of electrons stripped from the orbital radius in the field of positively charged ions located in the nodes of the crystal lattice of the metal. The exit and entry of free electrons into the connecting link of the crystal is carried out through the points “0” equidistant from the positive ions of atoms (Fig. 2).

The free movement of electrons in a metal was confirmed in 1916 by the experiments of Tolman and Stuart on the sharp braking of a rapidly rotating coil with a wire - free electrons continued to move by inertia, as a result of which the galvanometer recorded an electric current pulse. The free movement of electrons in a metal determines the high thermal conductivity of the metal and the tendency of metals to thermionic emission occurring at a moderate temperature.

The oscillation of the ions of the crystal lattice creates resistance to the movement of electrons through the metal, accompanied by heating of the metal. Currently, the most important sign of metals is considered to be the positive temperature coefficient of electrical conductivity , that is, a decrease in conductivity with increasing temperature. With decreasing temperature, the electrical resistance of metals decreases, due to a decrease in ion vibrations in the crystal lattice. In the process of studying the properties of matter at low temperatures, Kamerlingh Onnes discovers the phenomenon of superconductivity . In 1911, he manages to detect a decrease in the electrical resistance of mercury at the boiling point of liquid helium (4.2 K) to zero. In 1913, Kamerlingh Onnes was awarded the Nobel Prize in Physics with the following wording: "For the study of the properties of substances at low temperatures, which led to the production of liquid helium ."

However, the theory of superconductivity was created later. It is based on the concept of a Cooper electron pair - a correlated state of binding electrons with opposite spins and momenta, and, therefore, superconductivity can be considered as superfluidity of an electron gas consisting of Cooper pairs of electrons through an ionic crystal lattice. In 1972, the authors of the BCS theory — Bardin , Cooper, and Shriffer — were awarded the Nobel Prize in Physics “For the creation of the theory of superconductivity, usually called the BCS theory”.

Most metals form one of the following highly symmetric lattices with a dense packing of atoms: cubic volume-centered , cubic face-centered and hexagonal .

In a cubic body-centered lattice (BCC), the atoms are located at the vertices of the cube and one atom in the center of the volume of the cube. The cubic body-centered lattice has metals: Pb , K , Na , Li , β-Ti , β-Zr , Ta , W , V , α-Fe , Cr , Nb , Ba , etc.

In a cubic face-centered lattice (fcc), the atoms are located at the vertices of the cube and in the center of each face. Metals of this type have metals: α-Ca , Ce , α-Sr , Pb , Ni , Ag , Au , Pd , Pt , Rh , γ-Fe , Cu , α-Co , etc.

In the hexagonal lattice, atoms are located at the vertices and center of the hexagonal bases of the prism, and three atoms are located in the middle plane of the prism. Such a packing of atoms has metals: Mg , α-Ti , Cd , Re , Os , Ru , Zn , β-Co , Be , β-Ca , etc.

Freely moving electrons cause high electrical and thermal conductivity . Many metals have high hardness, such as chromium , molybdenum , tantalum , tungsten , etc. Substances with a metal bond often combine strength with ductility, since when the atoms are displaced relative to each other, bond breaking does not occur.

• Covalent bond
• Ionic bond