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3.2. Covalent communicationCovalent communication - This is a two-electron, two-center communication, carried out by the publication of the pair of electrons.
Consider the mechanism for the formation of a covalent bond on the example of hydrogen molecule H 2.
The kernel of each hydrogen atom is surrounded by a spherical electronic cloud of 1S-electron. Under the convergence of two atoms of the core of the first atom attracts the electron of the second, and the electron of the first atom is attracted by the second core. As a result, there is an overlap of their electronic clouds with the formation of a common molecular cloud. Thus, a covalent bond is formed as a result of overlapping electronic clouds of atoms.
Schematically, this can be depicted as follows:
N. + N N. : N.
Similarly, a covalent bond in the chlorine molecule is formed:
. . . . . . . .
: Cl. + Cl Cl. : Cl. :
. . . . . . . .
If the link form the same atoms (with the same electric negativity), the electronic cloud is symmetrically relative to the nuclei of two atoms. In this case, talk about covalent non-polar communication .
Covalent polar communication It is formed when atoms with different electronegitimia interact.
. . . .
N. + Cl N. : Cl. :
. . . .
The electronic cloud of communication is asymmetric, shifted to one of the atoms with greater electronegitability, in this case to chlorine.
The examples described characterize a covalent bond, which is formed by exchange mechanism.
The second mechanism for the formation of a covalent bond - donor-accept. In this case, the relationship is formed by an inequalized electronic pair of one atom (donor) and the free orbital of the other atom (acceptor):
H 3 N. : + H + +
Connections with covalent bond are called atomic.
Conditions of education chemical bond
1. The chemical bond is formed with sufficient convergence of atoms in the event that the total internal energy of the system decreases. Thus, the resulting molecule turns out to be more stable than individual atoms and has less energy.
2. The occurrence of chemical bond is always an exothermic process.
3. The prerequisite for the formation of a chemical bond is the presence of an increased electron density between the nuclei.
So, for example, the radius of the hydrogen atom is 0.053 nm. If hydrogen atoms only came closer to the formation of a molecule, then the inter-identical distance would be 0.106 nm. In fact, this distance is 0.074 nm, therefore, the rapprochement of the nuclei leads to an increase in electron density.
Quantitative characteristics of chemical
1. Communication Energy, E, KJ / Mole
Communication energy - This is the energy that is allocated during the formation of communication or the amount of energy that needs to be taken to break the communication.
The greater the bond energy, the stronger the connection. Most bond energy covalent compounds Located within 200 - 800 kJ / mol.
2. Length of communication, R 0, nm
Length communication - This is the distance between the centers of atoms (interstitial distance).
The smaller the length of the communication, the stronger the connection.
Table 3.1.
Energy values \u200b\u200band lengths of some connections
| Communication | r. 0 nm | E, kj / mole |
| C - S. | 0, 154 | 347 |
| C \u003d S. | 0,135 | 607 |
| C S. | 0,121 | 867 |
| H - F. | 0,092 | 536 |
| H - Cl. | 0,128 | 432 |
| H - Br. | 0,142 | 360 |
| H - I. | 0,162 | 299 |
3. Valental angles
depend on the spatial structure.
Covalent bond properties
1. The focus of covalent communications It occurs in the direction of the maximum overlapping of electronic orbitals of interacting atoms, which causes the spatial structure of molecules, i.e. their shape.
Distinguish - Communication - Communication formed along a line connecting atom centers. communication can form s. - s., s. - p. and p. - p. Electronic clouds.
communication can only be formed r - R. electronic clouds.
-Svyaz. - This is a connection formed on both sides of the line connecting the centers of atoms. This relationship is characteristic only for compounds with multiple connections (double and triple).
The formation schemes - and connections are presented in Fig. 3.1.
Fig. 3.1. Education schemes - and -links.
2. Saturability of covalent bond - Complete use by atom of valence orbitals.
3.3. Metal communication
Atoms of most metals at the external energy level contain a small number of electrons (1 e - 16 elements; 2 e - 58 elements,
3 e - 4 elements; 5 e in SB and Bi, and 6 e at RO). The last three elements are not typical metals.
Under normal conditions, the metals are solid crystalline substances (except mercury). In the nodes of the metal crystal lattice there are metal cations.
Fig. 3.2. Education scheme metal bond.
Valence electrons have a small ionization energy, and therefore weakly held in the atom. The electrons move across the entire crystal lattice and belong to all its atoms, representing the so-called "electronic gas" or "sea of \u200b\u200bvalence electrons". Thus, the chemical bond in metals is strongly delocalized. This is determined by such properties characteristic of metals as high heat and electrical conductivity, pupility, plasticity.
Metal bond is characteristic of metals and alloys in solid and liquid state. In a vapor state, metals consist of individual molecules (single-nuclear and diatomic), interconnected by covalent bonds.
For the first time about such a thing as covalent communication Chemist scientists spoke after the opening of Gilbert Newton Lewis, who described as the public company of two electrons. Later research was allowed to describe the principle of covalent communications. Word covalentit can be considered within the framework of chemistry as an atom ability to form connections with other atoms.
Let us explain on the example:
There are two atoms with minor differences in electronegativity (C and CL, C and H). As a rule, this is which as close as possible to the structure of the electronic shell of noble gases.
When performing these conditions, the nuclei of these atoms to the electronic pair, common to them, occurs. In this case, electronic clouds are not simply superimposed on each other, as with a covalent bond ensures a reliable connection of two atoms due to the fact that electron density is redistributed and the energy of the system is changed, which is caused by the "retracting" into the inter-identical space of one atom of the electronic cloud of another. The more extensive mutual overlap of electronic clouds, the connection is considered more durable.
Hence covalent communication - This is an education resulting from the mutual socialization of two electrons belonging to two atoms.
As a rule, substances with molecular crystal lattice They are formed by means of a covalent bond. Characteristic is melting and boiling at low temperatures, poor solubility in water and low electrical conductivity. From here we can conclude: the structure of such elements such as Germanium, silicon, chlorine, hydrogen is covalent.
Properties characteristic of this type of connection:
- Saturability.Under this property is usually understood maximum amount Relations that they can establish specific atoms. This number is determined by the total number of those orbitals in the atom that can participate in the formation of chemical bonds. Atom valence, on the other hand, can be determined by the number of orbitals already used for this purpose.
- Food. All atoms seek to form the most strong connections. The greatest strength is achieved in the case of coincidence of the spatial orientation of electronic clouds of two atoms, as they overlap each other. In addition, it is this property of a covalent bond as an orientation affects the spatial arrangement of molecules that is responsible for their "geometric shape".
- Polarizability.This provision is based on the idea that a covalent bond exists two types:
- polar or asymmetrical. The connection of this species can only form atoms of different types, i.e. Those whose electronegility differ significantly, or in cases where the total electronic pair is asymmetrically divided.
- It occurs between atoms, the electronegability of which is almost equal, and the distribution of electron density is uniformly.
In addition, there are certain quantitatives:
- Communication energy. This parameter characterizes polar communication From the point of view of its strength. Under energy it is understood that the amount of heat that was necessary for the destruction of the bond between the two atoms, as well as the amount of heat, which was allocated when they are connected.
- Under lengthand in molecular chemistry it is understood as the length of the straight between the cores of the two atoms. This parameter also characterizes communication strength.
- Dipole moment - The value that characterizes the polarity of the valence.
covalent communication
type of chemical communication; It is carried out by a pair of electrons common to two atoms forming communication. Atoms in the molecule can be connected by a single covalent bond (H2, H3C-CH3), dual (H2C \u003d CH2) or triple (N2, HCCH). Atoms that differ in electronegativity form the so-called. Polar covalent bond (HCl, H3C-CL).
Covalent communication
one of the types of chemical bond between two atoms, which is carried out by a common electron pair (one electron from each atom). K. s. exists in molecules (in any aggregate states) and between atoms forming the crystal lattice. K. s. It may bind the same atoms (in the H2, CL2 molecules, in diamond crystals) or different (in water molecules, in the crystals of carboard SiC). Almost all types of main bonds in molecules organic compounds are covalent (C ≈ C, C ≈ N, C ≈ N, etc.). K. s. very durable. This explains the small chemical activity of paraffin hydrocarbons. Many inorganic compounds whose crystals have an atomic lattice, that is, they are formed using K. with., Are refractory, they have high hardness and wear resistance. These include some carbides, silicides, borides, nitrides (in particular, the famous Boron BN), which have been used in new technique. See also valence and chemical bond.
═V. A. Kireev.
Wikipedia
Covalent communication
Covalent communication (from lat. co. - "together" and vales. - "With power") - a chemical bond formed by overlapping a pair of valence electronic clouds. Providing communication electronic clouds are called common electronic couple.
The term Covalent Communication was first introduced by the Nobel Prize Laureate Irving Langmur in 1919. This term was referred to a chemical bond due to the joint possession of electrons, in contrast to the metallic bond, in which the electrons were free, or from an ion connection, in which one of the atoms gave the electron and became a cation, and the other atom took an electron and became an anion.
Later (1927) F. London and V. Gaitler on 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 the statistical interpretation of the wave function of M. Born the density of the probability of finding binding electrons is concentrated in the space between the molecule nuclei (Fig. 1). In the theory of repulsion of electronic pairs, the geometric dimensions of these pairs are considered. So, for the elements of each period there is some average radius of an electronic pair:
0.6 for elements up to neon; 0.75 for elements up to argon; 0.75 for elements up to crypton and 0.8 for elements up to xenon.
The characteristic properties of a covalent bond - focus, saturation, polarity, polarizability - determine chemical and physical properties connections.
The focus of communication is due molecular structure substances and geometric shape of their molecule. Corners between two connections are called valence.
Saturability - the ability of atoms to form a limited number of covalent bonds. The number of connections formed by an atom is limited by the number of its external atomic orbitals.
The polarity of the communication is due to the uneven distribution of electron density due to differences in the electrical negativeness of atoms. On this basis, covalent bonds are divided into non-polar and polar (non-polar - ductomic molecule consists of identical atoms (H, CL, N) and the electronic clouds of each atom are distributed symmetrically with respect to these atoms; polar - ductomic molecule consists of different atoms chemical elements, and the general electron cloud shifts towards one of the atoms, thereby forming an asymmetry of the distribution of an electric charge in the molecule, generating a dipole moment of the molecule).
The polarizability of communication is expressed in the displacement of electrons of communication under the influence of an external electric field, including another reacting particle. Polarizability is determined by electron mobility. The polarity and polarizability of covalent bonds determines the reactivity of molecules with respect to polar reagents.
However, twice the laureate of the Nobel Prize L. Pauling indicated that "in some molecules there are covalent bonds due to one or three electrons instead common couple" One-electron chemical bond is implemented in the molecular ion of hydrogen H.
The molecular ion of hydrogen H contains two proton and one electron. The only electro molecular system compensates for the electrostatic repulsion of two protons and holds them at a distance of 1.06 Å (chemical bond length H). The center density center of the electronic cloud of the molecular system is equidal to both protons on the Borov radius α \u003d 0.53 A and is the center of symmetry of the molecular ion of hydrogen H.