The formation of a chemical bond involves a process for the redistribution of electron densities that originally belonged to different atoms. Due to the fact that the electrons at the external level with the nucleus are least tightly bound, they play the main role in the formation of the compound. The number of bonds that is formed by an atom in a compound is called valency. Accordingly, the electrons participating in the formation of the bond are called valence electrons. In the energy plane, there are atoms of different stability. The most stable is the one with two or eight electrons at its external level (maximum number). This level will then be considered completed. Such completed levels are characteristic of atoms contained in noble gases. In this regard, under ordinary conditions, they are in a state of a monatomic gas, chemically inert.
Atoms of other elements are characterized by incomplete levels. In a chemical reaction, the completion of external order levels occurs. This occurs due to the recoil or attachment of electrons, as well as through the formation of common pairs. So, covalent and ionic bonds are formed. The properties of atoms in this case are manifested in their desire to acquire a stable electron shell of external significance. It will be either two-electron or eight-electron. This pattern is considered the basis of the theory of the occurrence of chemical bonds.
The formation of the compound is accompanied by the release of a sufficiently large amount of energy. In other words, bond formation is exothermic. This is due to the fact that new particles (molecules) appear, which under ordinary conditions are more stable.
Electronegativity is one of the main signs indicating the nature of the emerging relationship. This feature is manifested in the ability of atoms to attract electrons from other atoms.
Electrostatic attraction between ions is an ionic chemical bond. Such an interaction is possible between atoms with sharply different electronegativity. The ionic bond forms the corresponding ionic compounds. They consist of individual molecules exclusively in a vaporous state. The ionic bond in the compounds of the crystalline (solid) state involves the interaction of ions (negative and positive) located naturally. In this case, the molecules are absent.
Compounds characterized by an ionic bond form elements of the main subgroups of 1,2,6,7 groups. There are relatively few such compounds. For example, inorganic salts (NH4Cl), salt-like organic substances (amine salts and others) should be attributed to them.
Nonpolar covalent and ionic bonds are two limiting cases in the distribution of electron density. The former is characterized by a uniform distribution between the same particles (atoms) of the binder of two electron clouds. In an ionic bond, the electron cloud belongs almost entirely to one of the atoms. However, in most of the compounds, the interactions are at an intermediate level. In other words, a polar covalent bond is formed in them .
The interaction arising from the formation of common electron pairs is covalent. A covalent bond resulting from the interaction of atoms with different electronegativity is considered polar. The electron density from the binding electron pair is shifted to the atom, in which the electronegativity is greater. Molecules such as H2S, NH3, H2O, and others can be cited as examples. In these compounds, the appearance of covalent (non-polar and polar) bonds is due to unpaired electrons of binding atoms.