Chemical bond

All interactions leading to the combination of chemical particles (atoms, molecules, ions, etc.) into substances are divided into chemical bonds and intermolecular bonds (intermolecular interactions).

Chemical bonds- bonds directly between atoms. There are ionic, covalent and metallic bonds.

Intermolecular bonds- connections between molecules. These are hydrogen bonds, ion-dipole bonds (due to the formation of this bond, for example, the formation of a hydration shell of ions occurs), dipole-dipole (due to the formation of this bond, molecules of polar substances are combined, for example, in liquid acetone), etc.

Ionic bond- a chemical bond formed due to the electrostatic attraction of oppositely charged ions. In binary compounds (compounds of two elements), it is formed when the sizes of the bonded atoms are very different from each other: some atoms are large, others are small - that is, some atoms easily give up electrons, while others tend to accept them (usually these are atoms of the elements that form typical metals and atoms of elements forming typical nonmetals); the electronegativity of such atoms is also very different.
Ionic bonding is non-directional and non-saturable.

Covalent bond- a chemical bond that occurs due to the formation of a common pair of electrons. A covalent bond is formed between small atoms with the same or similar radii. A necessary condition is the presence of unpaired electrons in both bonded atoms (exchange mechanism) or a lone pair in one atom and a free orbital in the other (donor-acceptor mechanism):

A)	H· + ·H H:H	H-H	H 2	(one shared pair of electrons; H is monovalent);
b)		NN	N 2	(three shared pairs of electrons; N is trivalent);
V)		H-F	HF	(one shared pair of electrons; H and F are monovalent);
G)			NH4+	(four shared pairs of electrons; N is tetravalent)

Based on the number of shared electron pairs, covalent bonds are divided into
• simple (single)- one pair of electrons,
• double- two pairs of electrons,
• triples- three pairs of electrons.

Double and triple bonds are called multiple bonds.

According to the distribution of electron density between the bonded atoms, a covalent bond is divided into non-polar And polar. A non-polar bond is formed between identical atoms, a polar one - between different ones.

Electronegativity- a measure of the ability of an atom in a substance to attract common electron pairs.
The electron pairs of polar bonds are shifted towards more electronegative elements. The displacement of electron pairs itself is called bond polarization. The partial (excess) charges formed during polarization are designated + and -, for example: .

Based on the nature of the overlap of electron clouds ("orbitals"), a covalent bond is divided into -bond and -bond.
-A bond is formed due to the direct overlap of electron clouds (along the straight line connecting the atomic nuclei), -a bond is formed due to lateral overlap (on both sides of the plane in which the atomic nuclei lie).

A covalent bond is directional and saturable, as well as polarizable.
The hybridization model is used to explain and predict the mutual direction of covalent bonds.

Hybridization of atomic orbitals and electron clouds- the supposed alignment of atomic orbitals in energy, and electron clouds in shape when an atom forms covalent bonds.
The three most common types of hybridization are: sp-, sp 2 and sp 3 -hybridization. For example:
sp-hybridization - in molecules C 2 H 2, BeH 2, CO 2 (linear structure);
sp 2-hybridization - in molecules C 2 H 4, C 6 H 6, BF 3 (flat triangular shape);
sp 3-hybridization - in molecules CCl 4, SiH 4, CH 4 (tetrahedral form); NH 3 (pyramidal shape); H 2 O (angular shape).

Metal connection- a chemical bond formed by sharing the valence electrons of all bonded atoms of a metal crystal. As a result, a single electron cloud of the crystal is formed, which easily moves under the influence of electrical voltage - hence the high electrical conductivity of metals.
A metallic bond is formed when the atoms being bonded are large and therefore tend to give up electrons. Simple substances with a metallic bond are metals (Na, Ba, Al, Cu, Au, etc.), complex substances are intermetallic compounds (AlCr 2, Ca 2 Cu, Cu 5 Zn 8, etc.).
The metal bond does not have directionality or saturation. It is also preserved in metal melts.

Hydrogen bond- an intermolecular bond formed due to the partial acceptance of a pair of electrons from a highly electronegative atom by a hydrogen atom with a large positive partial charge. It is formed in cases where one molecule contains an atom with a lone pair of electrons and high electronegativity (F, O, N), and the other contains a hydrogen atom bound by a highly polar bond to one of such atoms. Examples of intermolecular hydrogen bonds:

H—O—H OH 2 , H—O—H NH 3 , H—O—H F—H, H—F H—F.

Intramolecular hydrogen bonds exist in the molecules of polypeptides, nucleic acids, proteins, etc.

A measure of the strength of any bond is the bond energy.
Communication energy- the energy required to break a given chemical bond in 1 mole of a substance. The unit of measurement is 1 kJ/mol.

The energies of ionic and covalent bonds are of the same order, the energy of hydrogen bonds is an order of magnitude lower.

The energy of a covalent bond depends on the size of the bonded atoms (bond length) and on the multiplicity of the bond. The smaller the atoms and the greater the bond multiplicity, the greater its energy.

The ionic bond energy depends on the size of the ions and their charges. The smaller the ions and the greater their charge, the greater the binding energy.

Structure of matter

According to the type of structure, all substances are divided into molecular And non-molecular. Among organic substances, molecular substances predominate, among inorganic substances, non-molecular substances predominate.

Based on the type of chemical bond, substances are divided into substances with covalent bonds, substances with ionic bonds (ionic substances) and substances with metallic bonds (metals).

Substances with covalent bonds can be molecular or non-molecular. This significantly affects their physical properties.

Molecular substances consist of molecules connected to each other by weak intermolecular bonds, these include: H 2, O 2, N 2, Cl 2, Br 2, S 8, P 4 and other simple substances; CO 2, SO 2, N 2 O 5, H 2 O, HCl, HF, NH 3, CH 4, C 2 H 5 OH, organic polymers and many other substances. These substances do not have high strength, have low melting and boiling points, do not conduct electricity, and some of them are soluble in water or other solvents.

Non-molecular substances with covalent bonds or atomic substances (diamond, graphite, Si, SiO 2, SiC and others) form very strong crystals (with the exception of layered graphite), they are insoluble in water and other solvents, have high melting and boiling points, most of them they do not conduct electric current (except for graphite, which is electrically conductive, and semiconductors - silicon, germanium, etc.)

All ionic substances are naturally non-molecular. These are solid, refractory substances, solutions and melts of which conduct electric current. Many of them are soluble in water. It should be noted that in ionic substances, the crystals of which consist of complex ions, there are also covalent bonds, for example: (Na +) 2 (SO 4 2-), (K +) 3 (PO 4 3-), (NH 4 + )(NO 3-), etc. The atoms that make up complex ions are connected by covalent bonds.

Metals (substances with metallic bonds) very diverse in their physical properties. Among them there are liquid (Hg), very soft (Na, K) and very hard metals (W, Nb).

The characteristic physical properties of metals are their high electrical conductivity (unlike semiconductors, it decreases with increasing temperature), high heat capacity and ductility (for pure metals).

In the solid state, almost all substances are composed of crystals. Based on the type of structure and type of chemical bond, crystals (“crystal lattices”) are divided into atomic(crystals of non-molecular substances with covalent bonds), ionic(crystals of ionic substances), molecular(crystals of molecular substances with covalent bonds) and metal(crystals of substances with a metallic bond).

Tasks and tests on the topic "Topic 10. "Chemical bonding. Structure of matter."

• Types of chemical bond - Structure of matter grade 8–9

  Lessons: 2 Assignments: 9 Tests: 1

• Assignments: 9 Tests: 1

After working through this topic, you should understand the following concepts: chemical bond, intermolecular bond, ionic bond, covalent bond, metallic bond, hydrogen bond, simple bond, double bond, triple bond, multiple bonds, non-polar bond, polar bond, electronegativity, bond polarization , - and -bond, hybridization of atomic orbitals, binding energy.

You must know the classification of substances by type of structure, by type of chemical bond, the dependence of the properties of simple and complex substances on the type of chemical bond and the type of “crystal lattice”.

You must be able to: determine the type of chemical bond in a substance, the type of hybridization, draw up diagrams of bond formation, use the concept of electronegativity, a number of electronegativity; know how electronegativity changes in chemical elements of the same period and one group to determine the polarity of a covalent bond.

After making sure that everything you need has been learned, proceed to completing the tasks. We wish you success.

Recommended reading:

• O. S. Gabrielyan, G. G. Lysova. Chemistry 11th grade. M., Bustard, 2002.
• G. E. Rudzitis, F. G. Feldman. Chemistry 11th grade. M., Education, 2001.

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.

Of exceptionally great importance in biological systems is a special type of intermolecular interaction, the hydrogen bond, which occurs between hydrogen atoms chemically combined in one molecule and the electronegative atoms F, O, N, Cl, S belonging to another molecule. The concept of "hydrogen bond" was first introduced in 1920 by Latimer and Rodebush to explain the properties of water and other associated substances. Let's look at some examples of such a connection.

In paragraph 5.2 we talked about the pyridine molecule and it was noted that the nitrogen atom in it has two outer electrons with antiparallel spins that do not participate in the formation of a chemical bond. This "free" or "lone" pair of electrons will attract the proton and form a chemical bond with it. In this case, the pyridine molecule will go into an ionic state. If there are two pyridine molecules, they will compete to capture a proton, resulting in a compound

in which three dots indicate a new type of intermolecular interaction called hydrogen bonding. In this compound, the proton is closer to the left-handed nitrogen atom. With the same success, the proton may be closer to the right nitrogen atom. Therefore, the potential energy of a proton as a function of the distance to the right or left nitrogen atom at a fixed distance between them (approximately ) should be depicted by a curve with two minima. A quantum mechanical calculation of such a curve, carried out by Rhine and Harris, is shown in Fig. 4.

The quantum mechanical theory of the A-H...B hydrogen bond based on donor-acceptor interactions was one of the first to be developed by N. D. Sokolov. The reason for the bond is the redistribution of electron density between atoms A and B caused by the proton. Briefly, they say that a “lone pair” of electrons is shared. In fact, in

Rice. 4. Potential curve of proton energy as a function of the distance between the nitrogen atoms of two pyridine molecules.

Other electrons of molecules also participate in the formation of potential hydrogen bond curves, although to a lesser extent (see below).

Typical hydrogen bond energies range from 0.13 to 0.31 eV. It is an order of magnitude less than the energy of chemical covalent bonds, but an order of magnitude greater than the energy of van der Waals interactions.

The simplest intermolecular complex formed by hydrogen bonding is the complex. This complex has a linear structure. The distance between fluorine atoms is 2.79 A. The distance between atoms in a polar molecule is 0.92 A. When a complex is formed, an energy of about 0.26 eV is released.

With the help of hydrogen bonding, a water dimer is formed with a binding energy of about 0.2 eV. This energy is approximately one-twentieth the energy of the OH covalent bond. The distance between two oxygen atoms in the complex is approximately 2.76 A. It is less than the sum of the van der Waals radii of oxygen atoms, equal to 3.06 A. In Fig. Figure 5 shows the change in the electron density of water atoms calculated in the work during the formation of the complex. These calculations confirm that when a complex is formed, the distribution of electron density around all atoms of the reacting molecules changes.

The role of all atoms in the establishment of hydrogen bonds in the complex can also be judged by the mutual influence of two hydrogen bonds between the nitrogenous bases, thymine and adenine, that are part of the double helix of the DNA molecule. The location of the minima of the proton potential curves in two bonds reflects their mutual correlation (Fig. 6).

Along with the usual or weak hydrogen bond formed by hydrogen with an energy release of less than 1 eV, and characterized by a potential energy with two minima, hydrogen forms some complexes with a large energy release. For example, when creating a complex, an energy of 2.17 eV is released. This type of interaction is called strong

Rice. 5. Change in electron density around atoms in a complex formed by hydrogen bonds from two water molecules.

The charge of the electron is assumed to be equal to unity. In a free water molecule, the charge of 10 electrons is distributed so that near the oxygen atom there is a charge of 8.64, and at the hydrogen atoms

Rice. 6. Hydrogen bonds between nitrogenous bases: a - thymine (T) and adenip (A), which are part of the DNN molecules (arrows indicate the places of attachment of the bases to the chains of sugar and phosphoric acid molecules); - potential hydrogen bond curves; O - oxygen; - hydrogen; - carbon; - nitrogen.

hydrogen bond. When complexes with strong hydrogen bonds are formed, the configuration of the molecules changes significantly. The proton's potential energy has one relatively flat minimum located approximately at the center of the bond. Therefore, the proton is easily displaced. The easy displacement of the proton under the influence of an external field determines the high polarizability of the complex.

Strong hydrogen bonding does not occur in biological systems. As for the weak hydrogen bond, it is of decisive importance in all living organisms.

The exceptionally large role of hydrogen bonding in biological systems is due primarily to the fact that it determines the secondary structure of proteins, which is of fundamental importance for all life processes; with the help of hydrogen bonds, base pairs are held in DNA molecules and their stable structure in the form of double helices is ensured, and, finally, hydrogen bonds are responsible for the very unusual properties of water, which are important for the existence of living systems.

Water is one of the main components of all living things. Animal bodies are almost two-thirds water. The human embryo contains about 93% water during the first month. There would be no running water. Water serves as the main medium in which biochemical reactions occur in the cell. It forms the liquid part of blood and lymph. Water is necessary for digestion, since the breakdown of carbohydrates, proteins and fats occurs with the addition of water molecules. Water is released in the cell when proteins are built from amino acids. Physiological

Rice. 7. Ice structure. Each water molecule is connected by hydrogen bonds (three points) to four water molecules located at the vertices of the tetrahedron.

Rice. 8. Hydrogen bond in a dimer and “linear” hydrogen bond

the properties of biopolymers and many supramolecular structures (in particular, cell membranes) very significantly depend on their interaction with water.

Let's look at some properties of water. Each water molecule has a large electrical moment. Due to the high electronegativity of oxygen atoms, a water molecule can form hydrogen bonds with one, two, three, or four other water molecules. The result is relatively stable dimers and other polymer complexes. On average, each molecule in liquid water has four neighbors. The composition and structure of intermolecular complexes depend on water temperature.

Crystalline water (ice) has the most ordered structure at normal pressure and temperature below zero degrees Celsius. Its crystals have a hexagonal structure. The unit cell contains four water molecules. The cell structure is shown in Fig. 7. Around the central oxygen atom there are four other oxygen atoms located at the vertices of a regular tetrahedron at distances of 2.76 A. Each water molecule is connected to its neighbors by four hydrogen bonds. In this case, the angle between OH bonds in the molecule approaches the “tetrahedral” value of 109.1°. In a free molecule it is approximately 105°.

The structure of ice resembles that of diamond. However, in diamond there are chemical forces between the carbon atoms. A diamond crystal is a large molecule. Ice crystals are classified as molecular crystals. The molecules in a crystal retain essentially their individuality and hold each other together through hydrogen bonds.

Rice. 9. Experimental value of the shift in infrared vibration frequency in water during the formation of a hydrogen bond at an angle .

The ice lattice is very loose and contains many “voids”, since the number of nearest water molecules for each molecule (coordination number) is only four. When melting, the ice lattice is partially destroyed, at the same time some voids are filled and the density of water becomes greater than the density of ice. This is one of the main water anomalies. With further heating to 4° C, the compaction process continues. When heated above 4° C, the amplitude of anharmonic vibrations increases, the number of associated molecules in complexes (swarms) decreases, and the density of water decreases. According to rough estimates, the swarms at room temperature include about 240 molecules, at 37° C - about 150, at 45 and 100° C, 120 and 40, respectively.

The contribution of hydrogen bonding to the total energy of intermolecular interactions (11.6 kcal/mol) is about 69%. Due to hydrogen bonds, the melting points (0° C) and boiling points (100° C) of water differ significantly from the melting and boiling points of other molecular liquids, between the molecules of which only van der Waals forces act. For example, for methane these values ​​are respectively -186 and -161° C.

In liquid water, along with the remnants of the tetrahedral structure of ice, there are linear and cyclic dimers and other complexes containing 3, 4, 5, 6 or more molecules. It is important that the angle P formed between the OH bond and the hydrogen bond changes depending on the number of molecules in the cycle (Fig. 8). In a dimer this angle is 110°, in a five-membered ring it is 10°, and in a six-membered ring and hexagonal ice structure it is close to a bullet (“linear” hydrogen bond).

It turns out that the highest energy of one hydrogen bond corresponds to the angle. The energy of a hydrogen bond is proportional (Badger-Bauer rule) to the shift in the frequency of stretching infrared vibrations of the OH group in a water molecule compared to the vibration frequency of a free molecule. The maximum displacement is observed in the case of a “linear” hydrogen bond. In a water molecule in this case, the frequency decreases by , and the frequency decreases by . In Fig. Figure 9 shows a graph of the displacement ratio

frequency to maximum offset from angle . Consequently, this graph also characterizes the dependence of the hydrogen bond energy on the angle . This dependence is a manifestation of the cooperative nature of the hydrogen bond.

Multiple attempts have been made to theoretically calculate the structure and properties of water, taking into account hydrogen bonds and other intermolecular interactions. According to statistical physics, the thermodynamic properties of a system of interacting molecules located in volume V at constant pressure P in statistical equilibrium with a thermostat are determined through the partition function of states

Here V is the volume of the system; k - Boltzmann constant; T - absolute temperature; means that we need to take the trace of the statistical operator in curly brackets, where H is the quantum operator of the energy of the entire system. This operator is equal to the sum of the kinetic energy operators of the translational and rotational motions of molecules and the potential energy operator of the interaction of all molecules.

If all the eigenfunctions and the full energy spectrum E of the operator H are known, then (6.2) takes the form

Then the Gibbs free energy G of the system at pressure P and temperature T is determined by the simple expression

Knowing the Gibbs free energy, we find the total energy entropy volume.

Unfortunately, due to the complex nature of interactions between molecules in water (anisotropic dipole molecules, hydrogen bonds leading to complexes of variable composition, in which the energy of hydrogen bonds itself depends on the composition and structure of the complex, etc.), we cannot write the operator H in explicitly. Therefore, we have to resort to very large simplifications. Thus, Nameti and Scheraga calculated the partition function based on the fact that only five energy states of molecules in complexes can be taken into account, according to

with the number of hydrogen bonds they form (0, 1, 2, 3, 4) with neighboring molecules. Using this model, they even managed to show that the density of water is maximum at 4° C. However, later the authors themselves criticized the theory they developed, since it did not describe many experimental facts. Other attempts at theoretical calculations of the structure of water can be found in the review by Ben-Naim and Stillinger.

Due to the dipole nature of water molecules and the large role of hydrogen bonds, the interactions of water molecules with ions and neutral molecules in living organisms also play an extremely important role. Interactions leading to the hydration of ions and a special type of interaction called hydrophobic and hydrophilic will be discussed in the following sections of this chapter."

Speaking about the role of water in biological phenomena, it should be noted that all living organisms have very successfully adapted to a certain amount of hydrogen bonding between molecules. This is evidenced by the fact that the replacement of heavy water molecules has a very significant effect on biological systems. The solubility of polar molecules decreases, the speed of transmission of a nerve impulse decreases, the work of enzymes is disrupted, the growth of bacteria and fungi slows down, etc. Perhaps all these phenomena are due to the fact that the hydrogen interaction between molecules is stronger than the interaction between molecules. between the molecules of heavy water is indicated by its extremely high melting point (3.8 ° C) and high heat of fusion (1.51 kcal/mol). For ordinary water, the heat of fusion is 1.43 kcal/mol.

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CONNECTION

Synonyms:

consistency, coherence, continuity, foldability, consistency, harmony, interaction, connection, articulation, concatenation, coupling, communication, means of communication, intercourse, communication, contact, association, relation, relationship, dependence, binding, ties, romance, connecting link, union, causation, public relations, tomba, intimate relationships, intrigue, correlation, duplex, umbilical cord, intercourse, bonding, religion, cohabitation, parataxis, connecting thread, continuity, adhesion, interconnectedness, correlation, conditionality, connection, kinship, putty, bond , cupids, affair, synapse, context, love, thread, mail, message, quadruplex. Ant. fragmentation

CONNECTION synonyms, what are they? CONNECTION, CONNECTION this is the meaning of the word CONNECTION, origin (etymology) CONNECTION, CONNECTION stress, word forms in other dictionaries

+ CONNECTION synonym - Dictionary of Russian synonyms 4

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:

electronegativity, as a rule, increases in periods (“from left to right”) with increasing atomic number, and decreases in groups (“from top to bottom”).

The patterns of changes in electronegativity among d-block elements are much more complex.

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.

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.

Metals include most of the elements of the Periodic Table.

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.

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 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

1. Popkov V. A., Puzakov S. A. General chemistry: textbook. - M.: GEOTAR-Media, 2010. - 976 pp.: ISBN 978-5-9704-1570-2. [With. 35-37]
2. 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.