Graph of molecular forces versus distance between molecules. In what state is the distance between molecules in a liquid substance?

1. Structure of gaseous, liquid and solid bodies

The molecular kinetic theory makes it possible to understand why a substance can exist in gaseous, liquid and solid states.
Gases. In gases, the distance between atoms or molecules is on average many times greater than the size of the molecules themselves ( Fig.8.5). For example, at atmospheric pressure the volume of a vessel is tens of thousands of times greater than the volume of the molecules in it.

Gases are easily compressed, and the average distance between molecules decreases, but the shape of the molecule does not change ( Fig.8.6).

Molecules move at enormous speeds - hundreds of meters per second - in space. When they collide, they bounce off each other in different directions like billiard balls. The weak attractive forces of gas molecules are not able to hold them near each other. That's why gases can expand unlimitedly. They retain neither shape nor volume.
Numerous impacts of molecules on the walls of the vessel create gas pressure.

Liquids. The molecules of the liquid are located almost close to each other ( Fig.8.7), so a liquid molecule behaves differently than a gas molecule. In liquids, there is so-called short-range order, i.e., the ordered arrangement of molecules is maintained over distances equal to several molecular diameters. The molecule oscillates around its equilibrium position, colliding with neighboring molecules. Only from time to time she makes another “jump”, getting into a new equilibrium position. In this equilibrium position, the repulsive force is equal to the attractive force, i.e., the total interaction force of the molecule is zero. Time settled life water molecules, i.e., the time of its vibrations around one specific equilibrium position at room temperature, is on average 10 -11 s. The time of one oscillation is much less (10 -12 -10 -13 s). With increasing temperature, the residence time of molecules decreases.

The nature of molecular motion in liquids, first established by the Soviet physicist Ya.I. Frenkel, allows us to understand the basic properties of liquids.
Liquid molecules are located directly next to each other. As the volume decreases, the repulsive forces become very large. This explains low compressibility of liquids.
As is known, liquids are fluid, that is, they do not retain their shape. This can be explained this way. The external force does not noticeably change the number of molecular jumps per second. But jumps of molecules from one stationary position to another occur predominantly in the direction of the external force ( Fig.8.8). This is why liquid flows and takes the shape of the container.

Solids. Atoms or molecules of solids, unlike atoms and molecules of liquids, vibrate around certain equilibrium positions. For this reason, solids retain not only volume, but also shape. The potential energy of interaction between solid molecules is significantly greater than their kinetic energy.
There is another important difference between liquids and solids. A liquid can be compared to a crowd of people, where individual individuals are restlessly jostling in place, and a solid body is like a slender cohort of the same individuals who, although they do not stand at attention, maintain on average certain distances between themselves. If you connect the centers of the equilibrium positions of atoms or ions of a solid body, you get a regular spatial lattice called crystalline.
Figures 8.9 and 8.10 show the crystal lattices of table salt and diamond. The internal order in the arrangement of atoms in crystals leads to regular external geometric shapes.

Figure 8.11 shows Yakut diamonds.

In a gas, the distance l between molecules is much greater than the size of the molecules 0:" l>>r 0 .
For liquids and solids l≈r 0. The molecules of a liquid are arranged in disorder and from time to time jump from one settled position to another.
Crystalline solids have molecules (or atoms) arranged in a strictly ordered manner.

2. Ideal gas in molecular kinetic theory

The study of any field of physics always begins with the introduction of a certain model, within the framework of which further study takes place. For example, when we studied kinematics, the model of the body was a material point, etc. As you may have guessed, the model will never correspond to the actually occurring processes, but often it comes very close to this correspondence.

Molecular physics, and in particular MCT, is no exception. Many scientists have worked on the problem of describing the model since the eighteenth century: M. Lomonosov, D. Joule, R. Clausius (Fig. 1). The latter, in fact, introduced the ideal gas model in 1857. A qualitative explanation of the basic properties of a substance based on molecular kinetic theory is not particularly difficult. However, the theory that establishes quantitative connections between experimentally measured quantities (pressure, temperature, etc.) and the properties of the molecules themselves, their number and speed of movement, is very complex. In a gas at normal pressures, the distance between the molecules is many times greater than their dimensions. In this case, the interaction forces between molecules are negligible and the kinetic energy of the molecules is much greater than the potential energy of interaction. Gas molecules can be thought of as material points or very small solid balls. Instead of real gas, between the molecules of which complex interaction forces act, we will consider it The model is an ideal gas.

Ideal gas– a gas model, in which gas molecules and atoms are represented in the form of very small (vanishing sizes) elastic balls that do not interact with each other (without direct contact), but only collide (see Fig. 2).

It should be noted that rarefied hydrogen (under very low pressure) almost completely satisfies the ideal gas model.

Rice. 2.

Ideal gas is a gas in which the interaction between molecules is negligible. Naturally, when molecules of an ideal gas collide, a repulsive force acts on them. Since we can consider gas molecules, according to the model, as material points, we neglect the sizes of the molecules, considering that the volume they occupy is much less than the volume of the vessel.
Let us recall that in a physical model only those properties of a real system are taken into account, the consideration of which is absolutely necessary to explain the studied patterns of behavior of this system. No model can convey all the properties of a system. Now we have to solve a rather narrow problem: using molecular kinetic theory to calculate the pressure of an ideal gas on the walls of a vessel. For this problem, the ideal gas model turns out to be quite satisfactory. It leads to results that are confirmed by experience.

3. Gas pressure in molecular kinetic theory Let the gas be in a closed container. Pressure gauge shows gas pressure p 0. How does this pressure arise?
Each gas molecule hitting the wall acts on it with a certain force for a short period of time. As a result of random impacts on the wall, the pressure changes rapidly over time, approximately as shown in Figure 8.12. However, the effects caused by the impacts of individual molecules are so weak that they are not registered by a pressure gauge. The pressure gauge records the time-average force acting on each unit of surface area of ​​its sensitive element - the membrane. Despite small changes in pressure, the average pressure value p 0 practically turns out to be a completely definite value, since there are a lot of impacts on the wall, and the masses of the molecules are very small.

An ideal gas is a model of a real gas. According to this model, gas molecules can be considered as material points whose interaction occurs only when they collide. When the gas molecules collide with the wall, they exert pressure on it.

4. Micro- and macroparameters of gas

Now we can begin to describe the parameters of an ideal gas. They are divided into two groups:

Ideal gas parameters

That is, microparameters describe the state of a single particle (microbody), and macroparameters describe the state of the entire portion of gas (macrobody). Let us now write down the relationship that connects some parameters with others, or the basic MKT equation:

Here: - average speed of particle movement;

Definition. – concentration gas particles – the number of particles per unit volume; ; unit - .

5. Average value of the square of the speed of molecules

To calculate the average pressure, you need to know the average speed of the molecules (more precisely, the average value of the square of the speed). This is not a simple question. You are used to the fact that every particle has speed. The average speed of molecules depends on the movement of all particles.
Average values. From the very beginning, you need to give up trying to trace the movement of all the molecules that make up the gas. There are too many of them, and they move very difficult. We don't need to know how each molecule moves. We must find out what result the movement of all gas molecules leads to.
The nature of the movement of the entire set of gas molecules is known from experience. Molecules engage in random (thermal) motion. This means that the speed of any molecule can be either very large or very small. The direction of motion of molecules constantly changes as they collide with each other.
The speeds of individual molecules can be any, however average the value of the modulus of these speeds is quite definite. Similarly, the height of students in a class is not the same, but its average is a certain number. To find this number, you need to add up the heights of individual students and divide this sum by the number of students.
The average value of the square of the speed. In the future, we will need the average value not of the speed itself, but of the square of the speed. The average kinetic energy of molecules depends on this value. And the average kinetic energy of molecules, as we will soon see, is very important in the entire molecular kinetic theory.
Let us denote the velocity modules of individual gas molecules by . The average value of the square of the speed is determined by the following formula:

Where N- the number of molecules in the gas.
But the square of the modulus of any vector is equal to the sum of the squares of its projections on the coordinate axes OX, OY, OZ. That's why

Average values ​​of quantities can be determined using formulas similar to formula (8.9). Between the average value and the average values ​​of the squares of projections there is the same relationship as relationship (8.10):

Indeed, equality (8.10) is valid for each molecule. Adding these equalities for individual molecules and dividing both sides of the resulting equation by the number of molecules N, we arrive at formula (8.11).
Attention! Since the directions of the three axes OH, OH And OZ due to the random movement of molecules, they are equal, the average values ​​of the squares of the velocity projections are equal to each other:

You see, a certain pattern emerges from the chaos. Could you figure this out for yourself?
Taking into account relation (8.12), we substitute in formula (8.11) instead of and . Then for the mean square of the velocity projection we obtain:

i.e., the mean square of the velocity projection is equal to 1/3 of the mean square of the velocity itself. The 1/3 factor appears due to the three-dimensionality of space and, accordingly, the existence of three projections for any vector.
The speeds of molecules change randomly, but the average square of the speed is a well-defined value.

6. Basic equation of molecular kinetic theory
Let us proceed to the derivation of the basic equation of the molecular kinetic theory of gases. This equation establishes the dependence of gas pressure on the average kinetic energy of its molecules. After the derivation of this equation in the 19th century. and experimental proof of its validity began the rapid development of the quantitative theory, which continues to this day.
The proof of almost any statement in physics, the derivation of any equation can be done with varying degrees of rigor and convincingness: very simplified, more or less rigorous, or with the full rigor available to modern science.
A rigorous derivation of the equation of the molecular kinetic theory of gases is quite complex. Therefore, we will limit ourselves to a highly simplified, schematic derivation of the equation. Despite all the simplifications, the result will be correct.
Derivation of the basic equation. Let's calculate the gas pressure on the wall CD vessel ABCD area S, perpendicular to the coordinate axis OX (Fig.8.13).

When a molecule hits a wall, its momentum changes: . Since the modulus of the speed of molecules upon impact does not change, then . According to Newton's second law, the change in the momentum of a molecule is equal to the impulse of the force acting on it from the wall of the vessel, and according to Newton's third law, the magnitude of the impulse of the force with which the molecule acts on the wall is the same. Consequently, as a result of the impact of the molecule, a force was exerted on the wall, the momentum of which is equal to .

The molecular kinetic theory explains that all substances can exist in three states of aggregation: solid, liquid and gaseous. For example, ice, water and water vapor. Plasma is often considered the fourth state of matter.

Aggregate states of matter(from Latin aggrego– attach, connect) – states of the same substance, transitions between which are accompanied by a change in its physical properties. This is the change in the aggregate states of matter.

In all three states, the molecules of the same substance are no different from each other, only their location, the nature of thermal motion and the forces of intermolecular interaction change.

Movement of molecules in gases

In gases, the distance between molecules and atoms is usually much greater than the size of the molecules, and the attractive forces are very small. Therefore, gases do not have their own shape and constant volume. Gases are easily compressed because repulsive forces over large distances are also small. Gases have the property of expanding indefinitely, filling the entire volume provided to them. Gas molecules move at very high speeds, collide with each other, and bounce off each other in different directions. Numerous impacts of molecules on the walls of the vessel create gas pressure.

Movement of molecules in liquids

In liquids, molecules not only oscillate around the equilibrium position, but also make jumps from one equilibrium position to the next. These jumps occur periodically. The time interval between such jumps is called average time of settled life(or average relaxation time) and is designated by the letter ?. In other words, relaxation time is the time of oscillations around one specific equilibrium position. At room temperature this time averages 10 -11 s. The time of one oscillation is 10 -12 ... 10 -13 s.

The time of sedentary life decreases with increasing temperature. The distance between the molecules of a liquid is smaller than the size of the molecules, the particles are located close to each other, and the intermolecular attraction is strong. However, the arrangement of liquid molecules is not strictly ordered throughout the volume.

Liquids, like solids, retain their volume, but do not have their own shape. Therefore, they take the shape of the vessel in which they are located. The liquid has the following properties: fluidity. Thanks to this property, the liquid does not resist changing shape, is slightly compressed, and its physical properties are the same in all directions inside the liquid (isotropy of liquids). The nature of molecular motion in liquids was first established by the Soviet physicist Yakov Ilyich Frenkel (1894 - 1952).

Movement of molecules in solids

The molecules and atoms of a solid are arranged in a certain order and form crystal lattice. Such solids are called crystalline. Atoms perform vibrational movements around the equilibrium position, and the attraction between them is very strong. Therefore, solids under normal conditions retain their volume and have their own shape.

Physics

Interaction between atoms and molecules of matter. Structure of solid, liquid and gaseous bodies

Between the molecules of a substance, attractive and repulsive forces act simultaneously. These forces largely depend on the distances between molecules.

According to experimental and theoretical studies, intermolecular interaction forces are inversely proportional to the nth power of the distance between molecules:

where for attractive forces n = 7, and for repulsive forces .

The interaction of two molecules can be described using a graph of the projection of the resultant forces of attraction and repulsion of molecules on the distance r between their centers. Let us direct the r axis from molecule 1, the center of which coincides with the origin of coordinates, to the center of molecule 2 located at a distance from it (Fig. 1).

Then the projection of the repulsion force of molecule 2 from molecule 1 onto the r axis will be positive. The projection of the force of attraction of molecule 2 to molecule 1 will be negative.

Repulsive forces (Fig. 2) are much greater than attractive forces at short distances, but decrease much faster with increasing r. The attractive forces also decrease rapidly with increasing r, so that, starting from a certain distance, the interaction of molecules can be neglected. The greatest distance rm at which molecules still interact is called the radius of molecular action .

The repulsive forces are equal in magnitude to the attractive forces.

The distance corresponds to the stable equilibrium relative position of the molecules.

In different states of aggregation of a substance, the distance between its molecules is different. Hence the difference in the force interaction of molecules and a significant difference in the nature of the movement of molecules of gases, liquids and solids.

In gases, the distances between molecules are several times greater than the sizes of the molecules themselves. As a result, the interaction forces between gas molecules are small and the kinetic energy of the thermal motion of the molecules far exceeds the potential energy of their interaction. Each molecule moves freely from other molecules at enormous speeds (hundreds of meters per second), changing direction and velocity module when colliding with other molecules. The free path of gas molecules depends on the pressure and temperature of the gas. Under normal conditions.

In liquids, the distance between molecules is much smaller than in gases. The forces of interaction between molecules are large, and the kinetic energy of the movement of molecules is commensurate with the potential energy of their interaction, as a result of which the molecules of the liquid oscillate around a certain equilibrium position, then jump abruptly to new equilibrium positions after very short periods of time, which leads to the fluidity of the liquid. Thus, in a liquid, molecules perform mainly vibrational and translational movements. In solids, the interaction forces between molecules are so strong that the kinetic energy of motion of the molecules is much less than the potential energy of their interaction. Molecules perform only vibrations with small amplitude around a certain constant equilibrium position - a node of the crystal lattice.

This distance can be estimated by knowing the density of the substance and molar mass. Concentration – the number of particles per unit volume is related to density, molar mass and Avogadro's number by the relation.

Let us consider how the projection of the resulting force of interaction between them on the straight line connecting the centers of the molecules changes depending on the distance between the molecules. If molecules are located at distances several times greater than their sizes, then the interaction forces between them have practically no effect. The forces of interaction between molecules are short-range.

At distances exceeding 2-3 molecular diameters, the repulsive force is practically zero. Only the force of attraction is noticeable. As the distance decreases, the force of attraction increases and at the same time the force of repulsion begins to affect. This force increases very quickly when the electron shells of the molecules begin to overlap.

Figure 2.10 graphically shows the projection dependence F r the forces of interaction of molecules on the distance between their centers. On distance r 0, approximately equal to the sum of the molecular radii, F r = 0 , since the force of attraction is equal in magnitude to the force of repulsion. At r > r 0 there is an attractive force between the molecules. The projection of the force acting on the right molecule is negative. At r < r 0 there is a repulsive force with a positive projection value F r .

Origin of elastic forces

The dependence of the interaction forces between molecules on the distance between them explains the appearance of elastic force during compression and stretching of bodies. If you try to bring the molecules closer to a distance less than r0, then a force begins to act that prevents the approach. On the contrary, when molecules move away from each other, an attractive force acts, returning the molecules to their original positions after the cessation of external influence.

With a small displacement of molecules from equilibrium positions, the forces of attraction or repulsion increase linearly with increasing displacement. In a small area, the curve can be considered a straight segment (the thickened section of the curve in Fig. 2.10). That is why, at small deformations, Hooke’s law turns out to be valid, according to which the elastic force is proportional to the deformation. At large molecular displacements, Hooke's law is no longer valid.

Since the distances between all molecules change when a body is deformed, the neighboring layers of molecules account for an insignificant part of the total deformation. Therefore, Hooke's law is satisfied at deformations millions of times greater than the size of the molecules.

Atomic force microscope

The device of an atomic force microscope (AFM) is based on the action of repulsive forces between atoms and molecules at short distances. This microscope, unlike a tunnel microscope, allows you to obtain images of surfaces that do not conduct electrical current. Instead of a tungsten tip, AFM uses a small fragment of diamond, sharpened to atomic size. This fragment is fixed on a thin metal holder. As the tip approaches the surface under study, the electron clouds of diamond and surface atoms begin to overlap and repulsive forces arise. These forces deflect the tip of the diamond tip. The deviation is recorded using a laser beam reflected from a mirror mounted on a holder. The reflected beam drives a piezoelectric manipulator, similar to the manipulator of a tunnel microscope. The feedback mechanism ensures that the height of the diamond needle above the surface is such that the bend of the holder plate remains unchanged.

In Figure 2.11 you see an AFM image of the polymer chains of the amino acid alanine. Each tubercle represents one amino acid molecule.

At present, atomic microscopes have been constructed, the design of which is based on the action of molecular forces of attraction at distances several times greater than the size of an atom. These forces are approximately 1000 times less than the repulsive forces in AFM. Therefore, a more complex sensing system is used to record the forces.

Atoms and molecules are made up of electrically charged particles. Due to the action of electrical forces over short distances, molecules are attracted, but begin to repel when the electron shells of the atoms overlap.

Solids are those substances that are capable of forming bodies and have volume. They differ from liquids and gases in their shape. Solids retain their body shape due to the fact that their particles are not able to move freely. They differ in their density, plasticity, electrical conductivity and color. They also have other properties. For example, most of these substances melt during heating, acquiring a liquid state of aggregation. Some of them, when heated, immediately turn into gas (sublimate). But there are also those that decompose into other substances.

Types of solids

All solids are divided into two groups.

1. Amorphous, in which individual particles are arranged randomly. In other words: they do not have a clear (defined) structure. These solids are capable of melting within a certain temperature range. The most common of them include glass and resin.
2. Crystalline, which, in turn, are divided into 4 types: atomic, molecular, ionic, metallic. In them, particles are located only according to a certain pattern, namely at the nodes of the crystal lattice. Its geometry in different substances can vary greatly.

Solid crystalline substances predominate over amorphous substances in their numbers.

Types of Crystalline Solids

In the solid state, almost all substances have a crystalline structure. They are distinguished by their lattices at their nodes containing various particles and chemical elements. It is in accordance with them that they received their names. Each type has characteristic properties:

• In an atomic crystal lattice, particles of a solid are linked by covalent bonds. It is distinguished by its durability. Due to this, such substances have a high boiling point. This type includes quartz and diamond.
• In a molecular crystal lattice, the bonds between particles are characterized by their weakness. Substances of this type are characterized by ease of boiling and melting. They are characterized by volatility, due to which they have a certain smell. Such solids include ice and sugar. The movements of molecules in solids of this type are distinguished by their activity.
• Corresponding particles, charged positively and negatively, alternate at the nodes. They are held together by electrostatic attraction. This type of lattice exists in alkalis, salts. Many substances of this type are easily soluble in water. Due to the fairly strong bond between the ions, they are refractory. Almost all of them are odorless, since they are characterized by non-volatility. Substances with an ionic lattice are unable to conduct electric current because they do not contain free electrons. A typical example of an ionic solid is table salt. This crystal lattice gives it fragility. This is due to the fact that any shift of it can lead to the emergence of ion repulsive forces.
• In a metal crystal lattice, only positively charged chemical ions are present at the nodes. Between them there are free electrons, through which thermal and electrical energy passes perfectly. That is why any metals are distinguished by such a feature as conductivity.

General concepts about solids

Solids and substances are practically the same thing. These terms refer to one of 4 states of aggregation. Solids have a stable shape and a pattern of thermal motion of atoms. Moreover, the latter perform small oscillations near the equilibrium positions. The branch of science that studies the composition and internal structure is called solid state physics. There are other important areas of knowledge dealing with such substances. Changing shape under external influences and movement is called the mechanics of a deformable body.

Due to the different properties of solids, they have found application in various technical devices created by man. Most often, their use was based on properties such as hardness, volume, mass, elasticity, plasticity, and fragility. Modern science makes it possible to use other qualities of solids that can only be detected in laboratory conditions.

What are crystals

Crystals are solids with particles arranged in a certain order. Each has its own structure. Its atoms form a three-dimensional periodic arrangement called a crystal lattice. Solids have different structure symmetries. The crystalline state of a solid is considered stable because it has a minimum amount of potential energy.

The vast majority of solids consist of a huge number of randomly oriented individual grains (crystallites). Such substances are called polycrystalline. These include technical alloys and metals, as well as many rocks. Single natural or synthetic crystals are called monocrystalline.

Most often, such solids are formed from the state of the liquid phase, represented by a melt or solution. Sometimes they are obtained from the gaseous state. This process is called crystallization. Thanks to scientific and technological progress, the procedure for growing (synthesizing) various substances has reached an industrial scale. Most crystals have a natural shape like Their sizes vary greatly. Thus, natural quartz (rock crystal) can weigh up to hundreds of kilograms, and diamonds - up to several grams.

In amorphous solids, atoms are in constant vibration around randomly located points. They retain a certain short-range order, but lack long-range order. This is due to the fact that their molecules are located at a distance that can be compared with their size. The most common example of such a solid in our life is the glassy state. often considered as a liquid with infinitely high viscosity. The time of their crystallization is sometimes so long that it does not appear at all.

It is the above properties of these substances that make them unique. Amorphous solids are considered unstable because they can become crystalline over time.

The molecules and atoms that make up a solid are packed at high density. They practically retain their relative position relative to other particles and are held together due to intermolecular interaction. The distance between the molecules of a solid in different directions is called the crystal lattice parameter. The structure of a substance and its symmetry determine many properties, such as the electronic band, cleavage and optics. When a solid substance is exposed to a sufficiently large force, these qualities can be impaired to one degree or another. In this case, the solid body is subject to residual deformation.

Atoms of solids undergo vibrational movements, which determine their possession of thermal energy. Since they are negligible, they can only be observed under laboratory conditions. of a solid substance greatly affects its properties.

Study of solids

The features, properties of these substances, their qualities and the movement of particles are studied in various subfields of solid state physics.

The following methods are used for research: radio spectroscopy, structural analysis using X-rays and other methods. This is how the mechanical, physical and thermal properties of solids are studied. Hardness, load resistance, tensile strength, phase transformations are studied by materials science. It has a lot in common with solid state physics. There is other important modern science. The study of existing substances and the synthesis of new ones are carried out by solid state chemistry.

Features of solids

The nature of the movement of the outer electrons of the atoms of a solid substance determines many of its properties, for example, electrical ones. There are 5 classes of such bodies. They are set depending on the type of bond between atoms:

• Ionic, the main characteristic of which is the force of electrostatic attraction. Its features: reflection and absorption of light in the infrared region. At low temperatures, ionic bonds have low electrical conductivity. An example of such a substance is the sodium salt of hydrochloric acid (NaCl).
• Covalent, carried out by an electron pair that belongs to both atoms. Such a bond is divided into: single (simple), double and triple. These names indicate the presence of pairs of electrons (1, 2, 3). Double and triple bonds are called multiples. There is another division of this group. Thus, depending on the distribution of electron density, polar and nonpolar bonds are distinguished. The first is formed by different atoms, and the second by identical ones. This solid state of matter, examples of which are diamond (C) and silicon (Si), is distinguished by its density. The hardest crystals belong precisely to the covalent bond.
• Metallic, formed by combining the valence electrons of atoms. As a result, a general electron cloud appears, which shifts under the influence of electrical voltage. A metallic bond forms when the atoms being bonded are large. They are the ones who can donate electrons. In many metals and complex compounds, this bond forms a solid state of matter. Examples: sodium, barium, aluminum, copper, gold. The following non-metallic compounds can be noted: AlCr 2, Ca 2 Cu, Cu 5 Zn 8. Substances with metallic bonds (metals) have varied physical properties. They can be liquid (Hg), soft (Na, K), very hard (W, Nb).
• Molecular, occurring in crystals that are formed by individual molecules of a substance. It is characterized by gaps between molecules with zero electron density. The forces that bind atoms together in such crystals are significant. In this case, the molecules are attracted to each other only by weak intermolecular attraction. That is why the bonds between them are easily destroyed when heated. Connections between atoms are much more difficult to break down. Molecular bonding is divided into orientational, dispersive and inductive. An example of such a substance is solid methane.
• Hydrogen, which occurs between the positively polarized atoms of a molecule or part thereof and the negatively polarized smallest particle of another molecule or part. Such connections include ice.

Properties of solids

What do we know today? Scientists have long studied the properties of the solid state of matter. When it is exposed to temperatures, it also changes. The transition of such a body into liquid is called melting. The transformation of a solid into a gaseous state is called sublimation. As the temperature decreases, the solid crystallizes. Some substances under the influence of cold pass into the amorphous phase. Scientists call this process glass transition.

When the internal structure of solids changes. It acquires the greatest order as the temperature decreases. At atmospheric pressure and temperature T > 0 K, any substances existing in nature solidify. Only helium, which requires a pressure of 24 atm to crystallize, is an exception to this rule.

The solid state of a substance gives it various physical properties. They characterize the specific behavior of bodies under the influence of certain fields and forces. These properties are divided into groups. There are 3 methods of influence, corresponding to 3 types of energy (mechanical, thermal, electromagnetic). Accordingly, there are 3 groups of physical properties of solids:

• Mechanical properties associated with stress and deformation of bodies. According to these criteria, solids are divided into elastic, rheological, strength and technological. At rest, such a body retains its shape, but it can change under the influence of an external force. In this case, its deformation can be plastic (the original form does not return), elastic (returns to its original shape) or destructive (disintegration/breakage occurs when a certain threshold is reached). The response to the applied force is described by elastic moduli. A solid body resists not only compression and tension, but also shear, torsion and bending. The strength of a solid is its ability to resist destruction.
• Thermal, manifested when exposed to thermal fields. One of the most important properties is the melting point at which the body turns into a liquid state. It is observed in crystalline solids. Amorphous bodies have a latent heat of fusion, since their transition to a liquid state occurs gradually with increasing temperature. Upon reaching a certain heat, the amorphous body loses its elasticity and acquires plasticity. This state means it has reached the glass transition temperature. When heated, the solid body deforms. Moreover, it most often expands. Quantitatively, this state is characterized by a certain coefficient. Body temperature affects mechanical characteristics such as fluidity, ductility, hardness and strength.
• Electromagnetic, associated with the impact on solid matter of flows of microparticles and electromagnetic waves of high rigidity. These also include radiation properties.

Zone structure

Solids are also classified according to their so-called zone structure. So, among them there are:

• Conductors characterized in that their conduction and valence bands overlap. In this case, electrons can move between them, receiving the slightest energy. All metals are considered conductors. When a potential difference is applied to such a body, an electric current is formed (due to the free movement of electrons between points with the lowest and highest potential).
• Dielectrics whose zones do not overlap. The interval between them exceeds 4 eV. To conduct electrons from the valence band to the conduction band, large amounts of energy are required. Due to these properties, dielectrics practically do not conduct current.
• Semiconductors characterized by the absence of conduction and valence bands. The interval between them is less than 4 eV. To transfer electrons from the valence band to the conduction band, less energy is required than for dielectrics. Pure (undoped and intrinsic) semiconductors do not pass current well.

The movements of molecules in solids determine their electromagnetic properties.

Other properties

Solids are also classified according to their magnetic properties. There are three groups:

• Diamagnets, the properties of which depend little on temperature or state of aggregation.
• Paramagnets, which are a consequence of the orientation of conduction electrons and magnetic moments of atoms. According to Curie's law, their susceptibility decreases in proportion to temperature. So, at 300 K it is 10 -5.
• Bodies with an ordered magnetic structure, possessing long-range atomic order. Particles with magnetic moments are periodically located at the nodes of their lattice. Such solids and substances are often used in various fields of human activity.

The hardest substances in nature

What are they? The density of solids largely determines their hardness. In recent years, scientists have discovered several materials that claim to be the “strongest body.” The hardest substance is fullerite (a crystal with fullerene molecules), which is approximately 1.5 times harder than diamond. Unfortunately, it is currently only available in extremely small quantities.

Today, the hardest substance that may be used in industry in the future is lonsdaleite (hexagonal diamond). It is 58% harder than diamond. Lonsdaleite is an allotropic modification of carbon. Its crystal lattice is very similar to that of diamond. A cell of lonsdaleite contains 4 atoms, and a diamond - 8. Of the widely used crystals today, diamond remains the hardest.

Many natural phenomena indicate the chaotic movement of microparticles, molecules and atoms of matter. The higher the temperature of the substance, the more intense this movement. Therefore, the heat of a body is a reflection of the random movement of its constituent molecules and atoms.

Proof that all atoms and molecules of a substance are in constant and random movement can be diffusion - the interpenetration of particles of one substance into another (see Fig. 20a). Thus, the smell quickly spreads throughout the room even in the absence of air movement. A drop of ink quickly turns the entire glass of water uniformly black, although it would seem that gravity should help color the glass only in the top-to-bottom direction. Diffusion can also be detected in solids if they are pressed tightly together and left for a long time. The phenomenon of diffusion demonstrates that microparticles of a substance are capable of spontaneous movement in all directions. This movement of microparticles of a substance, as well as its molecules and atoms, is called thermal movement.

Obviously, all the water molecules in the glass are moving even if there is no drop of ink in it. Simply, the diffusion of ink makes the thermal movement of molecules noticeable. Another phenomenon that makes it possible to observe thermal motion and even evaluate its characteristics can be Brownian motion, which refers to the chaotic movement of any smallest particles in a completely calm liquid visible through a microscope. It was named Brownian in honor of the English botanist R. Brown, who in 1827, examining pollen spores of one of the plants suspended in water through a microscope, discovered that they were continuously and chaotically moving.

Brown's observation was confirmed by many other scientists. It turned out that Brownian motion is not associated either with flows in the liquid or with its gradual evaporation. The smallest particles (they were also called Brownian) behaved as if they were alive, and this “dance” of particles accelerated with heating of the liquid and with a decrease in the size of the particles and, conversely, slowed down when replacing water with a more viscous medium. Brownian motion was especially noticeable when it was observed in gas, for example, by following particles of smoke or droplets of fog in the air. This amazing phenomenon never stopped, and it could be observed for as long as desired.

An explanation of Brownian motion was given only in the last quarter of the 19th century, when it became obvious to many scientists that the motion of a Brownian particle is caused by random impacts of molecules of the medium (liquid or gas) undergoing thermal motion (see Fig. 20b). On average, the molecules of the medium impact a Brownian particle from all directions with equal force, however, these impacts never exactly cancel each other out, and as a result, the speed of the Brownian particle varies randomly in magnitude and direction. Therefore, the Brownian particle moves along a zigzag path. Moreover, the smaller the size and mass of a Brownian particle, the more noticeable its movement becomes.

In 1905, A. Einstein created the theory of Brownian motion, believing that at any given moment in time the acceleration of a Brownian particle depends on the number of collisions with molecules of the medium, which means it depends on the number of molecules per unit volume of the medium, i.e. from Avogadro's number. Einstein derived a formula by which it was possible to calculate how the mean square of the displacement of a Brownian particle changes over time, if you know the temperature of the medium, its viscosity, the size of the particle and Avogadro's number, which was still unknown at that time. The validity of this Einstein theory was confirmed experimentally by J. Perrin, who was the first to obtain the value of Avogadro's number. Thus, the analysis of Brownian motion laid the foundations of the modern molecular kinetic theory of the structure of matter.

Review questions:

· What is diffusion, and how is it related to the thermal movement of molecules?

· What is called Brownian motion, and is it thermal?

· How does the nature of Brownian motion change when heated?

Rice. 20. (a) – the upper part shows molecules of two different gases separated by a partition, which is removed (see lower part), after which diffusion begins; (b) in the lower left part there is a schematic representation of a Brownian particle (blue), surrounded by molecules of the medium, collisions with which cause the particle to move (see three trajectories of the particle).

§ 21. INTERMOLECULAR FORCES: STRUCTURE OF GASEOUS, LIQUID AND SOLID BODIES

We are accustomed to the fact that liquid can be poured from one vessel to another, and gas quickly fills the entire volume provided to it. Water can only flow along the riverbed, and the air above it knows no boundaries. If the gas did not try to occupy all the space around us, we would suffocate, because... The carbon dioxide we exhale would accumulate near us, preventing us from taking a breath of fresh air. Yes, and the cars would soon stop for the same reason, because... They also need oxygen to burn fuel.

Why does a gas, unlike a liquid, fill the entire volume provided to it? There are intermolecular attractive forces between all molecules, the magnitude of which decreases very quickly as the molecules move away from each other, and therefore at a distance equal to several molecular diameters they do not interact at all. It is easy to show that the distance between neighboring gas molecules is many times greater than that of a liquid. Using formula (19.3) and knowing the density of air (r=1.29 kg/m3) at atmospheric pressure and its molar mass (M=0.029 kg/mol), we can calculate the average distance between air molecules, which will be equal to 6.1.10- 9 m, which is twenty times the distance between water molecules.

Thus, between liquid molecules located almost close to each other, attractive forces act, preventing these molecules from scattering in different directions. On the contrary, the insignificant forces of attraction between gas molecules are not able to hold them together, and therefore gases can expand, filling the entire volume provided to them. The existence of intermolecular attractive forces can be verified by performing a simple experiment - pressing two lead bars against each other. If the contact surfaces are sufficiently smooth, the bars will stick together and will be difficult to separate.

However, intermolecular attractive forces alone cannot explain all the differences between the properties of gaseous, liquid and solid substances. Why, for example, is it very difficult to reduce the volume of a liquid or solid, but it is relatively easy to compress a balloon? This is explained by the fact that between molecules there are not only attractive forces, but also intermolecular repulsive forces, which act when the electron shells of the atoms of neighboring molecules begin to overlap. It is these repulsive forces that prevent one molecule from penetrating into a volume already occupied by another molecule.

When no external forces act on a liquid or solid body, the distance between their molecules is such (see r0 in Fig. 21a) at which the resultant forces of attraction and repulsion are equal to zero. If you try to reduce the volume of a body, the distance between the molecules decreases, and the resulting increased repulsive forces begin to act from the side of the compressed body. On the contrary, when a body is stretched, the elastic forces that arise are associated with a relative increase in the forces of attraction, because when molecules move away from each other, the repulsive forces fall much faster than the attractive forces (see Fig. 21a).

Gas molecules are located at distances tens of times greater than their sizes, as a result of which these molecules do not interact with each other, and therefore gases are much more easily compressed than liquids and solids. Gases do not have any specific structure and are a collection of moving and colliding molecules (see Fig. 21b).

A liquid is a collection of molecules that are almost closely adjacent to each other (see Fig. 21c). Thermal motion allows a liquid molecule to change its neighbors from time to time, jumping from one place to another. This explains the fluidity of liquids.

Atoms and molecules of solids are deprived of the ability to change their neighbors, and their thermal motion is only small fluctuations relative to the position of neighboring atoms or molecules (see Fig. 21d). The interaction between atoms can lead to the fact that a solid becomes a crystal, and the atoms in it occupy positions at the sites of the crystal lattice. Since the molecules of solid bodies do not move relative to their neighbors, these bodies retain their shape.

Review questions:

· Why don't gas molecules attract each other?

· What properties of bodies determine the intermolecular forces of repulsion and attraction?

How do you explain the fluidity of a liquid?

· Why do all solids retain their shape?

§ 22. IDEAL GAS. BASIC EQUATION OF THE MOLECULAR-KINETIC THEORY OF GASES.