For the first time, such a concept as a covalent bond was studied by chemists after the discovery of Gilbert Newton Lewis, who described a chemical bond as the socialization of two electrons. Later studies allowed us to describe the principle of covalent bonding itself. The word covalent can be considered in chemistry as the ability of an atom to form bonds with other atoms.
Let us illustrate with an example:
There are two atoms with slight differences in electronegativity (C and CL, C and H). As a rule, these are atoms whose structure of the electron shell is as close as possible to the structure of the electron shell of noble gases.
When these conditions are fulfilled, the attraction of the nuclei of these atoms to the electron pair common to them arises. In this case, electron clouds do not just overlap each other, as in ionic bonding. The covalent bond provides a reliable connection of two atoms due to the fact that the electron density is redistributed and the energy of the system changes, which is caused by the “retraction” of one atom of the electron cloud of the other into the internuclear space. The more extensive the mutual overlap of electronic clouds, the more reliable the connection.
Hence, a covalent bond is a formation that has arisen through the mutual socialization of two electrons belonging to two atoms.
As a rule, substances with a molecular crystal lattice are formed by means of a covalent bond. Characteristic of the molecular structure are melting and boiling at low temperatures, poor solubility in water and low electrical conductivity. From this we can conclude: the basis of the structure of such elements as germanium, silicon, chlorine, hydrogen is a covalent bond.
Properties specific to this type of compound:
- Saturability. This property is usually understood as the maximum number of bonds that they can establish specific atoms. This amount is determined by the total number of those orbitals in the atom that can participate in the formation of chemical bonds. The valency of an atom, on the other hand, can be determined by the number of orbitals already used for this purpose.
- Directivity . All atoms tend to form the most durable bonds. The greatest strength is achieved if the spatial orientation of the electron clouds of two atoms coincides, since they overlap each other. In addition, just such a property of covalent bonding as directivity affects the spatial arrangement of the molecules of organic matter, that is, is responsible for their "geometric shape".
- Polarizability. This provision is based on the idea that there are two types of covalent bonds:
- polar or asymmetric. A bond of a given species can be formed only by atoms of different species, i.e. those whose electronegativity varies significantly, or in cases where the total electron pair is asymmetrically separated.
- a nonpolar covalent bond arises between atoms whose electronegativity is almost equal and the distribution of electron density is uniform.
In addition, there are certain quantitative characteristics of the covalent bond :
- Communication energy . This parameter characterizes the polar bond in terms of its strength. Energy is understood as the amount of heat that was necessary to break the bond of two atoms, as well as the amount of heat that was released when they were combined.
- By the length of the bond in molecular chemistry is meant the length of the straight line between the nuclei of two atoms. This parameter also characterizes the bond strength.
- The dipole moment is a value that characterizes the polarity of the valence bond.