LECTURE No. 9. DIELECTRICS IN THE ELECTROSTATIC FIELD

INTRODUCTION

The material of this lecture is devoted to the study electrical properties such important materials as dielectrics.

Dielectric materials are widespread in our life, both in everyday life and in technology, this situation is explained by the uniqueness of the properties of these substances.

Dielectrics are substances that, under normal conditions, practically do not conduct electric current. There are no free charge carriers in dielectrics. Resistivity of dielectrics. For comparison, metals.

The main field of application of dielectrics is insulating materials in various electrical devices. The basic requirement that all insulating materials must meet is a high degree of leakage protection electric current by parts of the technical device. The fulfillment of this requirement is necessary to ensure the safe operation of equipment and humans, as well as to improve the efficiency of the device.

The lecture will show that special properties of all dielectrics are due to their internal structure, namely the electrical nature of the interaction of the molecules that make up the dielectric.

1. DIPOLE IN THE ELECTROSTATIC FIELD

1.1. Dipole moment electric dipole

All dielectric molecules are electrically neutral. However, molecules have electrical properties. In the first approximation, a molecule can be regarded as an electric dipole.

Consider a simple system of charges that is of great importance in electrostatics - an electric dipole.

An electric dipole is a combination of two opposite charges, equal in absolute value, located at a distance that is significantly less than the distance to the considered points of the field.

Straightconnecting the centers of charges is called the axis of the dipole.

Dipole shoulder Is a vector directed from a negative charge to a positive one, and equal in magnitude to the distance between the charges.

Fig. 1

The quantity that characterizes the electrical properties of a dipole is called the electrical dipole moment.

The electric dipole moment is a vector physical quantity equal to the product of the modulus of the dipole charge by its shoulder

Comment.

1) The electric dipole moment is always co-directional with the dipole arm, that is

2) The dimension of the dipole moment in the SI system is the coulomb multiplied by the meter.

a) Consider point A lying on the extension of the dipole axis. Let us find the strength of the electrostatic field created at a given point by an electric dipole:

Fig. 2

As can be seen from Fig. 2, the dipole field strength at a point is directed along the dipole axis and is equal in magnitude:

Then, based on the formula for the strength of the electrostatic field created by a point charge: , you can write:

where is the distance from the center of the dipole to the considered point A. By the definition of the dipole, therefore

b) Dipole field strengthat the point on the perpendicular, restored to the axis of the dipole from its middle.

Since the point is equidistant from the charges, then

, (1)

where is the distance from the point to the middle of the dipole. From the similarity of isosceles triangles resting on the dipole arm and the vector, we obtain

, (2)

where

(3)

Substituting (1) into (3), we obtain

The vector has a direction opposite to the vector of the electric moment of the dipole, that is

1.3. Electric dipole in a uniform electrostatic field.

Consider the behavior of an electric dipole in a uniform electric field... In an external electric field, a pair of forces acts on the ends of the dipole, which tends to rotate the dipole in such a way that the electric moment of the dipole turns along the direction of the field (Fig. 3).

Fig. 3

The electric field acts on the positive and negative charges of the dipole with a force equal in magnitude, but opposite in direction (Fig. 3)

These two forces are called a pair of forces, they create a torque relative to the point 0, which lies in the middle on the axis of the dipole. Under the action of this moment, the electric dipole rotates along the field so that its dipole moment will be codirectional with the strength of the external electric field:

The total (resultant) moment acting on the electric dipole from the side of the external electrostatic field is equal to:

As is known from mechanics, the moment of forces is always directed along the axis of rotation. In our case, the torque vector is directed from us perpendicular to the plane of the figure and passes through the middle of the dipole. The magnitude of the torque is equal to:

2. POLARIZATION VECTOR. CONNECTION OF DIELECTRIC PERMEABILITY

AND THE DIELECTRIC SENSITIVITY OF THE DIELECTRIC

Dielectrics are substances that poorly conduct electric current, since in dielectrics all electrons are bonded to the nuclei of atoms.

If we replace the positive charges of the nuclei of molecules with the total charge located in the "center of gravity" of positive charges, and the charge of all electrons with the total negative charge located in the "center of gravity" of negative charges, then dielectric molecules can be considered as electric dipoles.

There are three types of dielectrics.

1) Dielectrics with non-polar molecules symmetric molecules of which in the absence of an external electric field have zero dipole moment (for example ).

2) Dielectrics with polar molecules whose molecules, due to asymmetry, have a nonzero dipole moment even in the absence of an external electric field (for example ).

3) Ionic dielectrics (For example ). Ionic crystals are spatial lattices with correct alternation ions of different signs.

When dielectrics are placed in an external electric field, the dielectric is polarized - each molecule becomes an electric dipole, acquires an electric dipole momentand, most importantly, it is oriented (rotates along the field) in an external electric field.

According to the three types of dielectrics, three types of polarization are distinguished.

1) Electronicpolarization of a dielectric with non-polar molecules occurs due to the deformation of electron orbits, as a result of which a dipole moment arises in the atoms or molecules of the dielectric.

2) Orientation, or dipole polarization is inherent in dielectrics with polar molecules; in this case, the orientation of the already existing dipole moments of the molecules along the field occurs (this orientation is the stronger, the greater the external field strength and the lower the temperature).

3) Ionicpolarization is inherent in dielectrics with ionic crystal lattices- the displacement of the sublattice of positive ions along the field, and negative ions against the field leads to the appearance of dipole moments.

To quantitatively characterize the polarization of the dielectric, the polarization vector ( polarization).

PolarizationIs a vector physical quantity equal to the ratio of the total dipole electric moment of the entire dielectric to the volume of this dielectric:

Vector dimension polarization dielectric is easy to determine from this formula:

Note that the dimension polarization in the International system of units coincides with the dimension of the surface density of charges. This fact is very important, the meaning of which will be revealed below.

Dielectric polarization is the process of orienting the electric dipoles of the molecules of a substance.

It follows from experience that for a large class of dielectrics, polarization linearly depends on the electric field strength in the dielectric:

(4)

Formula (4) is valid only for isotropic dielectrics, that is, dielectrics whose properties are the same in all directions, located in electrostatic fields with not too high strength. The magnitude(kappa) is called dielectric susceptibility, it characterizes the properties of dielectrics. Dielectric susceptibility–it is a dimensionless, positive value.

So, it turns out that the properties of the same substance relative to the electric field are characterized by two quantities - dielectric constant and dielectric susceptibility... Therefore, there must be a relationship between these quantities.

To establish quantitative relationships between the dielectric susceptibility and permittivity, we introduce into a uniform electric field a dielectric in the form of a right parallelepiped, the area of ​​the side faces of which is S and width (fig. 4).

Under the action of an external field, the dielectric is polarized, i.e. the orientation of the dielectric molecules occurs, so that the positive charges of the molecule are displaced along the field, and the negative charges of the molecule are displaced against the field. As a result, on one face of the dielectric there will be an excess of bound positive charges, on the other - bound negative ones. Inside the dielectric, the bound charges are compensated and it can be assumed that there are no charges inside the dielectrics.

The appearance of bound charges on the lateral surfaces of dielectrics leads to the fact that an additional electrostatic field arises in the dielectric, created by bound charges. Let's designate the intensity of the electrostatic field of the bound charges. The electric field of bound charges is always directed against the external electric field and weakens it. The strength of the resulting electrostatic field, the field inside the dielectric, according to the principle of superpositions, is equal to the vector sum of the strengths of the external field and the strength of the field of bound charges:

.

In scalar form, this equality has the form: .

Fig. 4

Let us find the value of the field strength of the bound charges. A dielectric in an electric field can be viewed as a capacitor with a vacuum inside. The surface density of charges on the plates of such a capacitor is equal to the surface density of bound charges on the faces of the dielectric. The electric field strength of such a capacitor is known to be equal to:

In our case, then

Now let us determine the total dipole moment of the dielectric plate by the thickness and face area. For this we use the definition of the vector polarization dielectric:

And with a charge equal to: time, that is, the equality is true:

Comparing the two obtained formulas for the strength of the external electrostatic field, we can make the obvious conclusion that the relationship between the dielectric constant and the dielectric susceptibility has the form:

CONCLUSION

In conclusion of the lecture on dielectrics, it is necessary to emphasize once again the enormous practical significance of these materials in technology. Using the knowledge gained at the lecture, one can quantitatively and qualitatively study the processes of influence of dielectrics on external electric fields.

The practical significance of the influence of dielectric substances on the electric field, as you have seen, underlies the phenomena at the interface between two dielectric media, which are widely used in dielectric antennas.

On the other hand, dielectrics occupy a large place in the design of various types of capacitors to increase their electrical capacitance. We will deal with this issue already at the next lecture devoted to the problems of the behavior of conductors in an electrostatic field.

Electric dipole- an idealized electrically neutral system consisting of point and equal in absolute value positive and negative electrical charges.

In other words, an electric dipole is a combination of two equal in absolute value unlike point charges located at some distance from each other

The product of a vector drawn from a negative charge to a positive one by the absolute value of the charges is called the dipole moment:

In an external electric field, a moment of force acts on the electric dipole, which tends to rotate it so that the dipole moment is turned along the direction of the field.

The potential energy of an electric dipole in a (constant) electric field is (In the case of an inhomogeneous field, this means the dependence not only on the moment of the dipole - its magnitude and direction, but also on the location, the point of location of the dipole).

Far from the electric dipole, the strength of its electric polarity decreases with distance, which is faster than that of a point charge ().

Any electrically neutral system as a whole containing electric charges, in some approximation (that is, actually in dipole approximation) can be considered as an electric dipole with the moment where is the charge of the element, is its radius vector. In this case, the dipole approximation will be correct if the distance at which the electric field of the system is studied is large in comparison with its characteristic dimensions.

Magnetic dipole

Magnetic dipole- an analogue of an electric one, which can be imagined as a system of two "magnetic charges" (this analogy is conditional, since magnetic charges, from the point of view of modern electrodynamics, do not exist). As a model of a magnetic dipole, one can consider a small (compared to the distances at which the generated dipolemagnetic field is studied) flat closed conducting frame of the area through which the current flows. , when observing in which the current in the frame appears to flow in a clockwise direction.

The expressions for the torque acting on the magnetic dipole from the side of the magnetic field and the potential energy of a constant magnetic dipole in a magnetic field are similar to the corresponding formulas for the interaction of an electric dipole with an electric field, only the magnetic moment of the magnetic induction vector enters there:

Oscillating dipole field

This section deals with the field created by a point electric dipole located in set point space.

Let us consider, in relation to electrodynamics, what a dipole moment is. Elementary charge carriers flowing along a straight section of a system of conductors form a forward current. Accordingly, there is a current charge of the specified current (I * L, where I is the value of the current, L is the length of the section). In turn, he considers two parallel current charges with L tending to infinity. In a closed loop, its two halves have opposite sides, forming a current dipole. A vortex field is created around each such dipole, which is characterized by its own dipole current charge, oriented perpendicular to the plane in which the circuit is located. It is called the dipole moment. But since we are considering only the current component, then for the transition to electromagnetism the same term is called differently. Another name is magnetic dipole moment (Pm, sometimes just m).

It represents one of the key characteristics of any substance. It is believed that the dipole moment arises due to currents (both in the microcosm and in macrosystems). Under the microcosm in in this case the atom is understood: moving in circular orbits can be considered as an electric current. Since matter consists of elementary particles, each of them also has its own moment. Please note that elementary particles should be understood not only as molecules and atoms, but also protons, neutrons, electrons and, possibly, even smaller components. From the point of view, their magnetic dipole moment is determined by their own mechanical rotation - spin. However, this assumption has recently been increasingly called into question in the light of the latest field theory of particles. For example, the existence of the so-called anomalous dipole is generally accepted, the value of which differs from the calculations of the equation in quantum theory... But from the field point of view, in which the magnetic field is any elementary particle is generated not by the spin rotation of charge carriers, but is one of the constant components electromagnetic field, the anomalous dipole is easy to explain. The value is defined as a specific set with a corrective spin component. Thus, the magnetic moment for a neutron depends on the electric current that generates it and the energy of the changing electromagnetic field.

When calculating its value for an entire circuit, the method of integral addition of the dipole moments of the simplest current dipoles, creating a closed circular circuit, is used.

The dipole moment in electrodynamics is determined by the formula:

where I is the value of the flowing current; S - area closed loop(circular); n is a vector directed perpendicular to the plane in which the contour is located. Although the above formula does not show this, the value of Pm is also vector, the direction of which can be determined known in classical electrical engineering (right screw): if the rotation of an imaginary screw is compared with the direction of the flowing current, then the movement of the screw body coincides with the sought vector.

The electric field of a dipole differs from the field of a point charge, first of all, in the configuration ley lines... Since from the point of view of physics, such a dipole is a balanced system of two whose moduli are equal, and the polarity is opposite (+ and -), the corresponding lines of tension begin at one charge, and end at the other. In the case of only one point charge carrier, the lines diverge in all directions, like the light of a lamp.

Charge system:

Q = q 1 + q 2 +… + q n = Σq i

Dip moment syst.zar.

→ → → → → → → n → →

p = r 1 q 1 + r 2 q 2 +… + r n q n = Σr i q i

26. Gauss's theorem for the vector e.

Consider the field of a point charge q and calculate the flux of the vector E through the closed surface S containing the charge (Fig.). The number of lines of the vector E, starting at a point charge + q or ending at a charge –q, is numerically equal to q / ε0.

According to the formula Ф [a] (=) N [start] - N [end], the flux of the vector E through any closed surface is equal to the number of lines going out, that is, starting on the charge, if it is positive, and the number of lines entering inside, i.e. ending in a charge if it is negative. Taking into account that the number of lines beginning or ending at a point charge is numerically equal to q / ε0, we can write that Ф [E] = q / ε0.

The sign of the flux coincides with the sign of the charge q. The dimensions of both sides of this equality are the same.

Now let us assume that there are N point charges q1, q2, ..., q [N] inside the closed surface. By virtue of the principle of superposition, the field strength E created by all charges is equal to the sum of the strengths E [i] created by each charge separately: E = ∑E [i].

Therefore, Ф [E] = ∫ EdS = ∫ (∑E [i]) = ∑ ∫ E [i] dS. Each of the integrals under the sum sign is equal to q [i] / ε0. hence,

Ф [E] = ∫ EdS = 1 / ε0∑ q [i].

The assertion proved is called the Gauss theorem. This theorem states that the flux of the electric field strength vector through a closed surface is equal to the algebraic sum of charges contained inside this surface, divided by ε0.

27. Bulk, surface and linear charge density. Field of one and two charged planes. The field of charged cylindrical and spherical surfaces. Charged ball field.

1. The volumetric density of continuous distribution of charges is the ratio of charge to volume:

where ℮וֹ - elementary charges in the volume ∆Vph (taking into account their sign); ∆Q is the total charge contained in ∆Vph. The volume ∆Vph is small, but not infinitely small in the mathematical sense. ∆Vph depends on specific conditions.

2.Linear density electric charge- the limit of the ratio of the electric charge in a line element to the length of this line element that contains the given charge when the length of this element tends to zero.

3.Surface charge density

(σ = 1 / (∆Sph∑ [∆Sph] ℮1) = dQ / dS)

where dS is an infinitesimal area of ​​the surface.

Field of an infinite uniformly charged plane. Let the surface charge density at all points of the plane be the same and equal to σ; for definiteness, we will assume that the charge is positive. From considerations of symmetry, it follows that the field strength at any point has a direction perpendicular to the plane. Indeed, since the plane is infinite and uniformly charged, there is no reason for the vector E to deviate in any direction from the normal to the plane. Further, it is obvious that at points symmetric with respect to the plane, the field strength is the same in magnitude and opposite in direction. From the Gauss theorem it follows that at any distance from the plane, the field strength is the same

μ = δ l

Rice. 2.23. Diagram of the formation of a dipole in a heteronuclear molecule AB

2.3. Communication polarity. Dipole moment of a molecule

With the formation of a covalent chemical bond between different atoms (heteronuclear molecules), the electron density is not distributed symmetrically with respect to the nuclei. In the molecule, it is shifted towards the atom

the body charges of electrons do not coincide. There is a system of different signs, but the same magnitude of electricity

charges (δ + and δ−) - electric dipole(fig. 2.23).

The measure of the polarity of the bond (characteristic of the dipole) is the dipole

the moment µ is the product of the magnitude of the charge δ by the distance between the centers of gravity of positive and negative charges (the length of the dipole l).

The unit of measurement of the dipole moment in the SI system [Kl m] is more often

the off-system unit Debye (D) is used: 1D = 3.33 10-30 C m.

In heteronuclear molecules, the bond is always polar, but if the number of atoms in a molecule is three or more, then the resulting charge distribution system can lead to the fact that the molecule as a whole will not be a dipole - the centers of gravity of positive and negative charges coincide. As a rule, this is due to the symmetrical structure of the molecule.

If a molecule, even if it is not a dipole, is placed in an electric field of strength E, the centers of gravity of the charges are separated as a result of the displacement of electrons relative to the nuclei, and the displacement of atoms relative to each other in the molecule. In this case, the molecule acquires an induced (induced) dipole moment. The ability of molecules to acquire a dipole moment in an electric field is

called polarizability.

The dipole moment of the induced dipole is proportional to the electric field strength: μi = α ε 0 E, where α is the polarization coefficient

the bridging (polarizability) of an atom or molecule, ε 0 is an electrical constant.

3. CHEMICAL BOND IN SOLIDS

AND LIQUIDS

3.1. Aggregate states

Substances depending on external conditions(temperature and pressure) and their chemical composition can exist in three basic states of aggregation: gaseous, liquid and solid. At sufficiently low temperatures, substances are in a solid state, and at relatively high temperatures, in a liquid and gaseous state.

When heated, there is, as a rule, a sequential transition of substances from a solid to a liquid and gaseous state (melting and evaporation), and when cooled, reverse processes occur (condensation and crystallization). These transitions are carried out at a certain temperature (phase transition temperature), while the molar volume of the substance and the entropy (energy characteristic of the degree of disorder of the system) change abruptly, thermal energy is absorbed or released (phase transition enthalpy). The transition temperature from one state to another depends on chemical nature substances and pressure. The specific values ​​of the temperatures of phase transitions for various substances lie within wide limits (Table 3.1). It should be noted that under certain conditions a phase transition is possible solid state- gas (sublimation-crystallization).

Liquid and solid states of aggregation are referred to as condensed matter... It differs from the gaseous one in that the energy of interaction between the particles that form the substance is comparable in magnitude or exceeds the energy of their thermal motion. This leads to the fact that the average distance between particles (between the centers of particles) in a gas at normal conditions is ~ 10 of their diameters, while in the condensed state it is comparable to their diameter. The molar volume of any gas under normal conditions is 22.4 L / mol, while the molar volumes solids and liquids about 103 times less (0.01–0.05 l / mol).

Example. Calculation of the average dimensions of the space occupied by one particle at atmospheric pressure.

Gas Liquid, crystal

V = a 3 - volume of space a - edge of a cube

d - average particle diameter

ρ = 3.12 g / cm3 is the density of liquid bromine, V ν is the molar volume of liquid bromine.

Average size of space occupied by one particle:

M = 108 g / mol is the molar mass of silver, ρ = 10.50 g / cm3 is the density of silver, V ν is the molar volume of silver.

Average size of space occupied by one particle:

The size of the silver atom (two metal radii) d Ag 2.68 Ǻ.

In gases, the particles are in Brownian motion, with no short-range and long-range order in the position of the particles. Gas does not have its own volume and, accordingly, no shape. In liquids, Brownian motion is complicated by the presence of a more or less stable short-range order in the position of particles relative to each other due to the appearance of chemical bonds between individual particles. The liquid has its own volume, but due to the weak intermolecular interaction under the influence of gravity it takes the form of the vessel in which it is located. In the solid state of matter, the energy of interaction between particles is much higher than the energy of thermal motion, which leads to the fixation of the positions of particles in space, around which they perform oscillatory and rotational movements... This determines the presence of solid bodies of their own shape and volume and high shear resistance.

Comparison of the energy characteristics of phase transitions indicates a significantly lower rearrangement of the substance upon melting than upon evaporation. As you can see from the table. 3.1, for all crystals with different types the chemical bond, the heat (enthalpy) of fusion is much less than the heat of vaporization. The entropy of the phase transition, which characterizes the change in the degree of ordering of the system, is also much less for melting than for evaporation.

In the gaseous state, where there are weakly or not at all interacting molecules of a substance, the chemical bond inside them is considered using the models of the "classical" covalent bond.

When considering the condensed state of matter, the chemical bond is described using the models of covalent, ionic and metallic bonds. In this case, it is necessary to take into account proximity particles forming the system. This circumstance in a number of cases (liquids, molecular crystals) makes it necessary to take into account the significant contribution of intermolecular interactions to the energy of chemical bonds.

It should be noted that a number of substances may not have one of the states of aggregation. Most often this applies to liquid and gaseous states. This circumstance is associated with the ratio between the energy required to transfer a substance from one state of aggregation to another, and the energy sufficient to break intramolecular chemical bonds. For example, in many water-insoluble metal hydroxides, when heated, the dehydration reaction (Cu (OH) 2 → CuO + H2 O) occurs earlier, and then the substance melts.

3.2 Intermolecular interaction

As noted above, in the condensed state of matter, the value of the chemical bond energy is significantly affected by intermolecular interactions. They are associated with the electrostatic interaction of charges resulting from the violation of the symmetry of the electron density distribution in molecules.

3.2.1 Intermolecular interactions (van der Waals forces)

In condensed phases (liquid, solid) the distance between the molecules is commensurate with the size of the molecules themselves. At such small distances, the forces of electrostatic interaction of dipoles, both permanent and induced, manifest themselves. In this case, the energy of the system decreases.

Intermolecular interactions are characterized by a lack of electron exchange between particles, a lack of specificity and saturation. The energy of intermolecular interaction is relatively low, but it makes a significant contribution to energy state systems, determining to a large extent the physical and chemical properties of a substance.

At relatively large distances r between molecules, when the electron shells do not overlap, only attractive forces act. In this case, there are three possible mechanisms for the appearance of forces of attraction.

1. Orientation effect(dipole - dipole interaction). If the molecules are polar, then the electrostatic interaction of two permanent dipoles appears. Polar molecules are oriented relative to each other by oppositely charged parts, the energy of attraction is directly proportional to the dipole moments (µ i 2) and inversely proportional to the distance between them (r 6). An increase in temperature weakens this interaction, since the thermal interaction tends

disrupt the mutual orientation of molecules.

2. Induction effect(interaction dipole - induced dipole).

Non-polar molecules under the action of the field of a polar molecule are polarized, an induced dipole arises. The induced dipole moment is directly proportional to the polarizability of the molecules (µ and α µ d). The energy of attraction of such molecules is directly proportional to the dipole moments (α µ d 2) and inversely proportional to the distance between them (r 6). Since the guidance of dipoles occurs at any spatial arrangement of molecules, the induction effect does not depend on temperature.

3. Dispersion effect(interaction of instantaneous dipoles).

In contrast to the orientational and inductive interactions, the dispersion effect can be explained only within the framework of quantum mechanics... Its occurrence can be represented as follows: during the movement of electrons, the distribution of charges inside atoms can become asymmetric, which leads to the formation of "instantaneous dipoles", which are attracted to each other. Moreover, when molecules approach each other, the motion of electrons ceases to be independent and a "self-consistent" system of interacting instantaneous dipoles arises. The energy of attraction is directly proportional to the polarizabilities of the molecules (α i) and inversely proportional to the distance between them (r 6).

The dispersion effect, as the most universal, manifests itself in the interaction of both polar and non-polar molecules. Moreover, for non-polar molecules and molecules with a small dipole moment, it is the main one.

Induction and orientation effects play an essential role in the interaction of polar molecules. For molecules with great value the main dipole moment is the orientational effect. The induction effect is usually small and becomes significant only when polar molecules coexist with highly polarizable molecules (Table 3.2).

At small distances between molecules, when their electron shells overlap strongly, the electrostatic repulsion of nuclei and electrons becomes greater than their mutual attraction. The repulsive energy is much more dependent on the distance (r 12) than the energy of attraction. At large distances, the intermolecular interaction is determined by the forces of attraction, and at small distances, by the forces of repulsion.

Table 3. 2

Relative contribution of each component to the energy of intermolecular interaction for various molecules

3.2.2 Hydrogen bond

A special type of intermolecular interaction is the hydrogen bond. It arises between molecules that contain a hydrogen atom in their structure and a small-sized atom of an element with a large electronegativity value (oxygen, fluorine, nitrogen, etc.). Since the difference in the electronegativities of hydrogen and these elements is large, the bond is strongly polarized, and relatively large negative and positive charges appear on the atoms. At the same time, the small size of these atoms allows them to come close to each other in the dipole – dipole interaction. Therefore, the energy of the orientational interaction is much higher (by about an order of magnitude) than in other cases. In addition, the bond energy significantly increases due to the partial formation of the covalent component of the bond between the interacting atoms of neighboring molecules by the donor-acceptor mechanism. The 1s orbital of hydrogen is partially exposed due to the strong polarization of the bond (this is not yet H +, but already not H0), and there are lone electron pairs on the electronegative atom.

Both of these factors lead to an increase in the binding energy compared to the energy of intermolecular interaction. The energy of a hydrogen bond is about 100 kJ / mol, the energy of intermolecular interaction (van der Waals forces) is 10-20 kJ / mol.

During the condensation of molecules capable of forming hydrogen bonds, their mutual arrangement will be determined both by the direction in space of hydrogen atoms inside the molecule, and by the direction in space electronic orbitals an electronegative atom bonded to a hydrogen atom of a neighboring molecule.

The hydrogen bond determines many physical and chemical properties of substances, in particular, the melting and boiling points increase, and the density of the substance changes. The hydrogen bond plays a special role in biochemistry, organic molecules (including polymers) containing H-O, H-N communication, form big number hydrogen bonds.

Examples. Water H2 O.

In the condensed state, each water molecule can have four hydrogen bonds: two between the oxygen atom (donor function) and the hydrogen atoms of two neighboring water molecules; two more - due to two hydrogen atoms (acceptor function). In the crystalline state, a regular diamond-like structure is formed. The nodes contain large oxygen atoms, which are linked through a hydrogen atom. In the liquid state, some of the hydrogen bonds are broken (Figure 3.1).

Оδ -

Rice. 3.1. Diagram of the formation of the tetrahedral spatial structure of water in the crystalline and liquid states: - covalent bond, - hydrogen bond

Hydrogen fluoride HF.

In the gaseous state at low temperatures, due to the formation of hydrogen bonds, associates (HF) 2, (HF) 6 are formed. ... In the condensed state, in particular in the solid, HF forms zigzag chains (Fig. 3.2).