Passive transport is the transport of substances along a concentration gradient that does not require energy. Hydrophobic substances are passively transported through the lipid bilayer. All protein-channels and some carriers pass substances passively through themselves. Passive transport involving membrane proteins is called facilitated diffusion.

Other carrier proteins (sometimes called pump proteins) transport substances across the membrane at the expense of energy, which is usually supplied by ATP hydrolysis. This type of transport occurs against the concentration gradient of the transported substance and is called active transport.

Symport, antiport and uniport

Membrane transport of substances also differs in the direction of their movement and the amount of substances carried by this carrier:

1) Uniport - transport of one substance in one direction depending on the gradient

2) Symport - transport of two substances in one direction through one carrier.

3) Antiport - the movement of two substances in different directions through one carrier.

Uniport carries out, for example, a voltage-dependent sodium channel through which sodium ions move into the cell during the generation of an action potential.

Symport carries out a glucose transporter located on the outer (facing the intestinal lumen) side of the cells of the intestinal epithelium. This protein simultaneously captures a glucose molecule and a sodium ion and, changing its conformation, transfers both substances into the cell. In this case, the energy of the electrochemical gradient is used, which, in turn, is created due to the hydrolysis of ATP by sodium-potassium ATP-ase.

Antiport carries out, for example, sodium-potassium ATPase (or sodium-dependent ATPase). It transports potassium ions into the cell. and out of the cell - sodium ions.

Work of sodium-potassium atPase as an example of antiport and active transport

Initially, this carrier attaches three ions to the inside of the membrane Na+ . These ions change the conformation of the ATPase active site. After such activation, ATPase is able to hydrolyze one ATP molecule, and the phosphate ion is fixed on the surface of the carrier from the inside of the membrane.

The released energy is spent on changing the ATPase conformation, after which three ions Na+ and ion (phosphate) are on the outside of the membrane. Here the ions Na+ split off, and is replaced by two ions K+ . Then the conformation of the carrier changes to the original one, and the ions K+ appear on the inner side of the membrane. Here the ions K+ are split off, and the carrier is ready for work again.

More briefly, the actions of ATPase can be described as follows:

1) It “takes” three ions from inside the cell Na+ , then splits the ATP molecule and attaches phosphate to itself

2) "Throws out" ions Na+ and adds two ions K+ from the external environment.

3) Removes phosphate, two ions K+ throws into the cell

As a result, a high concentration of ions is created in the extracellular environment. Na+ , and inside the cell - a high concentration K+ . Work Na + , K+ - ATPase creates not only a difference in concentrations, but also a difference in charges (it works like an electrogenic pump). A positive charge is created on the outside of the membrane, and a negative charge on the inside.

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In animals with a closed vascular system, the extracellular fluid is conventionally divided into two components:

1) interstitial fluid
2) circulating blood plasma.

Interstitial fluid is the part of the extracellular fluid that is located outside the vascular system and bathes the cells.

About 1/3 of the total body water is extracellular fluid, the remaining 2/3 is intracellular fluid.

Concentrations of electrolytes and colloidal substances differ significantly in plasma, interstitial and intracellular fluids. The most pronounced differences are the relatively low content of anionic proteins in the interstitial fluid, compared with the intracellular fluid and blood plasma, and higher concentrations of sodium and chlorine in the interstitial fluid, and potassium in the intracellular fluid.

The unequal composition of various liquid media of the body is largely due to the nature of the barriers separating them. Cell membranes separate the intracellular fluid from the extracellular fluid, while capillary walls separate the interstitial fluid from the plasma. Transport of substances across these barriers can occur passively through diffusion, filtration and osmosis, as well as through active transport.

Passive transport

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Rice. 1.12 Types of passive and active transport of substances across the membrane.

Schematically, the main types of transport of substances through the cell membrane are shown in Fig. 1.12

Fig.1.12 Types of passive and active transport of substances through the membrane.

3 - facilitated diffusion,

Passive transfer of substances through cell membranes does not require the expenditure of metabolic energy.

Types of passive transport

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Types of passive transport of substances:

• simple diffusion
• Osmosis
• Diffusion of ions
• Facilitated diffusion

simple diffusion

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Diffusion is the process by which a gas or solute spreads and fills the entire available volume.

Molecules and ions dissolved in a liquid are in chaotic motion, colliding with each other, solvent molecules and the cell membrane. The collision of a molecule or ion with a membrane can have a twofold outcome: the molecule either "bounces" off the membrane or passes through it. When the probability of the last event is high, the membrane is said to permeable to thissubstances.

If the concentration of a substance on both sides of the membrane is different, a flow of particles occurs, directed from a more concentrated solution to a dilute one. Diffusion occurs until the concentration of the substance on both sides of the membrane is equalized. They pass through the cell membrane as highly soluble in water. (hydrophilic) substances, and hydrophobic, poorly or completely insoluble in it.

Hydrophobic, highly lipid-soluble substances diffuse due to dissolution in membrane lipids.

Water and substances soluble in it penetrate through temporary defects in the hydrocarbon region of the membrane, the so-called. kinky, and also through pores, permanently existing hydrophilic regions of the membrane.

In the case when the cell membrane is impermeable or poorly permeable to a solute, but permeable to water, it is subjected to osmotic forces. At a lower concentration of a substance in the cell than in the environment, the cell shrinks; if the concentration of the solute in the cell is higher, water rushes into the cell.

Osmosis

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Osmosis- the movement of water (solvent) molecules through the membrane from an area of ​​​​lower to an area of ​​\u200b\u200bhigher concentration of a solute.

Osmotic pressure called the smallest pressure that must be applied to the solution in order to prevent the solvent from flowing through the membrane into a solution with a higher concentration of the substance.

Solvent molecules, like the molecules of any other substance, are set in motion by a force arising from the difference in chemical potentials. When a substance dissolves, the chemical potential of the solvent decreases. Therefore, in the region where the solute concentration is higher, the chemical potential of the solvent is lower. Thus, solvent molecules, moving from a solution with a lower concentration to a solution with a higher concentration, move in the thermodynamic sense “down”, “along the gradient”.

The volume of cells is largely regulated by the amount of water they contain. The cell is never in a state of complete equilibrium with the environment. The continuous movement of molecules and ions across the plasma membrane changes the concentration of substances in the cell and, accordingly, the osmotic pressure of its contents. If a cell secretes a substance, then in order to maintain a constant value of osmotic pressure, it must either release an appropriate amount of water, or absorb an equivalent amount of another substance. Since the environment surrounding most cells is hypotonic, it is important for the cells to prevent large amounts of water from entering them. Maintaining a constant volume even in an isotonic environment requires energy consumption, therefore, the concentration of substances incapable of diffusion (proteins, nucleic acids, etc.) in the cell is higher than in the pericellular environment. In addition, metabolites constantly accumulate in the cell, which disrupts the osmotic balance. The need to expend energy to maintain a constant volume is easily demonstrated in experiments with cooling or metabolic inhibitors. Under such conditions, the cells swell rapidly.

To solve the "osmotic problem" cells use two methods: they pump out the components of their contents or the water entering them into the interstitium. In most cases, cells use the first opportunity - pumping out substances, more often ions, using for this sodium pump(see below).

In general, the volume of cells that do not have rigid walls is determined by three factors:

1) the amount of substances contained in them and incapable of penetrating through the membrane;
2) the concentration in the interstitium of compounds that can pass through the membrane;
3) the ratio of the rates of penetration and pumping of substances from the cell.

An important role in the regulation of the water balance between the cell and the environment is played by the elasticity of the plasma membrane, which creates hydrostatic pressure that prevents water from entering the cell. If there is a difference in hydrostatic pressures in two areas of the medium, water can be filtered through the pores of the barrier separating these areas.

The phenomena of filtration underlie many physiological processes, such as the formation of primary urine in the nephron, the exchange of water between the blood and tissue fluid in the capillaries.

Diffusion of ions

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Diffusion of ions occurs mainly through specialized protein structures of the membrane - ion kacash, when they are open. Depending on the type of tissue, cells can have a different set of ion channels.

Distinguish between sodium, potassium, calcium, sodium-calcium and chloride channels. The transport of ions through channels has a number of features that distinguish it from simple diffusion. This is especially true for calcium channels.

Ion channels may be in open, closed and inactivated states. The transition of a channel from one state to another is controlled either by a change in the electrical potential difference across the membrane, or by the interaction of physiologically active substances with receptors.

Accordingly, ion channels are divided into potential dependent and receptor-driven. The selective permeability of an ion channel for a particular ion is determined by the presence of special selective filters at its mouth.

Facilitated diffusion

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Through biological membranes, in addition to water and ions, many substances (from ethanol to complex drugs) penetrate by simple diffusion. At the same time, even relatively small polar molecules, such as glycols, monosaccharides, and amino acids, practically do not penetrate through the membrane of most cells due to simple diffusion. They are transferred through facilitated diffusion.

Diffusion is called light substances along its concentration gradient, which is carried out with the participation of special protein carrier molecules.

Transport Na + , K + , Cl - , Li + , Ca 2+ , HCO 3 - and H + can also carry out specific carriers. The characteristic features of this type of membrane transport are a high rate of substance transfer compared to simple diffusion, dependence on the structure of its molecules, saturation, competition, and sensitivity to specific inhibitors - compounds that inhibit facilitated diffusion.

All of the above features of facilitated diffusion are the result of the specificity of carrier proteins and their limited number in the membrane. When a certain concentration of the transferred substance is reached, when all carriers are occupied by the transported molecules or ions, its further increase will not lead to an increase in the number of transported particles - saturation phenomenon. Substances that are similar in molecular structure and transported by the same carrier will compete for the carrier - competition phenomenon.

There are several types of transport of substances through facilitated diffusion (Fig. 1.13):

Rice. 1.13 Classification of methods of transport through the membrane.

Uniport, when molecules or ions are transferred through the membrane, regardless of the presence or transfer of other compounds (transport of glucose, amino acids through the basement membrane of epithelial cells);

Symport, in which their transfer is carried out simultaneously and unidirectionally with other compounds (sodium-dependent transport of sugars and amino acids Na + K +, 2Cl - and co-transport);

Antiport - (transport of a substance is due to the simultaneous and oppositely directed transport of another compound or ion (Na + / Ca 2+, Na + / H + Cl - / HCO 3 - - exchanges).

Symport and antiport are species cotransport, in which the speed of transfer is controlled by all participants in the transport process.

The nature of the carrier proteins is unknown. According to the principle of action, they are divided into two types. Carriers of the first type make shuttle movements through the membrane, and of the second type they are embedded in the membrane, forming a channel. Their action can be simulated with the help of antibiotic ionophores, a carrier of alkali metals. So, one of them - (valinomycin) - acts as a true carrier, ferrying potassium across the membrane. Molecules of gramicidin A, another ionophore, are inserted into the membrane one after another, forming a "channel" for sodium ions.

Most cells have a facilitated diffusion system. However, the list of metabolites transported by this mechanism is rather limited. Basically, these are sugars, amino acids and some ions. Compounds that are intermediate products of metabolism (phosphorylated sugars, products of amino acid metabolism, macroergs) are not transported using this system. Thus, facilitated diffusion serves to transport those molecules that the cell receives from the environment. An exception is the transport of organic molecules through the epithelium, which will be considered separately.

active transport

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active transport carried out by transport adenosine triphosphatases (ATPases) and occurs due to the energy of ATP hydrolysis.

Figure 1.12 shows the types of passive and active transport of substances through the membrane.

1,2 - simple diffusion through the bilayer and ion channel,
3 - facilitated diffusion,
4 - primary active transport,
5 - secondary active transport.

Types of active transport

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Types of active transport of substances:

primary active transport,

secondary active transport.

primary active transport

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The transport of substances from a medium with a low concentration to a medium with a higher concentration cannot be explained by movement along a gradient, i.e. diffusion. This process is carried out due to the energy of ATP hydrolysis or energy due to the concentration gradient of any ions, most often sodium. If the source of energy for the active transport of substances is the hydrolysis of ATP, and not the movement of some other molecules or ions through the membrane, transport calledprimary active.

The primary active transfer is carried out by transport ATPases, which are called ion pumps. In animal cells, the most common Na +, K + - ATPase (sodium pump), which is an integral protein of the plasma membrane and Ca 2+ - ATPase, contained in the plasma membrane of the sarco-(endo)-plasmic reticulum. All three proteins have a common property - the ability to be phosphorylated and form an intermediate phosphorylated form of the enzyme. In the phosphorylated state, the enzyme can be in two conformations, which are commonly referred to as E 1 and E 2 .

Enzyme conformation - this is a way of spatial orientation (laying) of the polypeptide chain of its molecule. These two conformations of the enzyme are characterized by different affinities for transported ions, i.e. different ability to bind transported ions.

Na + /K + - ATPase provides conjugated active transport of Na + from the cell and K + into the cytoplasm. In the Na + /K + - ATPase molecule, there is a special area (site) in which the binding of Na and K ions occurs. With the conformation of the enzyme E 1, this area is turned inside the plasma reticulum. For the implementation of this stage of the conversion of Ca 2+ -ATPase, the presence of magnesium ions in the sarcoplasmic reticulum is necessary. Subsequently, the cycle of the enzyme is repeated.

secondary active transport

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secondary active transport is the transfer of a substance across the membrane against its concentration gradient due to the energy of the concentration gradient of another substance created in the process of active transport. In animal cells, the main source of energy for secondary active transport is the energy of the sodium ion concentration gradient, which is created due to the work of Na + /K + - ATPase. For example, the cell membrane of the mucous membrane of the small intestine contains a protein that carries out the transfer (symport) of glucose and Na + to epitheliocytes. Glucose transport is carried out only if Na +, simultaneously binding with glucose to the specified protein, is transferred along the electrochemical gradient. The electrochemical gradient for Na+ is maintained by the active transport of these cations out of the cell.

In the brain, the work of Na + -pump is associated with reverse absorption (reabsorption) of mediators - physiologically active substances that are released from nerve endings under the action of excitatory factors.

In cardiomyocytes and smooth muscle cells, the functioning of Na + , K + -ATPase is associated with the transport of Ca 2+ through the plasma membrane, due to the presence in the cell membrane of a protein that carries out countertransport (antiport) of Na + and Ca 2+. Calcium ions are transported across the cell membrane in exchange for sodium ions and due to the energy of the concentration gradient of sodium ions.

A protein was found in cells that exchanges extracellular sodium ions for intracellular protons - Na + /H + - exchanger. This carrier plays an important role in maintaining a constant intracellular pH. The rate at which Na + /Ca 2+ and Na + /H + - exchange is carried out is proportional to the electrochemical Na + gradient across the membrane. With a decrease in the extracellular concentration of Na + inhibition of Na + , K + -ATPase by cardiac glycosides or in a potassium-free environment, the intracellular concentration of calcium and protons is increased. This increase in intracellular concentration of Ca 2+ with inhibition of Na + , K + -ATPase underlies the use of cardiac glycosides in clinical practice to enhance heart contractions.

From consideration of the physical essence of the periodic law, it follows that periodic changes in the chemical properties of elements associated with the electronic structure of atoms, which, in accordance with the laws of wave mechanics, also changes periodically. All periodic changes in the chemical properties of elements, as well as changes in various properties of simple and complex substances, are associated with the properties of atomic orbitals.

The next most important conclusion, which follows from the analysis of the data given in Table 6, is the conclusion about the periodic change in the nature of the filling of external energy levels by electrons, which causes periodic changes in the chemical properties of elements and their compounds.

The atomic radius is the radius of the sphere that contains the nucleus of an atom and 95% of the density of the entire electron cloud surrounding the nucleus. This is a conditional concept, because. The electron cloud of an atom does not have a clear boundary; it allows one to judge the size of the atom.

The numerical values ​​of the atomic radii of different chemical elements are found experimentally by analyzing the lengths of chemical bonds, i.e. the distances between the nuclei of interconnected atoms. The radii of atoms are usually expressed in nanometers (nm), 1 nm = 10–9 m, picometers (pm), 1 pm = 10–12 m or angstroms (A), 1 A = 10–10 m.

The dependence of atomic radii on the charge of the atomic nucleus Z has a periodic character. Within one period of the periodic system of chemical elements, D.I. Mendeleev, the largest value of the atomic radius of an alkali metal atom. Further, with increasing Z, the value of the radius decreases, reaches a minimum at the atom of an element of the VIIA group, and then increases abruptly at the atom of an inert gas, and then even more - at the atom of the alkali metal of the next period.

Ionic radius.

The radii of ions differ from the atomic radii of the corresponding elements. The loss of electrons by atoms leads to a decrease in their effective sizes, and the addition of excess electrons leads to an increase. Therefore, the radius of a positively charged ion (cation) is always less, and the radius of a negatively charged ion (anion) is always greater than the radius of the corresponding electrically neutral atom. Thus, the radius of the potassium atom is 0.236 nm, and the radius of the K + ion is 0.133 nm; the radii of the chlorine atom and the chloride ion Cl are 0.099 and 0.181 nm, respectively. In this case, the radius of the ion differs the more from the radius of the atom, the greater the charge of the ion. For example, the radii of the chromium atom and the Cr 2+ and Cr 3+ ions are 0.127, 0.083, and 0.064 nm, respectively.

Within the main subgroup, the radii of ions of the same charge, like the radii of atoms, increase with increasing nuclear charge

Ionization energy(a measure of the manifestation of metallic properties) is the energy required to detach an electron from an atom.

(Ca 0 - Ca 2+ + 2e - - H).

The more electrons on the outer electron layer, the greater the ionization energy. As the atomic radius increases, the ionization energy decreases. This explains the decrease in metallic properties in periods from left to right and the increase in metallic properties in groups from top to bottom. Cesium (Cs) is the most active metal.

The energy of electron affinity (a measure of the manifestation of non-metallic properties) is the energy that is released as a result of the attachment of an electron to an atom (Cl 0 + 1e - -> Cl - + H). With an increase in the number of electrons on the outer electron layer, the energy of electron affinity increases, and with an increase in the radius of the atom, it decreases. This explains the increase in non-metallic properties in periods from left to right and the decrease in non-metallic properties in the main subgroups from top to bottom.

The affinity energy of an atom to an electron, or just him electron affinity(ε), is called the energy released in the process of addition electron to a free atom E in its ground state with its transformation into a negative ion E - (the affinity of an atom to an electron is numerically equal, but opposite in sign, to the ionization energy of the corresponding isolated singly charged anion).

E + e − = E − + ε

Electronegativity- chemical property of an atom, a quantitative characteristic of the ability of an atom in a molecule to attract electrons from atoms of other elements.

The strongest metallic properties are those elements whose atoms easily donate electrons. The values ​​of their electronegativity are small (χ ≤ 1).

Non-metallic properties are especially pronounced in those elements whose atoms vigorously add electrons.

In each period of the Periodic Table, the electronegativity of the elements increases with increasing serial number (from left to right), in each group of the Periodic Table, the electronegativity decreases with increasing serial number (from top to bottom).

Element fluorine F has the highest, and the element cesium Cs - the smallest electronegativity among the elements of 1-6 periods.

"

One of the most important characteristics of the chemical elements involved in the formation of a chemical bond is the size of an atom (ion): with its increase, the strength of interatomic bonds decreases. The size of an atom (ion) is usually determined by the value of its radius or diameter. Since an atom (ion) does not have clear boundaries, the concept of "atomic (ionic) radius" implies that 90–98% of the electron density of an atom (ion) is contained in the sphere of this radius. Knowing the values ​​of atomic (ionic) radii makes it possible to estimate internuclear distances in crystals (that is, the structure of these crystals), since for many problems the shortest distances between the nuclei of atoms (ions) can be considered the sum of their atomic (ionic) radii, although such additivity is approximate and holds not in all cases.

Under atomic radius chemical element (about the ionic radius, see below), participating in the formation of a chemical bond, in the general case, agreed to understand half the equilibrium internuclear distance between the nearest atoms in the crystal lattice of the element. This concept, which is quite simple if we consider atoms (ions) as rigid spheres, actually turns out to be complex and often ambiguous. The atomic (ionic) radius of a chemical element is not a constant value, but varies depending on a number of factors, the most important of which are the type of chemical bond

and coordination number.

If the same atom (ion) in different crystals forms different types of chemical bonds, then it will have several radii - covalent in a crystal with a covalent bond; ionic in a crystal with an ionic bond; metallic in metal; van der Waals in a molecular crystal. The influence of the type of chemical bond can be seen in the following example. In diamond, all four chemical bonds are covalent and are formed sp 3-hybrids, so all four neighbors of a given atom are on the same and

the same distance from it d= 1.54 A˚) and the covalent radius of carbon in diamond will be

is equal to 0.77 A˚. In an arsenic crystal, the distance between atoms bound by covalent bonds ( d 1 = 2.52 A˚), much less than between atoms bound by van der Waals forces ( d 2 = 3.12 A˚), so As will have a covalent radius of 1.26 A˚ and van der Waals of 1.56 A˚ .

The atomic (ionic) radius also changes very sharply with a change in the coordination number (this can be observed during polymorphic transformations of elements). The smaller the coordination number, the lower the degree of space filling with atoms (ions) and the smaller the internuclear distances. An increase in the coordination number is always accompanied by an increase in internuclear distances.

It follows from the foregoing that the atomic (ionic) radii of different elements involved in the formation of a chemical bond can only be compared when they form crystals in which the same type of chemical bond is realized, and these elements in the formed crystals have the same coordination numbers .

Let us consider the main features of atomic and ionic radii in more detail.

Under covalent radii of elements It is customary to understand half of the equilibrium internuclear distance between the nearest atoms connected by a covalent bond.

A feature of covalent radii is their constancy in different "covalent structures" with the same coordination number Z j. In addition, covalent radii are usually additively bonded to each other, that is, the A–B distance is half the sum of the A–A and B–B distances in the presence of covalent bonds and the same coordination numbers in all three structures.

There are normal, tetrahedral, octahedral, quadratic and linear covalent radii.

The normal covalent radius of an atom corresponds to the case when an atom forms as many covalent bonds as it corresponds to its place in the periodic table: for carbon - 2, for nitrogen - 3, etc. This results in different values ​​of normal radii depending on the multiplicity (order) bonds (single bond, double, triple). If the bond is formed when the hybrid electron clouds overlap, then they speak of tetrahedral

(Z k = 4, sp 3-hybrid orbitals), octahedral ( Z k = 6, d 2sp 3-hybrid orbitals), quadratic ( Z k = 4, dsp 2-hybrid orbitals), linear ( Z k = 2, sp-hybrid orbitals) covalent radii.

It is useful to know the following about covalent radii (the values ​​\u200b\u200bof covalent radii for a number of elements are given in).

1. Covalent radii, unlike ionic ones, cannot be interpreted as the radii of atoms that have a spherical shape. Covalent radii are used only to calculate the internuclear distances between atoms united by covalent bonds, and do not say anything about the distances between atoms of the same type that are not covalently bonded.

2. The value of the covalent radius is determined by the multiplicity of the covalent bond. A triple bond is shorter than a double bond, which in turn is shorter than a single bond, so the covalent radius of a triple bond is smaller than the covalent radius of a double bond, which is smaller

single. It should be borne in mind that the order of the multiplicity of the relationship does not have to be an integer. It can also be fractional if the bond is resonant (benzene molecule, Mg2 Sn compound, see below). In this case, the covalent radius has an intermediate value between the values ​​corresponding to integer orders of the bond multiplicity.

3. If the bond is of a mixed covalent-ionic nature, but with a high degree of the covalent component of the bond, then the concept of the covalent radius can be introduced, but the influence of the ionic component of the bond on its value cannot be neglected. In some cases, this effect can lead to a significant decrease in the covalent radius, sometimes down to 0.1 A˚. Unfortunately, attempts to predict the magnitude of this effect in various

cases have not yet been successful.

4. The value of the covalent radius depends on the type of hybrid orbitals that take part in the formation of a covalent bond.

Ionic radii, of course, cannot be defined as half the sum of the distances between the nuclei of the nearest ions, since, as a rule, the sizes of cations and anions differ sharply. In addition, the symmetry of the ions may differ somewhat from spherical. Nevertheless, for real ionic crystals under ionic radius It is customary to understand the radius of the ball, which approximates the ion.

Ionic radii are used for approximate estimates of internuclear distances in ionic crystals. It is assumed that the distance between the nearest cation and anion is equal to the sum of their ionic radii. The typical error in determining internuclear distances in terms of ionic radii in such crystals is ≈0.01 A˚.

There are several systems of ionic radii that differ in the values ​​of the ionic radii of individual ions, but lead to approximately the same internuclear distances. The first work on the determination of ionic radii was carried out by V. M. Goldshmit in the 1920s. In it, the author used, on the one hand, the internuclear distances in ionic crystals measured by X-ray structural analysis, and, on the other hand, the values ​​of the ionic radii F– and O2– determined by

refractometry method. Most other systems also rely on the internuclear distances in crystals determined by diffraction methods and on some "reference" values ​​of the ionic radius of a particular ion. In the most widely known system

Pauling, this reference value is the ionic radius of the O2− peroxide ion, equal to

1.40A˚. This value for O2– agrees well with theoretical calculations. In the system of G. B. Bokiya and N. V. Belov, which is considered one of the most reliable, the ionic radius O2– is taken equal to 1.36 A˚.

In the 1970s and 1980s, attempts were made to directly determine the radii of ions by measuring the electron density using X-ray structural analysis, provided that the minimum of the electron density on the line connecting the nuclei is taken as the boundary of the ions. It turned out that this direct method leads to overestimated values ​​of the ionic radii of cations and to underestimated values ​​of the ionic radii of anions. In addition, it turned out that the values ​​of ionic radii determined by a direct method cannot be transferred from one compound to another, and the deviations from additivity are too large. Therefore, such ionic radii are not used to predict internuclear distances.

It is useful to know the following about ionic radii (in the tables below, the values ​​\u200b\u200bof ionic radii according to Bokiy and Belov are given).

1. The ionic radius for ions of the same element varies depending on its charge, and for the same ion it depends on the coordination number. Depending on the coordination number, tetrahedral and octahedral ionic radii are distinguished.

2. Inside one vertical row, more precisely, inside one group, periodic

system, the radii of ions with the same charge increase with an increase in the atomic number of the element, since the number of shells occupied by electrons increases, and hence the size of the ion.

Radius, A˚

3. For positively charged ions of atoms from the same period, the ionic radii rapidly decrease with increasing charge. The rapid decrease is explained by the action of two main factors in one direction: the strong attraction of “own” electrons by the cation, the charge of which increases with increasing atomic number; an increase in the strength of interaction between the cation and the anions surrounding it with an increase in the charge of the cation.

Radius, A˚

4. For negatively charged ions of atoms from the same period, the ionic radii increase with increasing negative charge. The two factors discussed in the previous paragraph in this case act in opposite directions, and the first factor prevails (an increase in the negative charge of the anion is accompanied by an increase in its ionic radius), therefore, an increase in ionic radii with an increase in the negative charge occurs much more slowly than a decrease in the previous case.

Radius, A˚

5. For the same element, that is, with the same initial electronic configuration, the radius of the cation is less than that of the anion. This is due to a decrease in the attraction of external "additional" electrons to the anion nucleus and an increase in the screening effect due to internal electrons (the cation has a lack of electrons, and the anion has an excess).

Radius, A˚

6. The sizes of ions with the same charge follow the periodicity of the periodic table. However, the value of the ionic radius is not proportional to the charge of the nucleus Z, which is due to the strong attraction of electrons by the nucleus. In addition, the lanthanides and actinides, in the series of which the radii of atoms and ions with the same charge do not increase, but decrease with increasing atomic number (the so-called lanthanide contraction and actinide contraction), are an exception to the periodic dependence.11

11 Lanthanide contraction and actinide contraction are due to the fact that in lanthanides and actinides, electrons added with an increase in atomic number fill internal d and f-shells with a principal quantum number less than the principal quantum number of a given period. At the same time, according to quantum mechanical calculations in d and especially in f states, the electron is much closer to the nucleus than in s and p states of a given period with a large quantum number, therefore d and f-electrons are located in the inner regions of the atom, although the filling of these states with electrons (we are talking about electronic levels in the energy space) occurs differently.

metal radii are considered equal to half the shortest distance between the nuclei of atoms in the crystallizing structure of a metal element. They depend on the coordination number. If we take the metallic radius of any element at Z k \u003d 12 per unit, then with Z k = 8, 6 and 4 the metallic radii of the same element will be 0.98 respectively; 0.96; 0.88. Metallic radii have the property of additivity. Knowing their values ​​makes it possible to approximately predict the parameters of the crystal lattices of intermetallic compounds.

The atomic radii of metals are characterized by the following features (data on the values ​​of the atomic radii of metals can be found in).

1. The metallic atomic radii of transition metals are generally smaller than the metallic atomic radii of non-transition metals, reflecting the greater bond strength in transition metals. This feature is due to the fact that the metals of transition groups and the metals closest to them in the periodic system have electronic d-shells, and electrons in d-states can take part in the formation of a chemical bond. Strengthening of the bond may be due partly to the appearance of a covalent component of the bond and partly to the van der Waals interaction of the ionic cores. In crystals of iron and tungsten, for example, electrons in d-states make a significant contribution to the binding energy.

2. Within one vertical group, as we move from top to bottom, the atomic radii of metals increase, which is due to a sequential increase in the number of electrons (the number of shells occupied by electrons increases).

3. Within one period, more precisely, starting from the alkali metal to the middle of the transition metal group, in the direction from left to right, the atomic metal radii decrease. In the same sequence, the electric charge of the atomic nucleus increases and the number of electrons in the valence shell increases. With an increase in the number of binding electrons per atom, the metallic bond is strengthened, and at the same time, due to an increase in the charge of the nucleus, the attraction of core (internal) electrons by the nucleus increases, so the value of the metallic atomic radius decreases.

4. Transition metals of groups VII and VIII from the same period in the first approximation have almost the same metal radii. Apparently, when it comes to elements that have 5 or more d-electrons, an increase in the nuclear charge and the associated effects of attraction of core electrons, leading to a decrease in the atomic metallic radius, are compensated by the effects caused by the increasing number of electrons in the atom (ion) that do not participate in the formation of a metallic bond, and leading to an increase in the metallic radius (increasing the number of states occupied by electrons).

5. The increase in radii (see paragraph 2) for transition elements, which occurs during the transition from the fourth to the fifth period, is not observed for transition elements at

transition from the fifth to the sixth period; the metallic atomic radii of the corresponding (vertical comparison) elements in these last two periods are almost the same. Apparently, this is due to the fact that the elements located between them are completed with a relatively deep f-shell, so the increase in the charge of the nucleus and the associated attraction effects turn out to be more significant than the effects associated with an increasing number of electrons (lanthanide contraction).

Element from 4 periods

Radius, A˚

Element from period 5

Radius, A˚

Element from period 6

Radius, A˚

6. Usually, metallic radii are much larger than ionic radii, but they do not differ so significantly from the covalent radii of the same elements, although without exception they are all larger than covalent ones. The large difference in the values ​​of the metallic atomic and ionic radii of the same elements is explained by the fact that the bond, which owes its origin to almost free conduction electrons, is not strong (hence the observed relatively large interatomic distances in the metal lattice). A significantly smaller difference in the values ​​of the metallic and covalent radii of the same elements can be explained if we consider the metallic bond as some special "resonant" covalent bond.

Under van der Waals radius It is customary to understand half of the equilibrium internuclear distance between the nearest atoms connected by a van der Waals bond. Van der Waals radii determine the effective sizes of noble gas atoms. In addition, as follows from the definition, the van der Waals atomic radius can be considered to be half the internuclear distance between the nearest atoms of the same name, connected by a van der Waals bond and belonging to different molecules (for example, in molecular crystals). When atoms approach each other at a distance less than the sum of their van der Waals radii, a strong interatomic repulsion occurs. Therefore, van der Waals atomic radii characterize the minimum allowable contacts of atoms belonging to different molecules. Data on the values ​​of van der Waals atomic radii for some atoms can be found in).

Knowing the van der Waals atomic radii makes it possible to determine the shape of molecules and their packing in molecular crystals. The van der Waals radii are much larger than all the radii of the same elements listed above, which is explained by the weakness of the van der Waals forces.