CONNECTION
COMMUNICATION, -and, about communication, in connection and in connection, w.
1. (due). A relationship of mutual dependence, conditionality, commonality between something. C. theory and practice. Causal p.
2. (due). Close communication between someone or something. Friendly village Strengthen international relations.
3. (in connection and in connection). Love relationships, cohabitation. Lyubovnaya s. To be in touch with someone.
4. pl. h. Close acquaintance with someone, providing support, patronage, benefit. Have connections in influential circles. Great connections.
5. (due). Communication with someone, as well as means that make it possible to communicate, to communicate. Kosmicheskaya village Live s.(via contacts). Air village Intercity telephone s.
6. (due). A branch of the national economy related to the means of such communication (mail, telegraph, telephone, radio), as well as the totality of such means concentrated in the relevant institutions. Communication service. Communication workers.
7. (in connection), usually plural. h. Part of a building structure connecting its main elements (special).
Due to how, preposition with TV. n. as a result of something, because of something, being conditioned by something. Late due to skidding.
Due to, union for the reason that, on the basis of the fact that. I inquired because accurate information was needed.
CONNECTION What is it CONNECTION, meaning of the word CONNECTION, synonyms for CONNECTION, origin (etymology) CONNECTION, CONNECTION stress, word forms in other dictionaries
+ CONNECTION- T.F. Efremova New dictionary of the Russian language. Explanatory and word-formative
COMMUNICATION is
connection
and.
a) Mutual relations between someone or something.
b) Community, mutual understanding, internal unity.
a) Communication with someone.
b) Love relationships, cohabitation.
3) Relationships between someone that create mutual dependence, conditionality.
4) Consistency, coherence, harmony (in thoughts, presentation, etc.).
5) The ability to communicate with someone or something. on distance.
6) Means by which communication is carried out at a distance.
7) A set of institutions that provide means of communication at a distance (telegraph, mail, telephone, radio).
a) Connection, fastening of something.
b) Cohesion, mutual attraction (molecules, atoms, electrons, etc.).
+ CONNECTION- Modern explanatory dictionary ed. "Great Soviet Encyclopedia"
COMMUNICATION is
CONNECTION
1) transmission and reception of information using various technical means. In accordance with the nature of the means of communication used, it is divided into postal (see Mail) and electric (see Telecommunications). 2) The branch of the national economy that ensures the transmission and reception of postal, telephone, telegraph, radio and other messages. In the USSR in 1986 there were 92 thousand communications enterprises; 8.5 billion letters, 50.3 billion newspapers and magazines, 248 million parcels, 449 million telegrams were sent; the number of telephone sets on the general telephone network amounted to 33.0 million. 60s in the USSR, the Unified Automated Communications Network (EASC) is being introduced. 3) Military communications are provided by the Signal Corps. --- in philosophy - the interdependence of the existence of phenomena separated in space and time. Connections are classified according to objects of cognition, according to forms of determinism (unambiguous, probabilistic and correlational), according to their strength (rigid and corpuscular), according to the nature of the result that the connection gives (connection of generation, connection of transformation), according to the direction of action (direct and reverse), by the type of processes that define this connection (functioning connection, development connection, control connection), by the content that is the subject of the connection (connection that ensures the transfer of matter, energy or information).
+ CONNECTION- Small Academic Dictionary of the Russian Language
COMMUNICATION is
connection
AND, sentence about communication, in connection and in connection, and.
Mutual relationship between someone or something.
Connection between industry and agriculture. Connection between science and production. Trade connections. Economic connections between regions. Family ties.
Mutual dependence, conditionality.
Causality.
We only want to say that all sciences are closely connected with each other and that the lasting acquisitions of one science should not remain fruitless for others. Chernyshevsky, Grammar notes. V. Klassovsky.
The connection between Petrov-Vodkin’s work and the traditions of ancient Russian painting is obvious.
L. Mochalov, The uniqueness of talent.
Coherence, harmony, consistency (in connecting thoughts, in presentation, in speech).
Thoughts were confused in his head, and words had no connection. Pushkin, Dubrovsky.
There is not enough consistency in my thoughts, and when I put them on paper, it always seems to me that I have lost my sense of their organic connection. Chekhov, A boring story.
Closeness with someone, inner unity.
That invisible connection grew between them, which was not expressed in words, but only felt. Mamin-Sibiryak, Privalovsky millions.
When a writer deeply feels his blood connection with the people, it gives him beauty and strength. M. Gorky, Letter to D.N. Mamin-Sibiryak, October 18. 1912.
Communication (friendly or business), relations with someone or something.
Keep in touch with smb. Make connections in the literary world.
(Ivan Ivanovich and Ivan Nikiforovich) broke off all ties, while previously they were known as the most inseparable friends! Gogol, The story of how Ivan Ivanovich quarreled with Ivan Nikiforovich.
Drozdov's connections with one of the revolutionary organizations were established, and arrests were made. M. Gorky, Story about a hero.
Love relationship; cohabitation.
(Matvey) entered into a relationship with a bourgeois woman and had a child with her. Chekhov, Murder.
(Sophia:) What right do you have to talk about my infidelity?.. You had dozens of relationships. M. Gorky, The Last.
|| pl. h.(connections, -ey).
Close acquaintance with influential persons who can provide support and patronage.
Good B. decided to find a home for his stepfather. He already had great connections and immediately began to ask and recommend his poor comrade. Dostoevsky, Netochka Nezvanova.
Thanks to the connections of my late engineer father, I was enrolled in the Mikhailovsky School. Pertsov, From autobiography.
Communication, communication with someone or something. using various means.
In the cabin, using a speaking tube, the commander could communicate with the bridge, and by telephone with any department of the ship. Novikov-Priboy, Captain 1st Rank.
Morozka was among the cavalry assigned to communicate with the platoons during the battle. Fadeev, Defeat.
Now there was only one way of communication left - through the Volga. Simonov, Days and Nights.
|| Those.
Transmitting and receiving information using special means.
5. usually with a definition.
The means by which communication and transmission of information are carried out.
Radiotelephone communication. Telegraph communication. Dispatcher communication.
At night, the signalmen of the artillery regiment managed to establish a telephone connection to the tank. V. Kozhevnikov, Seven days.
A set of institutions that provide technical means of communication at a distance (telegraph, post office, telephone, radio).
Communication workers.
|| Military
A service that provides communication between military units (using telephone, radio, messengers, etc.).
Arkhip Khromkov became the head of intelligence and communications. Markov, Strogovs.
A liaison officer arrived from army headquarters with an urgent package. Popovkin, Rubanyuk Family.
Connection, fastening of something.
Bonding stones and bricks with clay.
In the Trinity Cathedral, he introduces iron into the masonry of the building to connect the corners. Pilyavsky, Works of V.P. Stasov in Leningrad.
Cohesion, mutual attraction (molecules, atoms, electrons, etc.).
The connection of electrons with the nucleus.
A device that binds or fastens parts of something. buildings or structures; clamp.
It was a huge decorative workshop - a dome intertwined at the top with iron rafters and braces. A. N. Tolstoy, Egor Abozov. logic, coherence, continuity, foldability, sequence, harmony, interaction, connection, articulation, concatenation, cohesion, communication, means of communication, intercourse, communication, contact, association, relation, relationship, dependence, binding, ties, romance, connecting link, union, causation, public relations, tomba, intimate relationships, intrigue, ratio, duplex, umbilical cord, intercourse, bonding, religion, cohabitation, parataxis, connecting thread, continuity, adhesion, interconnectedness, correlation, conditioning, connection, kinship, putty, bond, cupids, affair, synapse, context, love, thread, mail, message, quadruplex. Ant. fragmentation
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As a result of studying this topic, you will learn:
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5.1. Covalent bond
A chemical bond is formed when two or more atoms come together if, as a result of their interaction, the total energy of the system decreases. The most stable electronic configurations of the outer electron shells of atoms are those of noble gas atoms, consisting of two or eight electrons. The outer electron shells of atoms of other elements contain from one to seven electrons, i.e. are unfinished. When a molecule is formed, atoms tend to acquire a stable two-electron or eight-electron shell. The valence electrons of atoms take part in the formation of a chemical bond.
Covalent is a chemical bond between two atoms, which is formed by electron pairs that simultaneously belong to these two atoms.
There are two mechanisms for the formation of covalent bonds: exchange and donor-acceptor.
5.1.1. Exchange mechanism of covalent bond formation
Exchange mechanism The formation of a covalent bond is realized due to the overlap of electron clouds of electrons belonging to different atoms. For example, when two hydrogen atoms approach each other, the 1s electron orbitals overlap. As a result, a common pair of electrons appears, simultaneously belonging to both atoms. In this case, a chemical bond is formed by electrons having antiparallel spins, Fig. 5.1.
Rice. 5.1. Formation of a hydrogen molecule from two H atoms
5.1.2. Donor-acceptor mechanism for the formation of covalent bonds
With the donor-acceptor mechanism of covalent bond formation, the bond is also formed using electron pairs. However, in this case, one atom (donor) provides its electron pair, and the other atom (acceptor) participates in the formation of the bond with its free orbital. An example of the implementation of a donor-acceptor bond is the formation of ammonium ion NH 4 + during the interaction of ammonia NH 3 with the hydrogen cation H +.
In the NH 3 molecule, three electron pairs form three N – H bonds, the fourth electron pair belonging to the nitrogen atom is lone. This electron pair can form a bond with a hydrogen ion that has an unoccupied orbital. The result is ammonium ion NH 4 +, Fig. 5.2.
Rice. 5.2. The appearance of a donor-acceptor bond during the formation of ammonium ion
It should be noted that the four covalent N–H bonds existing in the NH 4 + ion are equivalent. In the ammonium ion it is impossible to identify a bond formed by the donor-acceptor mechanism.
5.1.3. Polar and non-polar covalent bond
If a covalent bond is formed by identical atoms, then the electron pair is located at the same distance between the nuclei of these atoms. Such a covalent bond is called nonpolar. Examples of molecules with a non-polar covalent bond are H2, Cl2, O2, N2, etc.
In the case of a polar covalent bond, the shared electron pair is shifted to the atom with higher electronegativity. This type of bond is realized in molecules formed by different atoms. A polar covalent bond occurs in molecules of HCl, HBr, CO, NO, etc. For example, the formation of a polar covalent bond in a HCl molecule can be represented by a diagram, Fig. 5.3:
Rice. 5.3. Formation of a covalent polar bond in the HC1 molecule
In the molecule under consideration, the electron pair is shifted to the chlorine atom, since its electronegativity (2.83) is greater than the electronegativity of the hydrogen atom (2.1).
5.1.4. Dipole moment and molecular structure
A measure of the polarity of a bond is its dipole moment μ:
μ = e l,
Where e– electron charge, l– the distance between the centers of positive and negative charges.
Dipole moment is a vector quantity. The concepts of “bond dipole moment” and “molecule dipole moment” coincide only for diatomic molecules. The dipole moment of a molecule is equal to the vector sum of the dipole moments of all bonds. Thus, the dipole moment of a polyatomic molecule depends on its structure.
In a linear CO 2 molecule, for example, each of the C–O bonds is polar. However, the CO 2 molecule is generally nonpolar, since the dipole moments of the bonds cancel each other out (Fig. 5.4). The dipole moment of the carbon dioxide molecule is m = 0.
In the angular H2O molecule, the polar H–O bonds are located at an angle of 104.5 o. The vector sum of the dipole moments of two H–O bonds is expressed by the diagonal of the parallelogram (Fig. 5.4). As a result, the dipole moment of the water molecule m is not equal to zero.
Rice. 5.4. Dipole moments of CO 2 and H 2 O molecules
5.1.5. Valency of elements in compounds with covalent bonds
The valence of atoms is determined by the number of unpaired electrons participating in the formation of common electron pairs with electrons of other atoms. Having one unpaired electron on the outer electron layer, the halogen atoms in the F 2, HCl, PBr 3 and CCl 4 molecules are monovalent. Elements of the oxygen subgroup contain two unpaired electrons in the outer layer, therefore in compounds such as O 2, H 2 O, H 2 S and SCl 2 they are divalent.
Since, in addition to ordinary covalent bonds, a bond can be formed in molecules by a donor-acceptor mechanism, the valence of atoms also depends on the presence of lone electron pairs and free electron orbitals. A quantitative measure of valence is the number of chemical bonds through which a given atom is connected to other atoms.
The maximum valence of elements, as a rule, cannot exceed the number of the group in which they are located. The exception is the elements of the secondary subgroup of the first group Cu, Ag, Au, whose valence in compounds is greater than one. The valence electrons primarily include the electrons of the outer layers, however, for elements of side subgroups, the electrons of the penultimate (pre-outer) layers also take part in the formation of a chemical bond.
5.1.6. Valency of elements in normal and excited states
The valency of most chemical elements depends on whether these elements are in a normal or excited state. Electronic configuration of the Li atom: 1s 2 2s 1. The lithium atom at the outer level has one unpaired electron, i.e. lithium is monovalent. A very large expenditure of energy is required associated with the transition of the 1s electron to the 2p orbital to obtain trivalent lithium. This energy expenditure is so great that it is not compensated by the energy released during the formation of chemical bonds. In this regard, there are no trivalent lithium compounds.
Configuration of the outer electronic layer of elements of the beryllium subgroup ns 2. This means that in the outer electron layer of these elements in the ns cell orbital there are two electrons with opposite spins. Elements of the beryllium subgroup do not contain unpaired electrons, so their valence in the normal state is zero. In the excited state, the electronic configuration of the elements of the beryllium subgroup is ns 1 nр 1, i.e. elements form compounds in which they are divalent.
Valence possibilities of the boron atom
Let's consider the electronic configuration of the boron atom in the ground state: 1s 2 2s 2 2p 1. The boron atom in the ground state contains one unpaired electron (Fig. 5.5), i.e. it is monovalent. However, boron is not characterized by the formation of compounds in which it is monovalent. When a boron atom is excited, one 2s electron transitions to a 2p orbital (Fig. 5.5). A boron atom in an excited state has 3 unpaired electrons and can form compounds in which its valency is three.
Rice. 5.5. Valence states of the boron atom in normal and excited states
The energy expended on the transition of an atom to an excited state within one energy level, as a rule, is more than compensated by the energy released during the formation of additional bonds.
Due to the presence of one free 2p orbital in the boron atom, boron in compounds can form a fourth covalent bond, acting as an electron pair acceptor. Figure 5.6 shows how the BF molecule interacts with the F – ion, resulting in the formation of the – ion, in which boron forms four covalent bonds.
Rice. 5.6. Donor-acceptor mechanism for the formation of the fourth covalent bond at the boron atom
Valence possibilities of the nitrogen atom
Let's consider the electronic structure of the nitrogen atom (Fig. 5.7).
Rice. 5.7. Distribution of electrons in the orbitals of the nitrogen atom
From the presented diagram it is clear that nitrogen has three unpaired electrons, it can form three chemical bonds and its valency is three. The transition of the nitrogen atom to an excited state is impossible, since the second energy level does not contain d-orbitals. At the same time, the nitrogen atom can provide a lone electron pair of outer electrons 2s 2 to an atom having a free orbital (acceptor). As a result, a fourth chemical bond of the nitrogen atom appears, as is the case, for example, in the ammonium ion (Fig. 5.2). Thus, the maximum covalency (the number of covalent bonds formed) of a nitrogen atom is four. In its compounds, nitrogen, unlike other elements of the fifth group, cannot be pentavalent.
Valence possibilities of phosphorus, sulfur and halogen atoms
Unlike the atoms of nitrogen, oxygen and fluorine, the atoms of phosphorus, sulfur and chlorine located in the third period have free 3d cells to which electrons can transfer. When a phosphorus atom is excited (Fig. 5.8), it has 5 unpaired electrons on its outer electron layer. As a result, in compounds the phosphorus atom can be not only tri-, but also pentavalent.
Rice. 5.8. Distribution of valence electrons in orbitals for a phosphorus atom in an excited state
In the excited state, sulfur, in addition to a valence of two, also exhibits a valence of four and six. In this case, 3p and 3s electrons are sequentially paired (Fig. 5.9).
Rice. 5.9. Valence possibilities of a sulfur atom in an excited state
In the excited state, for all elements of the main subgroup of group V, except fluorine, sequential pairing of first p- and then s-electron pairs is possible. As a result, these elements become tri-, penta- and heptavalent (Fig. 5.10).
Rice. 5.10. Valence possibilities of chlorine, bromine and iodine atoms in an excited state
5.1.7. Length, energy and direction of a covalent bond
Covalent bonds typically form between nonmetal atoms. The main characteristics of a covalent bond are length, energy and direction.
Covalent bond length
The length of a bond is the distance between the nuclei of the atoms forming this bond. It is determined by experimental physical methods. The bond length can be estimated using the additivity rule, according to which the bond length in the AB molecule is approximately equal to half the sum of the bond lengths in molecules A 2 and B 2:
.
From top to bottom along the subgroups of the periodic system of elements, the length of the chemical bond increases, since the radii of the atoms increase in this direction (Table 5.1). As the bond multiplicity increases, its length decreases.
Table 5.1.
Length of some chemical bonds
Chemical bond |
Link length, pm |
Chemical bond |
Link length, pm |
C – C |
|||
Communication energy
A measure of bond strength is the bond energy. Communication energy determined by the energy required to break a bond and remove the atoms forming that bond to an infinitely large distance from each other. The covalent bond is very strong. Its energy ranges from several tens to several hundred kJ/mol. For an IСl 3 molecule, for example, the Ebond is ≈40, and for N 2 and CO molecules the Ebond is ≈1000 kJ/mol.
From top to bottom along the subgroups of the periodic system of elements, the energy of a chemical bond decreases, since the bond length increases in this direction (Table 5.1). As the bond multiplicity increases, its energy increases (Table 5.2).
Table 5.2.
Energies of some chemical bonds
Chemical bond |
Communication energy, |
Chemical bond |
Communication energy, |
C – C |
|||
Saturation and directionality of covalent bonds
The most important properties of a covalent bond are its saturation and directionality. Saturability can be defined as the ability of atoms to form a limited number of covalent bonds. Thus, a carbon atom can form only four covalent bonds, and an oxygen atom can form two. The maximum number of ordinary covalent bonds that an atom can form (excluding bonds formed by the donor-acceptor mechanism) is equal to the number of unpaired electrons.
Covalent bonds have a spatial orientation, since the overlap of orbitals during the formation of a single bond occurs along the line connecting the atomic nuclei. The spatial arrangement of the electron orbitals of a molecule determines its geometry. The angles between chemical bonds are called bond angles.
The saturation and directionality of a covalent bond distinguishes this bond from an ionic bond, which, unlike a covalent bond, is unsaturated and non-directional.
Spatial structure of H 2 O and NH 3 molecules
Let us consider the direction of a covalent bond using the example of H 2 O and NH 3 molecules.
The H 2 O molecule is formed from an oxygen atom and two hydrogen atoms. The oxygen atom has two unpaired p electrons, which occupy two orbitals located at right angles to each other. Hydrogen atoms have unpaired 1s electrons. The angle between the bonds formed by p-electrons should be close to the angle between the orbitals of p-electrons. Experimentally, however, it was found that the angle between the O–H bonds in a water molecule is 104.50. The increase in the angle compared to the angle of 90 o can be explained by the repulsive forces that act between the hydrogen atoms, Fig. 5.11. Thus, the H 2 O molecule has an angular shape.
Three unpaired p-electrons of the nitrogen atom, whose orbitals are located in three mutually perpendicular directions, participate in the formation of the NH 3 molecule. Therefore, the three N–H bonds should be located at angles to each other close to 90° (Fig. 5.11). The experimental value of the angle between bonds in the NH 3 molecule is 107.3°. The difference between the angles between the bonds and the theoretical values is due, as in the case of the water molecule, to the mutual repulsion of hydrogen atoms. In addition, the presented schemes do not take into account the possibility of the participation of two electrons in the 2s orbitals in the formation of chemical bonds.
Rice. 5.11. Overlapping of electronic orbitals during the formation of chemical bonds in H 2 O (a) and NH 3 (b) molecules
Let's consider the formation of the BeC1 2 molecule. A beryllium atom in an excited state has two unpaired electrons: 2s and 2p. It can be assumed that the beryllium atom should form two bonds: one bond formed by the s-electron and one bond formed by the p-electron. These bonds must have different energies and different lengths. The BeCl 2 molecule in this case should not be linear, but angular. Experience, however, shows that the BeCl 2 molecule has a linear structure and both chemical bonds in it are equivalent. A similar situation is observed when considering the structure of the BCl 3 and CCl 4 molecules - all bonds in these molecules are equivalent. The BC1 3 molecule has a flat structure, CC1 4 has a tetrahedral structure.
To explain the structure of molecules such as BeCl 2, BCl 3 and CCl 4, Pauling and Slater(USA) introduced the concept of hybridization of atomic orbitals. They proposed replacing several atomic orbitals, which do not differ very much in their energy, with the same number of equivalent orbitals, called hybrid ones. These hybrid orbitals are composed of atomic orbitals as a result of their linear combination.
According to L. Pauling, when chemical bonds are formed by an atom having electrons of different types in one layer and, therefore, not very different in their energy (for example, s and p), it is possible to change the configuration of orbitals of different types, in which their alignment in shape and energy occurs . As a result, hybrid orbitals are formed that have an asymmetric shape and are highly elongated on one side of the nucleus. It is important to emphasize that the hybridization model is used when electrons of different types, for example s and p, are involved in the formation of bonds.
5.1.8.2. Various types of atomic orbital hybridization
sp hybridization
Hybridization of one s- and one R- orbitals ( sp- hybridization) is realized, for example, during the formation of beryllium chloride. As shown above, in an excited state, a Be atom has two unpaired electrons, one of which occupies the 2s orbital, and the other occupies the 2p orbital. When a chemical bond is formed, these two different orbitals are transformed into two identical hybrid orbitals, directed at an angle of 180° to each other (Fig. 5.12). The linear arrangement of two hybrid orbitals corresponds to their minimal repulsion from each other. As a result, the BeCl 2 molecule has a linear structure - all three atoms are located on the same line.
Rice. 5.12. Diagram of electron orbital overlap during the formation of a BeCl 2 molecule
The structure of the acetylene molecule; sigma and pi bonds
Let's consider a diagram of the overlap of electronic orbitals during the formation of an acetylene molecule. In an acetylene molecule, each carbon atom is in an sp-hybrid state. Two sp-hybrid orbitals are located at an angle of 1800 to each other; they form one σ bond between carbon atoms and two σ bonds with hydrogen atoms (Fig. 5.13).
Rice. 5.13. Scheme of formation of s-bonds in an acetylene molecule
A σ bond is a bond formed as a result of overlapping electron orbitals along a line connecting the nuclei of atoms.
Each carbon atom in the acetylene molecule contains two more p-electrons, which do not take part in the formation of σ bonds. The electron clouds of these electrons are located in mutually perpendicular planes and, overlapping each other, form two more π bonds between carbon atoms due to the lateral overlap of non-hybrid R–clouds (Fig. 5.14).
A π bond is a covalent chemical bond formed as a result of an increase in electron density on either side of the line connecting the nuclei of atoms.
Rice. 5.14. Scheme of the formation of σ - and π - bonds in the acetylene molecule.
Thus, in the acetylene molecule, a triple bond is formed between the carbon atoms, which consists of one σ - bond and two π - bonds; σ -bonds are stronger than π-bonds.
sp2 hybridization
The structure of the BCl 3 molecule can be explained in terms of sp 2- hybridization. A boron atom in an excited state on the outer electron layer contains one s-electron and two p-electrons, i.e. three unpaired electrons. These three electron clouds can be converted into three equivalent hybrid orbitals. The minimum repulsion of three hybrid orbitals from each other corresponds to their location in the same plane at an angle of 120 o to each other (Fig. 5.15). Thus, the BCl 3 molecule has a flat shape.
Rice. 5.15. Flat structure of the BCl 3 molecule
sp 3 - hybridization
The valence orbitals of the carbon atom (s, р x, р y, р z) can be converted into four equivalent hybrid orbitals, which are located in space at an angle of 109.5 o to each other and directed to the vertices of the tetrahedron, in the center of which is the nucleus of the carbon atom (Fig. 5.16).
Rice. 5.16. Tetrahedral structure of the methane molecule
5.1.8.3. Hybridization involving lone electron pairs
The hybridization model can be used to explain the structure of molecules that, in addition to bonding ones, also contain lone pairs of electrons. In water and ammonia molecules, the total number of electron pairs of the central atom (O and N) is four. At the same time, a water molecule has two, and an ammonia molecule has one lone pair of electrons. The formation of chemical bonds in these molecules can be explained by assuming that lone pairs of electrons can also fill hybrid orbitals. Lone electron pairs take up much more space in space than bonding ones. As a result of the repulsion that occurs between lone and bonding electron pairs, the bond angles in water and ammonia molecules decrease, which turn out to be less than 109.5 o.
Rice. 5.17. sp 3 – hybridization involving lone electron pairs in H 2 O (A) and NH 3 (B) molecules
5.1.8.4. Establishing the type of hybridization and determining the structure of molecules
To establish the type of hybridization, and, consequently, the structure of molecules, the following rules must be used.
1. The type of hybridization of the central atom, which does not contain lone pairs of electrons, is determined by the number of sigma bonds. If there are two such bonds, sp-hybridization occurs, three - sp 2 -hybridization, four - sp 3 -hybridization. Lone electron pairs (in the absence of bonds formed by the donor-acceptor mechanism) are absent in molecules formed by atoms of beryllium, boron, carbon, silicon, i.e. in elements of the main subgroups II - IV groups.
2. If the central atom contains lone electron pairs, then the number of hybrid orbitals and the type of hybridization are determined by the sum of the number of sigma bonds and the number of lone electron pairs. Hybridization involving lone electron pairs occurs in molecules formed by atoms of nitrogen, phosphorus, oxygen, sulfur, i.e. elements of the main subgroups of groups V and VI.
3. The geometric shape of the molecules is determined by the type of hybridization of the central atom (Table 5.3).
Table 5.3.
Bond angles, geometric shape of molecules depending on the number of hybrid orbitals and the type of hybridization of the central atom
5.2. Ionic bond
Ionic bonding occurs through electrostatic attraction between oppositely charged ions. These ions are formed as a result of the transfer of electrons from one atom to another. An ionic bond is formed between atoms that have large differences in electronegativity (usually greater than 1.7 on the Pauling scale), for example, between alkali metal and halogen atoms.
Let us consider the occurrence of an ionic bond using the example of the formation of NaCl. From the electronic formulas of the atoms Na 1s 2 2s 2 2p 6 3s 1 and Cl 1s 2 2s 2 2p 6 3s 2 3p 5 it is clear that to complete the outer level, it is easier for the sodium atom to give up one electron than to add seven, and it is easier for the chlorine atom to add one, than to give away seven. In chemical reactions, the sodium atom gives up one electron, and the chlorine atom takes it. As a result, the electronic shells of sodium and chlorine atoms are transformed into stable electronic shells of noble gases (the electronic configuration of the sodium cation is Na + 1s 2 2s 2 2p 6, and the electronic configuration of the chlorine anion Cl – - 1s 2 2s 2 2p 6 3s 2 3p 6). The electrostatic interaction of ions leads to the formation of a NaCl molecule.
Basic characteristics of ionic bonds and properties of ionic compounds
1. An ionic bond is a strong chemical bond. The energy of this bond is on the order of 300 – 700 kJ/mol.
2. Unlike a covalent bond, an ionic bond is non-directional, since an ion can attract ions of the opposite sign to itself in any direction.
3. Unlike a covalent bond, an ionic bond is unsaturated, since the interaction of ions of opposite sign does not lead to complete mutual compensation of their force fields.
4. During the formation of molecules with an ionic bond, complete transfer of electrons does not occur, therefore, one hundred percent ionic bonds do not exist in nature. In the NaCl molecule, the chemical bond is only 80% ionic.
5. Compounds with ionic bonds are crystalline solids that have high melting and boiling points.
6. Most ionic compounds are soluble in water. Solutions and melts of ionic compounds conduct electric current.
5.3. Metal connection
Metal atoms at the outer energy level contain a small number of valence electrons. Since the ionization energy of metal atoms is low, valence electrons are weakly retained in these atoms. As a result, positively charged ions and free electrons appear in the crystal lattice of metals. In this case, metal cations are located in the nodes of their crystal lattice, and electrons move freely in the field of positive centers forming the so-called “electron gas”. The presence of a negatively charged electron between two cations causes each cation to interact with this electron. Thus, metallic bonding is the bonding between positive ions in metal crystals, which occurs through the attraction of electrons moving freely throughout the crystal.
Since the valence electrons in a metal are evenly distributed throughout the crystal, a metallic bond, like an ionic bond, is a non-directional bond. Unlike a covalent bond, a metallic bond is an unsaturated bond. From covalent bond metal connection It also differs in strength. The energy of a metallic bond is approximately three to four times less than the energy of a covalent bond.
Due to the high mobility of the electron gas, metals are characterized by high electrical and thermal conductivity.
5.4. Hydrogen bond
In the molecules of the compounds HF, H 2 O, NH 3, there are hydrogen bonds with a strongly electronegative element (H–F, H–O, H–N). Between the molecules of such compounds can form intermolecular hydrogen bonds. In some organic molecules containing H–O, H–N bonds, intramolecular hydrogen bonds.
The mechanism of hydrogen bond formation is partly electrostatic, partly donor-acceptor in nature. In this case, the electron pair donor is an atom of a strongly electronegative element (F, O, N), and the acceptor is the hydrogen atoms connected to these atoms. Like covalent bonds, hydrogen bonds are characterized by focus in space and saturability.
Hydrogen bonds are usually denoted by dots: H ··· F. The stronger the hydrogen bond, the greater the electronegativity of the partner atom and the smaller its size. It is characteristic primarily of fluorine compounds, as well as oxygen, to a lesser extent nitrogen, and to an even lesser extent chlorine and sulfur. The energy of the hydrogen bond also changes accordingly (Table 5.4).
Table 5.4.
Average values of hydrogen bond energies
Intermolecular and intramolecular hydrogen bonding
Thanks to hydrogen bonds, molecules combine into dimers and more complex associates. For example, the formation of a formic acid dimer can be represented by the following diagram (Fig. 5.18).
Rice. 5.18. Formation of intermolecular hydrogen bonds in formic acid
Long chains of (H 2 O) n associates can appear in water (Fig. 5.19).
Rice. 5.19. Formation of a chain of associates in liquid water due to intermolecular hydrogen bonds
Each H2O molecule can form four hydrogen bonds, but an HF molecule can form only two.
Hydrogen bonds can occur both between different molecules (intermolecular hydrogen bonding) and within a molecule (intramolecular hydrogen bonding). Examples of the formation of intramolecular bonds for some organic substances are presented in Fig. 5.20.
Rice. 5.20. Formation of intramolecular hydrogen bonds in molecules of various organic compounds
The influence of hydrogen bonding on the properties of substances
The most convenient indicator of the existence of intermolecular hydrogen bonds is the boiling point of a substance. The higher boiling point of water (100 o C compared to hydrogen compounds of elements of the oxygen subgroup (H 2 S, H 2 Se, H 2 Te) is explained by the presence of hydrogen bonds: additional energy must be expended to destroy intermolecular hydrogen bonds in water.
Hydrogen bonding can significantly affect the structure and properties of substances. The existence of intermolecular hydrogen bonds increases the melting and boiling points of substances. The presence of an intramolecular hydrogen bond causes the deoxyribonucleic acid (DNA) molecule to be folded into a double helix in water.
Hydrogen bonding also plays an important role in dissolution processes, since solubility also depends on the ability of a compound to form hydrogen bonds with the solvent. As a result, substances containing OH groups such as sugar, glucose, alcohols, and carboxylic acids are, as a rule, highly soluble in water.
5.5. Types of crystal lattices
Solids usually have a crystalline structure. The particles that make up crystals (atoms, ions or molecules) are located at strictly defined points in space, forming a crystal lattice. The crystal lattice consists of elementary cells that retain the structural features characteristic of a given lattice. The points at which particles are located are called crystal lattice nodes. Depending on the type of particles located at the lattice sites and on the nature of the connection between them, 4 types of crystal lattices are distinguished.
5.5.1. Atomic crystal lattice
At the nodes of atomic crystal lattices there are atoms connected to each other by covalent bonds. Substances that have an atomic lattice include diamond, silicon, carbides, silicides, etc. In the structure of an atomic crystal it is impossible to isolate individual molecules; the entire crystal is considered as one giant molecule. The structure of diamond is shown in Fig. 5.21. Diamond is made up of carbon atoms, each of which is bonded to four neighboring atoms. Due to the fact that covalent bonds are strong, all substances with atomic lattices are refractory, hard and low-volatile. They are slightly soluble in water.
Rice. 5.21. Diamond crystal lattice
5.5.2. Molecular crystal lattice
At the nodes of molecular crystal lattices there are molecules connected to each other by weak intermolecular forces. Therefore, substances with a molecular lattice have low hardness, they are fusible, characterized by significant volatility, are slightly soluble in water, and their solutions, as a rule, do not conduct electric current. A lot of substances with a molecular crystal lattice are known. These are solid hydrogen, chlorine, carbon monoxide (IV) and other substances that are in a gaseous state at ordinary temperatures. Most crystalline organic compounds have a molecular lattice.
5.5.3. Ionic crystal lattice
Crystal lattices containing ions at their nodes are called ionic. They are formed by substances with ionic bonds, for example, alkali metal halides. In ionic crystals, individual molecules cannot be distinguished; the entire crystal can be considered as one macromolecule. The bonds between ions are strong, therefore substances with an ionic lattice have low volatility and high melting and boiling points. The crystal lattice of sodium chloride is shown in Fig. 5.22.
Rice. 5.22. Crystal lattice of sodium chloride
In this figure, the light balls are Na + ions, the dark balls are Cl – ions. On the left in Fig. Figure 5.22 shows the unit cell of NaCI.
5.5.4. Metal crystal lattice
Metals in the solid state form metallic crystal lattices. The sites of such lattices contain positive metal ions, and valence electrons move freely between them. The electrons electrostatically attract cations, thereby imparting stability to the metal lattice. This lattice structure determines the high thermal conductivity, electrical conductivity and plasticity of metals - during mechanical deformation there is no breaking of bonds and destruction of the crystal, since the ions that make it up seem to float in a cloud of electron gas. In Fig. Figure 5.23 shows the sodium crystal lattice.
Rice. 5.23. Sodium crystal lattice
Why can atoms combine with each other and form molecules? What is the reason for the possible existence of substances that contain atoms of completely different chemical elements? These are global questions affecting the fundamental concepts of modern physical and chemical science. You can answer them by having an idea of the electronic structure of atoms and knowing the characteristics of the covalent bond, which is the basic basis for most classes of compounds. The purpose of our article is to become familiar with the mechanisms of formation of various types of chemical bonds and compounds containing them in their molecules.
Electronic structure of the atom
Electrically neutral particles of matter, which are its structural elements, have a structure that mirrors the structure of the Solar system. Just as the planets revolve around the central star - the Sun, so the electrons in an atom move around a positively charged nucleus. To characterize a covalent bond, the electrons located at the last energy level and furthest from the nucleus will be significant. Since their connection with the center of their own atom is minimal, they can easily be attracted by the nuclei of other atoms. This is very important for the occurrence of interatomic interactions leading to the formation of molecules. Why is the molecular form the main type of existence of matter on our planet? Let's figure it out.
Basic property of atoms
The ability of electrically neutral particles to interact, leading to a gain in energy, is their most important feature. Indeed, under normal conditions, the molecular state of a substance is more stable than the atomic state. The basic principles of modern atomic-molecular science explain both the principles of molecular formation and the characteristics of covalent bonds. Let us recall that there can be from 1 to 8 electrons per atom; in the latter case, the layer will be complete, and therefore very stable. The atoms of noble gases: argon, krypton, xenon - inert elements that complete each period in D.I. Mendeleev’s system - have this structure of the external level. The exception here would be helium, which has not 8, but only 2 electrons at the last level. The reason is simple: in the first period there are only two elements, the atoms of which have a single electron layer. All other chemical elements have from 1 to 7 electrons on the last, incomplete layer. In the process of interaction with each other, the atoms will tend to be filled with electrons to the octet and restore the configuration of the atom of the inert element. This state can be achieved in two ways: by losing one’s own or accepting someone else’s negatively charged particles. These forms of interaction explain how to determine which bond - ionic or covalent - will arise between the atoms entering the reaction.
Mechanisms of formation of a stable electronic configuration
Let's imagine that two simple substances enter into a compound reaction: sodium metal and chlorine gas. A substance of the salt class is formed - sodium chloride. It has an ionic type of chemical bond. Why and how did it arise? Let us again turn to the structure of the atoms of the starting substances. Sodium has only one electron in the last layer, weakly bound to the nucleus due to the large radius of the atom. The ionization energy of all alkali metals, which includes sodium, is low. Therefore, the electron of the outer level leaves the energy level, is attracted by the nucleus of the chlorine atom and remains in its space. This sets a precedent for the Cl atom to become a negatively charged ion. Now we are no longer dealing with electrically neutral particles, but with charged sodium cations and chlorine anions. In accordance with the laws of physics, electrostatic attraction forces arise between them, and the compound forms an ionic crystal lattice. The mechanism of formation of an ionic type of chemical bond that we have considered will help to more clearly clarify the specifics and main characteristics of a covalent bond.
Common electron pairs
If an ionic bond occurs between atoms of elements that differ greatly in electronegativity, i.e., metals and nonmetals, then the covalent type appears during the interaction of atoms of both the same and different nonmetallic elements. In the first case, it is customary to talk about a nonpolar, and in the other, about a polar form of a covalent bond. The mechanism of their formation is common: each of the atoms partially gives up electrons for common use, which are combined in pairs. But the spatial arrangement of electron pairs relative to the atomic nuclei will be different. On this basis, types of covalent bonds are distinguished - non-polar and polar. Most often, in chemical compounds consisting of atoms of non-metallic elements, there are pairs consisting of electrons with opposite spins, i.e., rotating around their nuclei in opposite directions. Since the movement of negatively charged particles in space leads to the formation of electron clouds, which ultimately ends in their mutual overlap. What are the consequences of this process for atoms and what does it lead to?
Physical properties of covalent bond
It turns out that a two-electron cloud with a high density appears between the centers of two interacting atoms. The electrostatic forces of attraction between the negatively charged cloud itself and the nuclei of atoms increase. A portion of energy is released and the distances between atomic centers decrease. For example, at the beginning of the formation of the H 2 molecule, the distance between the nuclei of hydrogen atoms is 1.06 A, after the clouds overlap and the formation of a common electron pair - 0.74 A. Examples of covalent bonds formed according to the mechanism described above can be found among both simple and among complex inorganic substances. Its main distinguishing feature is the presence of common electron pairs. As a result, after the emergence of a covalent bond between atoms, for example, hydrogen, each of them acquires the electronic configuration of inert helium, and the resulting molecule has a stable structure.
Spatial shape of the molecule
Another very important physical property of a covalent bond is directionality. It depends on the spatial configuration of the molecule of the substance. For example, when two electrons overlap with a spherical cloud shape, the appearance of the molecule is linear (hydrogen chloride or hydrogen bromide). The shape of the water molecules in which the s- and p-clouds hybridize is angular, and the very strong particles of nitrogen gas have the shape of a pyramid.
The structure of simple substances - nonmetals
Having found out what kind of bond is called covalent, what characteristics it has, now is the time to understand its varieties. If atoms of the same non-metal - chlorine, nitrogen, oxygen, bromine, etc. - interact with each other, then the corresponding simple substances are formed. Their common electron pairs are located at the same distance from the centers of the atoms, without moving. Compounds with a non-polar type of covalent bond have the following characteristics: low boiling and melting points, insolubility in water, dielectric properties. Next, we will find out which substances are characterized by a covalent bond, in which a displacement of common electron pairs occurs.
Electronegativity and its effect on the type of chemical bond
The property of a certain element to attract electrons to itself from an atom of another element in chemistry is called electronegativity. The scale of values for this parameter, proposed by L. Pauling, can be found in all textbooks on inorganic and general chemistry. Fluorine has its highest value - 4.1 eV, other active non-metals have a smaller value, and the lowest value is characteristic of alkali metals. If elements that differ in their electronegativity react with each other, then inevitably one, more active, will attract negatively charged particles of the atom of a more passive element to its nucleus. Thus, the physical properties of a covalent bond directly depend on the ability of the elements to donate electrons for common use. The common pairs formed in this case are no longer located symmetrically relative to the nuclei, but are shifted towards the more active element.
Features of connections with polar coupling
Substances in whose molecules the shared electron pairs are asymmetrical with respect to the atomic nuclei include hydrogen halides, acids, compounds of chalcogens with hydrogen, and acid oxides. These are sulfate and nitrate acids, oxides of sulfur and phosphorus, hydrogen sulfide, etc. For example, a hydrogen chloride molecule contains one common electron pair formed by unpaired electrons of hydrogen and chlorine. It is shifted closer to the center of the Cl atom, which is a more electronegative element. All substances with polar bonds in aqueous solutions dissociate into ions and conduct electric current. The compounds we have given also have higher melting and boiling points compared to simple non-metallic substances.
Methods for breaking chemical bonds
In organic chemistry, saturated hydrocarbons and halogens follow a radical mechanism. A mixture of methane and chlorine reacts in light and at ordinary temperatures in such a way that chlorine molecules begin to split into particles carrying unpaired electrons. In other words, the destruction of the common electron pair and the formation of very active radicals -Cl are observed. They are able to influence methane molecules in such a way that they break the covalent bond between carbon and hydrogen atoms. An active species -H is formed, and the free valency of the carbon atom accepts a chlorine radical, and the first reaction product is chloromethane. This mechanism of molecular breakdown is called homolytic. If the common pair of electrons is completely transferred to one of the atoms, then they speak of a heterolytic mechanism, characteristic of reactions taking place in aqueous solutions. In this case, polar water molecules will increase the rate of destruction of the chemical bonds of the soluble compound.
Double and triple bonds
The vast majority of organic substances and some inorganic compounds contain not one, but several common electron pairs in their molecules. The multiplicity of covalent bonds reduces the distance between atoms and increases the stability of compounds. They are usually referred to as chemically resistant. For example, a nitrogen molecule has three pairs of electrons; they are designated in the structural formula by three dashes and determine its strength. The simple substance nitrogen is chemically inert and can only react with other compounds, such as hydrogen, oxygen or metals, when heated or under elevated pressure, or in the presence of catalysts.
Double and triple bonds are inherent in such classes of organic compounds as unsaturated diene hydrocarbons, as well as substances of the ethylene or acetylene series. Multiple bonds determine the basic chemical properties: addition and polymerization reactions that occur at the places where they are broken.
In our article, we gave a general description of covalent bonds and examined its main types.
Main function telecommunication networks (TCN) is to ensure information exchange between all subscriber systems of a computer network. The exchange is carried out through communication channels, which constitute one of the main components of telecommunication networks.
A communication channel is a combination of a physical medium (communication line) and data transmission equipment (DTE) that transmits information signals from one network switching node to another or between nodes switching and subscriber system.
Thus, communication channel and physical communication line are not the same thing. In general, several logical channels can be organized on the basis of one communication line by means of time, frequency, phase and other types of separation.
Used in computer networks telephone, telegraph, television, satellite communication networks. Wired (aerial), cable, radio channels of terrestrial and satellite communications are used as communication lines. The difference between them is determined by the data transmission medium. The physical medium of data transmission can be a cable, as well as the earth's atmosphere or outer space through which electromagnetic waves propagate.
Computer networks use telephone, telegraph, television, and satellite communication networks. Wired (aerial), cable, radio channels of terrestrial and satellite communications are used as communication lines. The difference between them is determined by the data transmission medium. The physical medium of data transmission can be a cable, as well as the earth's atmosphere or outer space through which electromagnetic waves propagate.
Wired (overhead) communication lines- these are wires without insulating or shielding braids, laid between poles and hanging in the air. Traditionally they are used to transmit telephone and telegraph signals, but in the absence of other possibilities they are used to transmit computer data. Wired communication lines are characterized by low bandwidth and low noise immunity, so they are quickly being replaced by cable lines.
Cable lines include a cable consisting of conductors with several layers of insulation - electrical, electromagnetic, mechanical, and connectors for connecting various equipment to it. Cable networks mainly use three types of cable: a cable based on twisted pairs of copper wires (this is a twisted pair in a shielded version, when a pair of copper wires is wrapped in an insulating screen, and unshielded, when there is no insulating wrapper), coaxial cable (consists of an internal copper core and braiding, separated from the core by a layer of insulation) and fiber-optic cable (consists of thin - 5-60 microns fibers, through which light signals propagate).
Among cable communication lines Light guides have the best performance. Their main advantages: high throughput (up to 10 Gbit/s and higher), due to the use of electromagnetic waves in the optical range; insensitivity to external electromagnetic fields and the absence of its own electromagnetic radiation, low labor intensity of laying an optical cable; spark, explosion and fire safety; increased resistance to aggressive environments; low specific gravity (ratio of linear mass to bandwidth); wide areas of application (creation of public access highways, communication systems between computers and peripheral devices of local networks, in microprocessor technology, etc.).
Disadvantages of fiber optic communication lines: connecting additional computers to the light guide significantly weakens the signal; high-speed modems required for light guides are still expensive; light guides connecting computers must be equipped with converters of electrical signals to light and vice versa.
Terrestrial and satellite radio channels are formed using a transmitter and receiver of radio waves. Different types of radio channels differ in the frequency range used and the range of information transmission. Radio channels operating in the short, medium and long wave bands (HF, MF, DV) provide long-distance communication, but at a low data transfer rate. These are radio channels that use amplitude modulation of signals. Channels operating on ultrashort waves (VHF) are faster and are characterized by frequency modulation of signals. Ultra-high-speed channels are those operating in ultra-high frequency (microwave) ranges, i.e. over 4 GHz. In the microwave range, signals are not reflected by the Earth's ionosphere, so stable communication requires direct visibility between the transmitter and receiver. For this reason, microwave signals are used either in satellite channels or in radio relays, where this condition is met.
Characteristics of communication lines. The main characteristics of communication lines include the following: amplitude-frequency response, bandwidth, attenuation, throughput, noise immunity, crosstalk at the near end of the line, reliability of data transmission, unit cost.
The characteristics of a communication line are often determined by analyzing its responses to certain reference influences, which are sinusoidal oscillations of various frequencies, since they are often encountered in technology and can be used to represent any function of time. The degree of distortion of sinusoidal signals of a communication line is assessed using the amplitude-frequency response, bandwidth and attenuation at a certain frequency.
Amplitude-frequency response(Afrequency response) gives the most complete picture of the communication line; it shows how the amplitude of the sinusoid at the output of the line attenuates compared to the amplitude at its input for all possible frequencies of the transmitted signal (instead of the amplitude of the signal, its power is often used). Consequently, the frequency response allows you to determine the shape of the output signal for any input signal. However, it is very difficult to obtain the frequency response of a real communication line, so in practice other, simplified characteristics are used instead - bandwidth and attenuation.
Communication bandwidth represents a continuous range of frequencies over which the ratio of the amplitude of the output signal to the input signal exceeds a predetermined limit (usually 0.5). Therefore, bandwidth determines the range of frequencies of a sinusoidal signal at which this signal is transmitted over a communication line without significant distortion. The bandwidth that most influences the maximum possible speed of information transmission along a communication line is the difference between the maximum and minimum frequencies of the sinusoidal signal in a given bandwidth. The bandwidth depends on the type of line and its length.
Distinctions should be made between bandwidth and the width of the spectrum of transmitted information signals. The spectrum width of the transmitted signals is the difference between the maximum and minimum significant harmonics of the signal, i.e. those harmonics that make the main contribution to the resulting signal. If significant signal harmonics fall within the line passband, then such a signal will be transmitted and received by the receiver without distortion. Otherwise, the signal will be distorted, the receiver will make mistakes when recognizing information, and, therefore, information will not be able to be transmitted with the given bandwidth.
Attenuation is a relative decrease in the amplitude or power of a signal when transmitting a signal of a certain frequency along a line.
Attenuation A is measured in decibels (dB, dB) and is calculated by the formula:
A = 10?lg(P out / P in)
where P out, P in - signal power at the output and input of the line, respectively.
For a rough estimate distortion of signals transmitted along the line, it is enough to know the attenuation of the fundamental frequency signals, i.e. frequency whose harmonic has the greatest amplitude and power. A more accurate estimate is possible if we know the attenuation at several frequencies close to the main one.
The throughput of a communication line is its characteristic, which determines (like the bandwidth) the maximum possible data transfer rate along the line. It is measured in bits per second (bps), as well as in derived units (Kbps, Mbps, Gbps).
Bandwidth a communication line depends on its characteristics (frequency response, bandwidth, attenuation) and on the spectrum of transmitted signals, which, in turn, depends on the chosen method of physical or linear coding (i.e., on the method of representing discrete information in the form of signals). For one coding method, a line may have one capacity, and for another, another.
When encoding usually a change in some parameter of a periodic signal (for example, sinusoidal oscillations) is used - frequency, amplitude and phase, sinusoids or the sign of the pulse sequence potential. A periodic signal whose parameters change is called a carrier signal or carrier frequency if a sinusoid is used as such a signal. If the received sinusoid does not change any of its parameters (amplitude, frequency or phase), then it does not carry any information.
The number of changes in the information parameter of a periodic carrier signal per second (for a sinusoid this is the number of changes in amplitude, frequency or phase) is measured in baud. The transmitter operating cycle is the period of time between adjacent changes in the information signal.
In general The line capacity in bits per second is not the same as the baud rate. Depending on the encoding method, it may be higher, equal to or lower than the baud number. If, for example, with this coding method, a single bit value is represented by a pulse of positive polarity, and a zero value by a pulse of negative polarity, then when transmitting alternately changing bits (there are no series of bits of the same name), the physical signal changes its state twice during the transmission of each bit. Therefore, with this encoding, the line capacity is half the number of bauds transmitted along the line.
For throughput line is affected not only by physical, but also by so-called logical encoding, which is performed before physical encoding and consists of replacing the original sequence of information bits with a new sequence of bits that carries the same information, but has additional properties (for example, the ability for the receiving side to detect errors in received data or ensure the confidentiality of transmitted data by encrypting it). Logical coding, as a rule, is accompanied by the replacement of the original bit sequence with a longer sequence, which negatively affects the transmission time of useful information.
There is a certain connection between the capacity of a line and its bandwidth. With a fixed physical encoding method, the line capacity increases with increasing frequency of the periodic carrier signal, since this increase is accompanied by an increase in information transmitted per unit time. But as the frequency of this signal increases, the width of its spectrum also increases, which is transmitted with distortions determined by the bandwidth of the line. The greater the discrepancy between the line bandwidth and the spectrum width of the transmitted information signals, the more the signals are subject to distortion and the more likely errors are in the recognition of information by the receiver. As a result, the speed of information transfer turns out to be less than expected.
C=2F log 2 M, (4)
where M is the number of different states of the information parameter of the transmitted signal.
The Nyquist relation, which is also used to determine the maximum possible throughput of a communication line, does not explicitly take into account the presence of noise on the line. However, its influence is indirectly reflected in the choice of the number of states of the information signal. For example, to increase the throughput of a line, it was possible to use not 2 or 4 levels, but 16, when encoding data. But if the noise amplitude exceeds the difference between adjacent 16 levels, then the receiver will not be able to consistently recognize the transmitted data. Therefore, the number of possible signal states is effectively limited by the ratio of signal power to noise.
The Nyquist formula determines the limiting value of the channel capacity for the case when the number of states of the information signal has already been selected taking into account the capabilities of their stable recognition by the receiver.
Noise immunity of the communication line- this is its ability to reduce the level of interference created in the external environment on internal conductors. It depends on the type of physical medium used, as well as on the line equipment that screens and suppresses interference. The most noise-resistant and insensitive to external electromagnetic radiation are fiber-optic lines, the least noise-resistant are radio lines, and cable lines occupy an intermediate position. Reducing interference caused by external electromagnetic radiation is achieved by shielding and twisting the conductors.
Crosstalk at the near end of the line - determines the cable's noise immunity to internal sources of interference. They are usually assessed in relation to a cable consisting of several twisted pairs, when the mutual interference of one pair to another can reach significant values and create internal interference commensurate with the useful signal.
Reliability of data transmission(or bit error rate) characterizes the probability of corruption for each transmitted bit of data. The reasons for the distortion of information signals are interference on the line, as well as limited bandwidth. Therefore, increasing the reliability of data transmission is achieved by increasing the degree of noise immunity of the line, reducing the level of crosstalk in the cable, and using more broadband communication lines.
For conventional cable communication lines without additional means of error protection, the reliability of data transmission is, as a rule, 10 -4 -10 -6. This means that on average, out of 10 4 or 10 6 transmitted bits, the value of one bit will be distorted.
Communication line equipment(data transmission equipment - ATD) is edge equipment that directly connects computers to the communication line. It is part of the communication line and usually operates at the physical level, ensuring the transmission and reception of a signal of the required shape and power. Examples of ADFs are modems, adapters, analog-to-digital and digital-to-analog converters.
The ADF does not include the user's data terminal equipment (DTE), which generates data for transmission over the communication line and is connected directly to the ADF. A DTE includes, for example, a local network router. Note that the division of equipment into APD and DOD classes is quite arbitrary.
On communication lines over long distances, intermediate equipment is used, which solves two main problems: improving the quality of information signals (their shape, power, duration) and creating a permanent composite channel (end-to-end channel) for communication between two network subscribers. In a LCS, intermediate equipment is not used if the length of the physical medium (cables, radio air) is short, so that signals from one network adapter to another can be transmitted without intermediate restoration of their parameters.
Global networks ensure high-quality transmission of signals over hundreds and thousands of kilometers. Therefore, amplifiers are installed at certain distances. To create an end-to-end line between two subscribers, multiplexers, demultiplexers and switches are used.
The intermediate equipment of the communication channel is transparent to the user (he does not notice it), although in reality it forms a complex network, called the primary network, which serves as the basis for building computer, telephone and other networks.
Distinguish analog And digital communication lines, which use various types of intermediate equipment. In analog lines, intermediate equipment is designed to amplify analog signals having a continuous range of values. In high-speed analog channels, a technique of frequency multiplexing is implemented, when several low-speed analog subscriber channels are multiplexed into one high-speed channel. In digital communication channels, where rectangular information signals have a finite number of states, intermediate equipment improves the shape of the signals and restores their repetition period. It provides the formation of high-speed digital channels, working on the principle of time multiplexing of channels, when each low-speed channel is allocated a certain share of the time of the high-speed channel.
When transmitting discrete computer data over digital communication lines, the physical layer protocol is defined, since the parameters of the information signals transmitted by the line are standardized, but when transmitting over analog lines, it is not defined, since the information signals have an arbitrary shape and there is nothing to do with the method of representing ones and zeros by data transmission equipment. there are no requirements.
The following have found application in communication networks: re information transfer presses :
Simplex, when the transmitter and receiver are connected by one communication channel, through which information is transmitted only in one direction (this is typical for television communication networks);
Half-duplex, when two communication nodes are also connected by one channel, through which information is transmitted alternately in one direction and then in the opposite direction (this is typical for information-reference, request-response systems);
Duplex, when two communication nodes are connected by two channels (a forward communication channel and a reverse channel), through which information is simultaneously transmitted in opposite directions. Duplex channels are used in systems with decision and information feedback.
Switched and dedicated communication channels. In TSS, a distinction is made between dedicated (non-switched) communication channels and those with switching for the duration of information transmission over these channels.
When using dedicated communication channels, the transceiver equipment of communication nodes is constantly connected to each other. This ensures a high degree of readiness of the system for information transmission, higher quality of communication, and support for a large volume of traffic. Due to the relatively high costs of operating networks with dedicated communication channels, their profitability is achieved only if the channels are sufficiently fully loaded.
For switched communication channels, created only for the duration of the transfer of a fixed amount of information, they are characterized by high flexibility and relatively low cost (with a small volume of traffic). Disadvantages of such channels: loss of time for switching (to establish communication between subscribers), the possibility of blocking due to the occupancy of certain sections of the communication line, lower quality of communication, high cost with a significant volume of traffic.
Electronegativity is the ability of atoms to displace electrons in their direction when forming a chemical bond. This concept was introduced by the American chemist L. Pauling (1932). Electronegativity characterizes the ability of an atom of a given element to attract a common electron pair in a molecule. Electronegativity values determined by various methods differ from each other. In educational practice, they most often use relative rather than absolute values of electronegativity. The most common is a scale in which the electronegativity of all elements is compared with the electronegativity of lithium, taken as one.
Among the elements of groups IA - VIIA:
The patterns of changes in electronegativity among d-block elements are much more complex.electronegativity, as a rule, increases in periods (“from left to right”) with increasing atomic number, and decreases in groups (“from top to bottom”).
These include: hydrogen, carbon, nitrogen, phosphorus, oxygen, sulfur, selenium, fluorine, chlorine, bromine and iodine. According to a number of characteristics, a special group of noble gases (helium-radon) is also classified as nonmetals.Elements with high electronegativity, the atoms of which have high electron affinity and high ionization energy, i.e., prone to the addition of an electron or the displacement of a pair of bonding electrons in their direction, are called nonmetals.
Metals include most of the elements of the Periodic Table.
This set of physical properties that distinguish metals from non-metals is explained by the special type of bond that exists in metals. All metals have a clearly defined crystal lattice. Along with atoms, its nodes contain metal cations, i.e. atoms that have lost their electrons. These electrons form a socialized electron cloud, the so-called electron gas. These electrons are in the force field of many nuclei. This bond is called metallic. The free migration of electrons throughout the volume of the crystal determines the special physical properties of metals.Metals are characterized by low electronegativity, i.e., low ionization energy and electron affinity. Metal atoms either donate electrons to nonmetal atoms or mix pairs of bonding electrons from themselves. Metals have a characteristic luster, high electrical conductivity and good thermal conductivity. They are mostly durable and malleable.
Metals include all d and f elements. If from the Periodic Table you mentally select only blocks of s- and p-elements, i.e., elements of group A and draw a diagonal from the upper left corner to the lower right corner, then it turns out that non-metallic elements are located on the right side of this diagonal, and metallic ones - in the left. Adjacent to the diagonal are elements that cannot be unambiguously classified as either metals or non-metals. These intermediate elements include: boron, silicon, germanium, arsenic, antimony, selenium, polonium and astatine.
Ideas about covalent and ionic bonds played an important role in the development of ideas about the structure of matter, however, the creation of new physical and chemical methods for studying the fine structure of matter and their use showed that the phenomenon of chemical bonding is much more complex. It is currently believed that any heteroatomic bond is both covalent and ionic, but in different proportions. Thus, the concept of covalent and ionic components of a heteroatomic bond is introduced. The greater the difference in electronegativity of the bonding atoms, the greater the polarity of the bond. When the difference is more than two units, the ionic component is almost always predominant. Let's compare two oxides: sodium oxide Na 2 O and chlorine oxide (VII) Cl 2 O 7. In sodium oxide, the partial charge on the oxygen atom is -0.81, and in chlorine oxide -0.02. This effectively means that the Na-O bond is 81% ionic and 19% covalent. The ionic component of the Cl-O bond is only 2%.
List of used literature
- Popkov V. A., Puzakov S. A. General chemistry: textbook. - M.: GEOTAR-Media, 2010. - 976 pp.: ISBN 978-5-9704-1570-2. [With. 35-37]
- Volkov, A.I., Zharsky, I.M. Big chemical reference book / A.I. Volkov, I.M. Zharsky. - Mn.: Modern School, 2005. - 608 with ISBN 985-6751-04-7.