For the first time about such a concept as covalent bond Chemical scientists started talking after the discovery of Gilbert Newton Lewis, which he described as the socialization of two electrons. More later studies made it possible to describe the principle of covalent bonding itself. Word covalent can be considered within the framework of chemistry as the ability of an atom to form bonds with other atoms.
Let's explain with an example:
There are two atoms with slight differences in electronegativity (C and CL, C and H). As a rule, these are as close as possible to the building electron shell noble gases.
When these conditions are met, an attraction of the nuclei of these atoms to the electron pair common to them occurs. In this case, electron clouds do not simply overlap each other, as with Covalent bond ensures 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 “pulling” of the electron cloud of another atom into the internuclear space. The more extensive the mutual overlap of electron clouds, the stronger the connection is considered.
From here, covalent bond- this is a formation that arose through the mutual socialization of two electrons belonging to two atoms.
As a rule, substances with a molecular crystal lattice are formed through covalent bonds. Characteristics are melting and boiling at low temperatures, poor solubility in water and low electrical conductivity. From this we can conclude: the structure of elements such as germanium, silicon, chlorine, and hydrogen is based on a covalent bond.
Properties characteristic of this type of connection:
- Saturability. This property is usually understood as maximum amount the bonds they can establish with specific atoms. This quantity is determined total number those orbitals in an 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.
- Focus. All atoms strive to form as much as possible strong connections. The greatest strength is achieved when the spatial orientation of the electron clouds of two atoms coincides, since they overlap each other. In addition, it is precisely this property of a covalent bond, such as directionality, that affects the spatial arrangement of molecules, that is, it is responsible for their “geometric shape”.
- Polarizability. This position is based on the idea that there are two types of covalent bonds:
- polar or asymmetrical. A bond of this type can only be formed by atoms of different types, i.e. those whose electronegativity varies significantly, or in cases where the shared electron pair is asymmetrically shared.
- arises between atoms whose electronegativity is practically equal, and the distribution electron density evenly.
In addition, there are certain quantitative ones:
- Communication energy. This parameter characterizes the polar bond in terms of its strength. Energy refers to the amount of heat that was necessary to break the bond between two atoms, as well as the amount of heat that was released during their connection.
- Under bond length and in molecular chemistry the length of a straight line between the nuclei of two atoms is understood. This parameter also characterizes the strength of the connection.
- Dipole moment- a quantity that characterizes the polarity of the valence bond.
covalent bond
type of chemical bond; carried out by a pair of electrons shared by the two atoms forming the bond. Atoms in a molecule can be connected by a single covalent bond (H2, H3C-CH3), double (H2C=CH2) or triple (N2, HCCH). Atoms that differ in electronegativity form the so-called. polar covalent bond (HCl, H3C-Cl).
Covalent bond
one of the types of chemical bonds between two atoms, which is carried out by a common electron pair (one electron from each atom). K. s. exists both in molecules (in any states of aggregation), and between the atoms that form the crystal lattice. K. s. can bind identical atoms (in H2, Cl2 molecules, in diamond crystals) or different (in water molecules, in SiC carborundum crystals). Almost all types of basic bonds in molecules organic compounds are covalent (C ≈ C, C ≈ H, C ≈ N, etc.). K. s. very durable. This explains the low chemical activity of paraffin hydrocarbons. Many inorganic compounds, the crystals of which have an atomic lattice, that is, are formed with the help of crystallization, are refractory, have high hardness and wear resistance. These include some carbides, silicides, borides, nitrides (in particular, the well-known borazone BN), which have found application in new technology. See also Valency and Chemical Bond.
══V. A. Kireev.
Wikipedia
Covalent bond
Covalent bond(from lat. co- “together” and vales- “having force”) - a chemical bond formed by the overlap of a pair of valence electron clouds. Electronic clouds that provide communication are called shared electron pair.
The term covalent bond was first introduced by the laureate Nobel Prize Irving Langmuir in 1919. The term referred to a chemical bond due to the shared possession of electrons, as opposed to metal connection, in which electrons were free, or from an ionic bond, in which one of the atoms gave up an electron and became a cation, and the other atom accepted an electron and became an anion.
Later (1927), F. London and W. Heitler, using the example of a hydrogen molecule, gave the first description of a covalent bond from the point of view of quantum mechanics.
Taking into account statistical interpretation wave function M. Born, the probability density of finding bonding electrons is concentrated in the space between the nuclei of the molecule (Fig. 1). The theory of electron pair repulsion considers the geometric dimensions of these pairs. Thus, for elements of each period there is a certain average radius of an electron pair:
0.6 for elements up to neon; 0.75 for elements up to argon; 0.75 for elements up to krypton and 0.8 for elements up to xenon.
The characteristic properties of a covalent bond - directionality, saturation, polarity, polarizability - determine the chemical and physical properties connections.
The direction of the connection is determined molecular structure substances and geometric shape their molecules. The angles between two bonds are called bond angles.
Saturability is the ability of atoms to form a limited number of covalent bonds. The number of bonds formed by an atom is limited by the number of its outer atomic orbitals.
The polarity of the bond is due to the uneven distribution of electron density due to differences in the electronegativity of the atoms. On this basis, covalent bonds are divided into non-polar and polar (non-polar - a diatomic molecule consists of identical atoms (H, Cl, N) and the electron clouds of each atom are distributed symmetrically relative to these atoms; polar - a diatomic molecule consists of different atoms chemical elements, and the total electron cloud shifts towards one of the atoms, thereby forming an asymmetry of the distribution electric charge in a molecule, generating a dipole moment of the molecule).
Bond polarizability is expressed in the displacement of bond electrons under the influence of external electric field, including another reacting particle. Polarizability is determined by electron mobility. The polarity and polarizability of covalent bonds determines reactivity molecules in relation to polar reagents.
However, two-time Nobel Prize winner L. Pauling pointed out that “in some molecules there are covalent bonds due to one or three electrons instead of a common pair.” A one-electron chemical bond is realized in the molecular hydrogen ion H.
The molecular hydrogen ion H contains two protons and one electron. Single electron molecular system compensates for the electrostatic repulsion of two protons and keeps them at a distance of 1.06 Å (chemical bond length H). The center of the electron density of the electron cloud of the molecular system is equidistant from both protons at the Bohr radius α = 0.53 A and is the center of symmetry molecular ion hydrogen H.
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3.2. Covalent bondCovalent bond- This is a two-electron, two-center connection, carried out by sharing a pair of electrons.
Let us consider the mechanism of covalent bond formation using the example of the hydrogen molecule H2.
The nucleus of each hydrogen atom is surrounded by a spherical electron cloud of a 1s electron. When two atoms come together, the nucleus of the first atom attracts the electron of the second, and the electron of the first atom is attracted by the nucleus of the second. As a result, their electron clouds overlap to form a common molecular cloud. Thus, as a result of the overlapping electron clouds of atoms, a covalent bond is formed.
Schematically this can be depicted as follows:
N + N N : N
A covalent bond is formed in a similar way in a chlorine molecule:
. . . . . . . .
: Cl + Cl Cl : Cl :
. . . . . . . .
If the bond is formed by identical atoms (with the same electronegativity), then the electron cloud is located symmetrically relative to the nuclei of the two atoms. In this case they talk about covalent nonpolar bond.
Covalent polar connection formed when atoms with different electronegativity interact.
. . . .
N + Cl H : Cl :
. . . .
The electron cloud of communication is asymmetrical, shifted to one of the atoms with higher electronegativity, in in this case to chlorine.
The given examples characterize the covalent bond that is formed by metabolic mechanism .
The second mechanism for the formation of covalent bonds is donor-acceptor. In this case, the bond is formed due to the lone electron pair of one atom (donor) and the free orbital of another atom (acceptor):
N 3 N : + H + +
Compounds with covalent bonds are called atomic.
Conditions for the formation of a chemical bond
1. A chemical bond is formed when the atoms are sufficiently close together in the event that complete internal energy system goes down. Thus, the resulting molecule turns out to be more stable than individual atoms and has less energy.
2. The formation of a chemical bond is always an exothermic process.
3. Required condition the formation of a chemical bond is the presence of increased electron density between the nuclei.
For example, the radius of a hydrogen atom is 0.053 nm. If the hydrogen atoms only came closer together during the formation of the molecule, then the internuclear distance would be 0.106 nm. In fact, this distance is 0.074 nm, therefore, bringing the nuclei closer leads to an increase in the electron density.
Quantitative characteristics of chemical bonds
1. Bond energy, E, kJ/mol
Communication energy- this is the energy that is released when a bond is formed or the amount of energy that must be spent to break the bond.
The higher the binding energy, the stronger the connection. Majority bond energy covalent compounds is in the range of 200 - 800 kJ/mol.
2. Bond length, r 0 , nm
Link length is the distance between the centers of atoms (internuclear distance).
The shorter the bond length, the stronger the connection.
Table 3.1.
Energy values and lengths of some bonds
| Connection | r 0 , nm | E, kJ/mol |
| S - S | 0, 154 | 347 |
| C = C | 0,135 | 607 |
| C C | 0,121 | 867 |
| H - F | 0,092 | 536 |
| H-Cl | 0,128 | 432 |
| H-Br | 0,142 | 360 |
| H - I | 0,162 | 299 |
3. Bond angles depend on the spatial structure.
Properties of covalent bonds
1. Directionality of covalent bond occurs in the direction of maximum overlap electron orbitals interacting atoms, which determines the spatial structure of molecules, i.e. their shape.
Distinguish -communications- bonds formed along the line connecting the centers of atoms. -bonds can form s - s, s - p And p - p electronic clouds.
An bond can only be formed r - r electron clouds.
-connection is a bond formed on both sides of a line connecting the centers of atoms. This bond is characteristic only for compounds with multiple bonds (double and triple).
Schemes for the formation of - and - bonds are presented in Fig. 3.1.
Rice. 3.1. Schemes for the formation of - and - bonds.
2. Covalent bond saturation - full use atom valence orbitals.
3.3. Metal connection
The atoms of most metals at the outer energy level contain a small number of electrons (1 e - 16 elements; 2 e - 58 elements,
3rd - 4 elements; 5 e for Sb and Bi, and 6 e for Po). The last three elements are not typical metals.
IN normal conditions metals are hard crystalline substances(except mercury). Metal cations are located at the nodes of the metal crystal lattice.
Rice. 3.2. Scheme of metal bond formation.
Valence electrons have low ionization energy and are therefore weakly retained in the atom. Electrons move throughout crystal lattice and belong to all its atoms, representing the so-called “electron gas” or “sea of valence electrons”. Thus, chemical bonding in metals is highly delocalized. This determines such properties characteristic of metals as high thermal and electrical conductivity, malleability, and plasticity.
Metallic bonding is characteristic of metals and alloys in solid and liquid states. In the vapor state, metals consist of individual molecules (monatomic and diatomic) connected to each other by covalent bonds.