In the process of forming a chemical bond, electron densities that originally belonged to different atoms are redistributed in space. The interaction with the core of electrons of the external level is less stable. It is they who play the main role in the formation of chemical bonds. The electrons that take part in this process are called valence. And the chemical bond itself is valency. For p- and s-elements, external electrons are valence, for d-elements - the last (external) s- and penultimate d-electrons. Atoms are less or more stable. The latter include those whose external level contains the maximum number of electrons (two and eight). This level is considered complete. Completed levels are more durable. They are inherent in the atoms of noble gases, and therefore, under ordinary conditions, their state is an inert monatomic gas.
Atoms of other elements are characterized by incomplete levels. In the course of the chemical reaction , the completion of the external value levels occurs. These are achieved either by recoil, or by the addition of electrons. The completion of levels can be carried out by the formation of common pairs. These methods contribute to the formation of two main types of bonds: ionic and covalent. In the formation of a molecule, in this way, the atoms try to acquire a stable electronic shell (external): either eight-electron or two-electron. This pattern underlies the theory of the formation of chemical bonds. The formation of valency by completing the external level in the corresponding atoms is accompanied by the release of a significant amount of energy. The formation of a chemical bond, in other words, always proceeds exothermically, due to the fact that molecules (new particles) appear that are more stable under ordinary conditions.
Electronegativity is one of the significant indicators determining the nature of the connection. This is the ability of an atom to attract the electrons of other atoms. The change in electronegativity occurs gradually. So, in the periods of the system the value of this indicator increases from left to right. The decrease in value occurs in groups from top to bottom. Covalent compounds include formations due to the formation of binding (common) electron pairs.
In a liquid, solid state in metals there is a metallic chemical bond. According to the situation in the periodic system, the atoms of the elements in question have a small amount of valence electrons (from one to three). In addition, they also have low energy of "electron detachment" (ionization). In this regard, the metal bond is a rather weak connection. The electrons in the atom are held weakly, come off quite easily, while having the ability to move around the entire crystal. A metal bond implies the presence of free atoms. Part of the valence electrons, endowed with the ability to move freely within the crystal lattice, form the "electron gas". With its help, a metal bond is formed. It arises due to the "socialization" of valence electrons by atoms. It should be noted that the metal bond has its own characteristics. So, the electrons forming it have the ability to move throughout the entire volume of the element. In this regard, metals acquire a number of characteristic features. In particular, they include gloss, good electrical conductivity, thermal conductivity, ductility, ductility and others. The metal bond provides a relatively high reduction ability in the elements.