An ionic chemical bond occurs between. The meaning of the phrase “ionic bond.” Ions. Ionic bond

Ionic bond

Chemical bond theory takes the most important place in modern chemistry. She explains why atoms combine to form chemical particles, And allows you to compare the stability of these particles. Using chemical bond theory, Can predict the composition and structure of various compounds. Concept of the breaking of some chemical bonds and the formation of others is the basis of modern ideas about the transformations of substances during chemical reactions.

Chemical bond- This interaction of atoms, determining the stability of a chemical particle or crystal as a whole. Chemical bond is formed due to electrostatic interaction between charged particles: cations and anions, nuclei and electrons. When atoms come together, attractive forces begin to act between the nucleus of one atom and the electrons of another, as well as repulsive forces between nuclei and between electrons. On some distance these forces balance each other, And a stable chemical particle is formed.

When a chemical bond is formed, a significant redistribution of the electron density of atoms in the compound can occur in comparison with free atoms.

In the extreme case, this leads to the formation of charged particles - ions (from the Greek "ion" - going).

1 Ion interaction

If atom loses one or several electrons, then he turns into a positive ion - cation(translated from Greek - “ going down"). This is how they are formed cations hydrogen H + , lithium Li + , barium Ba 2+. By acquiring electrons, atoms turn into negative ions - anions(from the Greek "anion" - going up). Examples of anions are fluoride ion F−, sulfide ion S 2−.

Cations And anions able attract each other. In this case, there arises chemical bond, And chemical compounds are formed. This type of chemical bond is called ionic bond:

2 Definition of Ionic Bond

Ionic bond is a chemical bond educated due to electrostatic attraction between cations And anions.

The mechanism of ionic bond formation can be considered using the example of a reaction between sodium and chlorine. An alkali metal atom easily loses an electron, A halogen atom - acquires. As a result of this there is sodium cation And chloride ion. They form a connection due to electrostatic attraction between them.

Interaction between cations And anions independent of direction, That's why about ionic bonding they talk like non-directional. Every cation Maybe attract any number of anions, And vice versa. That's why ionic bond is unsaturated. Number interactions between ions in the solid state are limited only by the size of the crystal. That's why " molecule" ionic compound should be considered the entire crystal.

For the occurrence ionic bond necessary, to sum of ionization energy values E i(to form a cation) And electron affinity A e(for anion formation) must be energetically favorable. This limits the formation of ionic bonds by active metal atoms(elements of IA and IIA groups, some elements of IIIA group and some transition elements) and active nonmetals(halogens, chalcogens, nitrogen).

There is practically no ideal ionic bond. Even in those compounds that are usually classified as ionic, There is no complete transfer of electrons from one atom to another; electrons remain partially in common use. Yes, the connection is lithium fluoride by 80% ionic, and by 20% - covalent. Therefore, it is more correct to talk about degree of ionicity (polarity) covalent chemical bond. It is believed that with the difference electronegativities elements 2.1 communication is on 50% ionic. At greater difference compound can be considered ionic.

The ionic model of chemical bonding is widely used to describe the properties of many substances., first of all, connections alkaline And alkaline earth metals with nonmetals. This is due simplicity of description of such connections: believed to be built from incompressible charged spheres, answering cations and anions. In this case, the ions tend to arrange themselves in such a way that the attractive forces between them are maximum and the repulsive forces are minimal.

Ionic bond- a strong chemical bond formed between atoms with large difference (>1.7 on the Pauling scale) electronegativity, with which the shared electron pair is completely transferred to the atom with higher electronegativity. This is the attraction of ions as oppositely charged bodies. An example is the compound CsF, in which the “degree of ionicity” is 97%.

Ionic bond- extreme case polarization of covalent polar bond. Formed between typical metal and non-metal. In this case, the electrons in the metal completely switch to non-metal. Ions are formed.

If a chemical bond is formed between atoms that have very large electronegativity difference (EO > 1.7 according to Pauling), then the total electron pair is completely moves to an atom with greater EO. The result of this is the formation of a compound oppositely charged ions:

Between the formed ions there arises electrostatic attraction which is called ionic bond. Or rather, this look convenient. In practice ionic bond between atoms in in its pure form is not realized anywhere or almost nowhere, usually in reality the connection is partially ionic, and partially covalent in nature. At the same time communication complex molecular ions can often be considered purely ionic. The most important differences between ionic bonds and other types of chemical bonds are lack of direction and saturation. That is why crystals formed due to ionic bonds gravitate towards various dense packings of the corresponding ions.

3 Ionic radii

In simple electrostatic model of ionic bonding the concept is used ionic radii. The sum of the radii of neighboring cation and anion must be equal to the corresponding internuclear distance:

r 0 = r + + r −

At the same time it remains unclear where to spend boundary between cation and anion. Today it is known, that there is no purely ionic bond, as always there is some overlap of electron clouds. For calculations of ion radii use research methods, which allow you to determine the electron density between two atoms. The internuclear distance is divided at the point, Where electron density is minimal.

Ion sizes depend on many factors. At constant charge of the ion with increasing atomic number(and consequently, core charge) ionic radius decreases. This is especially noticeable in the lanthanide series, Where ionic radii vary monotonically from 117 pm for (La 3+) to 100 pm (Lu 3+) with a coordination number of 6. This effect is called lanthanide compression.

IN groups of elements ionic radii generally increase with increasing atomic number. However For d-elements of the fourth and fifth periods due to lanthanide compression even a decrease in ionic radius may occur(for example, from 73 pm for Zr 4+ to 72 pm for Hf 4+ with a coordination number of 4).

During the period there is a noticeable decrease in the ionic radius related to increased attraction of electrons to the nucleus with a simultaneous increase in the charge of the nucleus and the charge of the ion itself: 116 pm for Na +, 86 pm for Mg 2+, 68 pm for Al 3+ (coordination number 6). For the same reason an increase in the charge of an ion results in a decrease in the ionic radius for one element: Fe 2+ 77 pm, Fe 3+ 63 pm, Fe 6+ 39 pm (coordination number 4).

Comparison ionic radii Can carry out only with the same coordination number, because the it affects the size of the ion due to repulsive forces between counterions. This is clearly seen in the example Ag+ ion; its ionic radius is 81, 114 and 129 pm For coordination numbers 2, 4 and 6, respectively.

Structure ideal ionic compound, conditioned maximum attraction between unlike ions and minimal repulsion between like ions, in many ways determined by the ratio of the ionic radii of cations and anions. This can be shown simple geometric constructions.

4 Ionic bond energy

Energy communications And for ionic compound- This energy, which is in released during its formation from gaseous counterions infinitely distant from each other. Considering only electrostatic forces corresponds to about 90% of the total interaction energy, which also includes the contribution of non-electrostatic forces(For example, electron shell repulsion).

Electrons from one atom can completely transfer to another. This redistribution of charges leads to the formation of positively and negatively charged ions (cations and anions). A special type of interaction arises between them - an ionic bond. Let us consider in more detail the method of its formation, the structure and properties of substances.

Electronegativity

Atoms differ in electronegativity (EO) - the ability to attract electrons from the valence shells of other particles. For quantitative determination, the relative electronegativity scale (dimensionless value) proposed by L. Polling is used. The ability to attract electrons from fluorine atoms is more pronounced than other elements; its EO is 4. On the Pauling scale, fluorine is immediately followed by oxygen, nitrogen, and chlorine. The EO values ​​of hydrogen and other typical nonmetals are equal to or close to 2. Of the metals, most have electronegativity between 0.7 (Fr) and 1.7. There is a dependence of the ionicity of the bond on the difference in the EO of chemical elements. The larger it is, the higher the likelihood that an ionic bond will occur. This type of interaction is more common when the difference is EO = 1.7 and higher. If the value is less, then the compounds are polar covalent.

Ionization energy

To remove external electrons weakly bound to the nucleus, ionization energy (IE) is required. The unit of change of this physical quantity is 1 electron volt. There are patterns of changes in EI in the rows and columns of the periodic table, depending on the increase in the charge of the nucleus. In periods from left to right, the ionization energy increases and acquires the greatest values ​​for non-metals. In groups it decreases from top to bottom. The main reason is an increase in the radius of the atom and the distance from the nucleus to the outer electrons, which are easily detached. A positively charged particle appears - the corresponding cation. The value of EI can be used to determine whether an ionic bond occurs. Properties also depend on ionization energy. For example, alkali and alkaline earth metals have low EI values. They have pronounced restorative (metallic) properties. Inert gases are chemically inactive, which is due to their high ionization energy.

Electron affinity

In chemical interactions, atoms can add electrons to form a negative particle - an anion; the process is accompanied by the release of energy. The corresponding physical quantity is electron affinity. The unit of measurement is the same as ionization energy (1 electron volt). But its exact values ​​are not known for all elements. Halogens have the highest electron affinity. At the outer level of atoms of elements there are 7 electrons, only one is missing to reach the octet. The electron affinity of halogens is high and they have strong oxidizing (non-metallic) properties.

Interactions of atoms during the formation of ionic bonds

Atoms that have an incomplete outer level are in an unstable energy state. The desire to achieve a stable electronic configuration is the main reason that leads to the formation of chemical compounds. The process is usually accompanied by the release of energy and can lead to molecules and crystals that differ in structure and properties. Strong metals and nonmetals differ significantly from each other in a number of indicators (EO, EI and electron affinity). A type of interaction more suitable for them is an ionic chemical bond, in which the unifying molecular orbital (shared electron pair) moves. It is believed that when metals form ions, they completely transfer electrons to nonmetals. The strength of the resulting bond depends on the work required to destroy the molecules that make up 1 mole of the substance under study. This physical quantity is known as binding energy. For ionic compounds, its values ​​range from several tens to hundreds of kJ/mol.

Ion formation

An atom that donates its electrons during chemical interactions becomes a cation (+). The receiving particle is an anion (-). To find out how atoms will behave and whether ions will appear, it is necessary to establish the difference between their EOs. The easiest way to carry out such calculations is for a compound of two elements, for example, sodium chloride.

Sodium has only 11 electrons, the configuration of the outer layer is 3s 1. To complete it, it is easier for an atom to give away 1 electron than to add 7. The structure of the valence layer of chlorine is described by the formula 3s 2 3p 5. In total, an atom has 17 electrons, 7 external ones. One thing is missing to achieve an octet and a stable structure. Chemical properties confirm the assumption that the sodium atom donates and chlorine accepts electrons. Ions appear: positive (sodium cation) and negative (chlorine anion).

Ionic bond

By losing an electron, sodium acquires a positive charge and a stable shell of the inert gas atom neon (1s 2 2s 2 2p 6). As a result of interaction with sodium, chlorine receives an additional negative charge, and the ion repeats the structure of the atomic shell of the noble gas argon (1s 2 2s 2 2p 6 3s 2 3p 6). The acquired electrical charge is called the charge of the ion. For example, Na +, Ca 2+, Cl -, F -. The ions may contain atoms of several elements: NH 4 +, SO 4 2-. Within such complex ions, particles are bound by a donor-acceptor or covalent mechanism. Electrostatic attraction occurs between differently charged particles. Its value in the case of an ionic bond is proportional to the charges, and with increasing distance between the atoms it weakens. Characteristic features of an ionic bond:

• strong metals react with active non-metallic elements;
• electrons move from one atom to another;
• the resulting ions have a stable configuration of outer shells;
• Electrostatic attraction occurs between oppositely charged particles.

Crystal lattices of ionic compounds

In chemical reactions, metals of groups 1, 2 and 3 of the periodic table usually lose electrons. Single-, double- and triple-charged positive ions are formed. Nonmetals of groups 6 and 7 usually gain electrons (with the exception of reactions with fluorine). Single and doubly charged negative ions appear. Energy costs for these processes, as a rule, are compensated when creating a crystal of the substance. Ionic compounds are usually in a solid state, forming structures consisting of oppositely charged cations and anions. These particles attract and form giant crystal lattices in which positive ions are surrounded by negative particles (and vice versa). The total charge of a substance is zero, because the total number of protons is balanced by the number of electrons of all atoms.

Properties of substances with ionic bonds

Ionic crystalline substances are characterized by high boiling and melting points. Typically these connections are heat resistant. The following feature can be detected when such substances are dissolved in a polar solvent (water). The crystals are easily destroyed, and the ions pass into the solution, which is electrically conductive. Ionic compounds are also destroyed when melted. Free charged particles appear, which means the melt conducts electric current. Substances with ionic bonds are electrolytes - conductors of the second kind.

Oxides and halides of alkali and alkaline earth metals belong to the group of ionic compounds. Almost all of them find wide application in science, technology, chemical production, and metallurgy.

Definition 1

When studying the structure of a molecule, the question arises about the nature of the forces that provide the connection between the neutral atoms that make up their composition. Such bonds between atoms in a molecule are called chemical bond.

Classified into two types:

• ionic bond;
• covalent bond.

The division is made conditionally. Most cases are characterized by the presence of features of both types of connections. With the help of detailed and empirical studies, it is possible to establish in each case the relationship between the degree of “ionicity” and “covalence” of the bond.

It has been experimentally proven that when a molecule is separated into its constituent atoms, work must be done. That is, the process of its formation must be accompanied by the release of energy. If two hydrogen atoms are in a free state, they have greater energy compared to the atoms in a diatomic H 2 molecule. The energy released during the formation of a molecule is considered a measure of the work of the interaction forces that bind atoms into a molecule.

Experiments prove that the appearance of the interaction force between atoms is due to the presence of external valence electrons of the atoms. This is possible due to a sharp change in the optical spectrum of atoms entering into chemical reactions while maintaining the X-ray characteristic spectrum of the atoms without changing, regardless of the type of chemical compound.

Line optical spectra are determined by the state of the valence electrons, and characteristic X-ray radiation is determined using the internal electrons, that is, their state. Chemical interactions involve electrons, requiring little energy to undergo their changes. The outer electrons have this function. They have a lower ionization potential compared to electrons in the inner shells.

Ionic bond

There is an assumption about the nature of the chemical bond of atoms in a molecule, which indicates the emergence of an interaction force of an electrical nature between external electrons. To fulfill the stability condition, there must be two interacting atoms with electric charges of opposite signs. The type of chemical bond can be realized only in some molecules. After the interaction of atoms, transformation into ions occurs. When an atom gains one or more electrons then it becomes a negative ion and the other becomes a positive ion.

Ionic bonding is similar to the forces of attraction between charges of opposite signs. If the positively charged sodium ion N a + is attracted to the negative chlorine C l -, then we get the molecule N a Cl, which serves as a clear example of an ionic bond.

Definition 2

In other words, ionic chemical bond called heteropolar (hetero - different). Molecules and ionic bond types - ionic or heteropolar molecules.

The concept of an ionic bond does not make it possible to explain the structures and structures of all molecules. It is inexplicable why a molecule can be formed from two neutral hydrogen atoms. Due to the identical polarity of the hydrogen atoms, it is unacceptable to assume that one of the hydrogen ions has a positive charge and the other has a negative charge. The bond that hydrogen atoms have (between neutral atoms) can only be explained by quantum mechanics. It is called covalent.

Covalent bond

Definition 3

A chemical bond between neutral atoms in a molecule is called covalent or homeopolar(homeo – same). Molecules formed on the basis of such bonds are called homeopolar or atomic.

Classical physics considers only one type of interaction where its implementation between two bodies is possible - gravity. Since gravitational forces are small, it is difficult to explain the interaction in a homeopolar molecule with their help.

A covalent bond consists of being in a certain quantum state with a certain energy of an electron in the field of the nucleus. If the distances between nuclei change, this is reflected in the state of motion of the electron and its energy. As the energy between atoms decreases, the energy of interaction between nuclei increases, which is explained by the action of the repulsive force.

When the electron energy decreases with decreasing distance faster than the nuclear interaction energy increases, then the value of the total energy of the system decreases significantly. This is explained by the action of forces tending to reduce the distance between nuclei in a system composed of two repelling nuclei and an electron. The existing attractive forces are involved in the generation of a covalent bond of the molecule. Their appearance is provoked by the presence of a common electron, in other words, due to the electronic exchange between atoms, which means they are considered exchange quantum forces.

A covalent bond has the property of saturation. Its manifestation is possible due to a certain valence of atoms. That is, a hydrogen atom bonds with one hydrogen atom, and a carbon atom with no more than 4 hydrogen atoms.

The proposed connection contributes to the explanation of the valency of atoms, which was not received in classical physics. That is, the saturation property is not clear from the point of view of the nature of interaction in classical theory.

The presence of covalent bonds is observed not only in diatomic molecules. It is characteristic of a large number of molecules of inorganic compounds (nitric oxide, ammonia and others).

In 1927, a quantitative theory of covalent bonding for the hydrogen molecule was created by W. Heitler and F. London, based on the concepts of quantum mechanics. They proved the reason that causes the appearance of a molecule with a covalent bond, namely: the quantum mechanical effect associated with the indistinguishability of electrons. The determination of the main binding energy occurs in the presence of an exchange integral. The total spin of a hydrogen molecule is 0, it has no orbital momentum, so it is diamagnetic. When two hydrogen atoms collide, a molecule appears only when the spins of both electrons are parallel. This condition promotes the repulsion of hydrogen atoms, meaning molecules will not be able to form.

When two identical atoms are connected by a covalent bond, the arrangement of the electron cloud in the molecule becomes symmetrical. If a bond unites two different atoms, then the electron cloud is located asymmetrically. A molecule with an asymmetric distribution of the electron cloud has a permanent dipole moment, that is, it is polar. When the probability of localizing an electron near one of the atoms prevails over the probability of finding this electron near another atom, a transition from a covalent bond to an ionic bond occurs. There is no clear boundary between ionic and covalent bonds.

Example 1

Describe the state when two atoms approach each other.

Solution

When the distance between two atoms is reduced, several situations may arise:

1. One pair of electrons or more become shared between the atoms in question. They can move between atoms and stay there longer than in other places. This helps create a force of attraction.
2. The emergence of ionic bonds. One or more electrons are capable of moving to another. That is, this contributes to the appearance of attractive positive and negative ions.
3. No connection occurs. The electronic structures of the two atoms overlap and form a single system. According to the Pauli principle, such a system is inappropriate only for the quantum state of two electrons. When moving to a higher energy level, the system will receive more energy, which will lead to instability. Even if the Pauli principle is satisfied, without increasing the energy of the system, an electrical repulsive force will appear between different electrons. According to the condition, there is much less influence on the creation of the connection than with the Pauli principle.

Example 2

The ionization energy (ionization potential) of an element is the energy required to remove an electron from one atom. It is considered a measure of the binding strength of the outer electron or electrons. Explain why the ionization energy of lithium is greater than sodium, sodium is greater than potassium, and potassium is greater than rubidium.

Solution

All of the above elements have the properties of alkali metals and belong to the first group. Any of their atoms has a single outer electron in the s state. The electrons of the inner shells partially shield the outer electron from the nuclear charge + Z q e as a consequence of the effective charge holding the outer electron equal to + q e . To remove an outer electron from such an atom, work must be done to transform alkali metal atoms into positive ions. The larger the size of the atom, the greater the distance of the valence electron from the nucleus, but the less force of its attraction. This group is characterized by a decrease in ionization energy from top to bottom according to the periodic table of Mendeleev. Its growth in each period from left to right is associated with an increase in charge and a constant number of internal screening electrons.

If you notice an error in the text, please highlight it and press Ctrl+Enter

The first of these is the formation of ionic bonds. (The second is education, which will be discussed below). When an ionic bond is formed, a metal atom loses electrons, and a non-metal atom gains electrons. For example, consider the electronic structure of sodium and chlorine atoms:

Na 1s 2 2s 2 2 p 6 3 s 1 - one electron in the outer level

Cl 1s 2 2s 2 2 p 6 3 s 2 3 p 5 — seven electrons in the outer level

If a sodium atom donates its only 3s electron to a chlorine atom, the octet rule will be satisfied for both atoms. The chlorine atom will have eight electrons on the outer third layer, and the sodium atom will also have eight electrons on the second layer, which has now become the outer layer:

Na+1s2 2s 2 2 p 6

Cl - 1s 2 2s 2 2 p 6 3 s 2 3 p 6 - eight electrons in the outer level

In this case, the nucleus of the sodium atom still contains 11 protons, but the total number of electrons has decreased to 10. This means that the number of positively charged particles is one more than the number of negatively charged ones, so the total charge of the sodium “atom” is +1.
The chlorine “atom” now contains 17 protons and 18 electrons and has a charge of -1.
Charged atoms formed by the loss or gain of one or more electrons are called ions. Positively charged ions are called cations, and negatively charged ones are called anions.
Cations and anions, having opposite charges, are attracted to each other by electrostatic forces. This attraction of oppositely charged ions is called ionic bonding. . It occurs in compounds formed by a metal and one or more nonmetals. The following compounds satisfy this criterion and are ionic in nature: MgCl 2, Fel 2, CuF, Na 2 0, Na 2 S0 4, Zn(C 2 H 3 0 2) 2.

There is another way to depict ionic compounds:

In these formulas, dots show only electrons located in the outer shells ( valence electrons ). Such formulas are called Lewis formulas in honor of the American chemist G. N. Lewis, one of the founders (along with L. Pauling) of the theory of chemical bonding.

The transfer of electrons from a metal atom to a non-metal atom and the formation of ions are possible due to the fact that non-metals have high electronegativity, and metals have low electronegativity.

Due to the strong attraction of ions to each other, ionic compounds are mostly solid and have a fairly high melting point.

An ionic bond is formed by the transfer of electrons from a metal atom to a non-metal atom. The resulting ions are attracted to each other by electrostatic forces.

Ionic bond

Chemical bond theory takes the most important place in modern chemistry. She explains why atoms combine to form chemical particles, And allows you to compare the stability of these particles. Using chemical bond theory, Can predict the composition and structure of various compounds. Concept of the breaking of some chemical bonds and the formation of others is the basis of modern ideas about the transformations of substances during chemical reactions .

Chemical bond- This interaction of atoms , determining the stability of a chemical particle or crystal as a whole . Chemical bond is formed due to electrostatic interaction between charged particles : cations and anions, nuclei and electrons. When atoms come together, attractive forces begin to act between the nucleus of one atom and the electrons of another, as well as repulsive forces between nuclei and between electrons. On some distance these forces balance each other, And a stable chemical particle is formed .

When a chemical bond is formed, a significant redistribution of the electron density of atoms in the compound can occur in comparison with free atoms.

In the extreme case, this leads to the formation of charged particles - ions (from the Greek "ion" - going).

1 Ion interaction

If atom loses one or several electrons, then he turns into a positive ion - cation(translated from Greek - “ going down"). This is how they are formed cations hydrogen H + , lithium Li + , barium Ba 2+ . By acquiring electrons, atoms turn into negative ions - anions(from the Greek "anion" - going up). Examples of anions are fluoride ion F−, sulfide ion S 2− .

Cations And anions able attract each other. In this case, there arises chemical bond, And chemical compounds are formed. This type of chemical bond is called ionic bond :

2 Definition of Ionic Bond

Ionic bond is a chemical bond educated due to electrostatic attraction between cations And anions .

The mechanism of ionic bond formation can be considered using the example of a reaction between sodium and chlorine . An alkali metal atom easily loses an electron, A halogen atom - acquires. As a result of this there is sodium cation And chloride ion. They form a connection due to electrostatic attraction between them .

Interaction between cations And anions independent of direction, That's why about ionic bonding they talk like non-directional. Every cation Maybe attract any number of anions, And vice versa. That's why ionic bond is unsaturated. Number interactions between ions in the solid state are limited only by the size of the crystal. That's why " molecule " ionic compound should be considered the entire crystal .

For the occurrence ionic bond necessary, to sum of ionization energy values E i(to form a cation) And electron affinity A e(for anion formation) must be energetically favorable. This limits the formation of ionic bonds by active metal atoms(elements of IA and IIA groups, some elements of IIIA group and some transition elements) and active nonmetals(halogens, chalcogens, nitrogen).

There is practically no ideal ionic bond. Even in those compounds that are usually classified as ionic , There is no complete transfer of electrons from one atom to another ; electrons remain partially in common use. Yes, the connection is lithium fluoride by 80% ionic, and by 20% - covalent. Therefore, it is more correct to talk about degree of ionicity (polarity) covalent chemical bond. It is believed that with the difference electronegativities elements 2.1 communication is on 50% ionic. At greater difference compound can be considered ionic .

The ionic model of chemical bonding is widely used to describe the properties of many substances., first of all, connections alkaline And alkaline earth metals with nonmetals. This is due simplicity of description of such connections: believed to be built from incompressible charged spheres, answering cations and anions. In this case, the ions tend to arrange themselves in such a way that the attractive forces between them are maximum and the repulsive forces are minimal.

Ionic bond- a strong chemical bond formed between atoms with large difference (>1.7 on the Pauling scale) electronegativity, with which the shared electron pair is completely transferred to the atom with higher electronegativity. This is the attraction of ions as oppositely charged bodies. An example is the compound CsF, in which the “degree of ionicity” is 97%.

Ionic bond- extreme case polarization of covalent polar bond. Formed between typical metal and non-metal. In this case, the electrons in the metal completely switch to non-metal . Ions are formed.

If a chemical bond is formed between atoms that have very large electronegativity difference (EO > 1.7 according to Pauling), then the total electron pair is completely moves to an atom with greater EO. The result of this is the formation of a compound oppositely charged ions :

Between the formed ions there arises electrostatic attraction which is called ionic bond. Or rather, this look convenient. In practice ionic bond between atoms in in its pure form is not realized anywhere or almost nowhere, usually in reality the connection is partially ionic , and partially covalent in nature. At the same time communication complex molecular ions can often be considered purely ionic. The most important differences between ionic bonds and other types of chemical bonds are lack of direction and saturation. That is why crystals formed due to ionic bonds gravitate towards various dense packings of the corresponding ions.

3 Ionic radii

In simple electrostatic model of ionic bonding the concept is used ionic radii . The sum of the radii of neighboring cation and anion must be equal to the corresponding internuclear distance :

r 0 = r + + r −

At the same time it remains unclear where to spend boundary between cation and anion . Today it is known , that there is no purely ionic bond, as always there is some overlap of electron clouds. For calculations of ion radii use research methods, which allow you to determine the electron density between two atoms . The internuclear distance is divided at the point, Where electron density is minimal .

Ion sizes depend on many factors. At constant charge of the ion with increasing atomic number(and consequently, core charge) ionic radius decreases. This is especially noticeable in the lanthanide series, Where ionic radii vary monotonically from 117 pm for (La 3+) to 100 pm (Lu 3+) with a coordination number of 6. This effect is called lanthanide compression .

IN groups of elements ionic radii generally increase with increasing atomic number. However For d-elements of the fourth and fifth periods due to lanthanide compression even a decrease in ionic radius may occur(for example, from 73 pm for Zr 4+ to 72 pm for Hf 4+ with a coordination number of 4).

During the period there is a noticeable decrease in the ionic radius related to increased attraction of electrons to the nucleus with a simultaneous increase in the charge of the nucleus and the charge of the ion itself: 116 pm for Na +, 86 pm for Mg 2+, 68 pm for Al 3+ (coordination number 6). For the same reason an increase in the charge of an ion results in a decrease in the ionic radius for one element: Fe 2+ 77 pm, Fe 3+ 63 pm, Fe 6+ 39 pm (coordination number 4).

Comparison ionic radii Can carry out only with the same coordination number, because the it affects the size of the ion due to repulsive forces between counterions. This is clearly seen in the example Ag+ ion; its ionic radius is 81, 114 and 129 pm For coordination numbers 2, 4 and 6 , respectively .

Structure ideal ionic compound, conditioned maximum attraction between unlike ions and minimal repulsion between like ions, in many ways determined by the ratio of the ionic radii of cations and anions. This can be shown simple geometric constructions.

4 Ionic bond energy

Energy communications And for ionic compound- This energy, which is in released during its formation from gaseous counterions infinitely distant from each other . Considering only electrostatic forces corresponds to about 90% of the total interaction energy, which also includes the contribution of non-electrostatic forces(For example, electron shell repulsion).

Whenever ionic bond between two free ion energy their attraction is determined by Coulomb's law :

E(adv.) = q+ q− / (4π r ε),

Where q+ And q−- charges interacting ions , r - distance between them , ε - dielectric constant of the medium .

Since one of the charges negative, That energy value Also will be negative .

According to Coulomb's law, on At infinitely small distances, the energy of attraction must become infinitely large. However, this not happening, because ions are not point charges. At bringing ions closer together repulsive forces arise between them, conditioned interaction of electronic clouds . Ion repulsion energy described Born equation :

E(ott.) = B / rn,

Where IN - some constant , n Maybe take values ​​from 5 to 12(depends on ion size). The total energy is determined by the sum of the energies of attraction and repulsion :

E = E(in.) + E(out.)

Its meaning passes through minimum . The coordinates of the minimum point correspond to the equilibrium distance r 0 And equilibrium energy of interaction between ions E 0 :

E0 = q+ q− (1 - 1 / n) / (4π r0 ε)

IN crystal lattice Always there are more interactions, how between a pair of ions. This number determined primarily by the type of crystal lattice. For accounting for all interactions(weakening with increasing distance) into the expression for ionic energy crystal lattice introduce the so-called constant Madelunga A :

E(adv.) = A q+ q− / (4π r ε)

Constant value Madelunga determined only lattice geometry and not depends on the radius and charge of the ions. For example, for sodium chloride it is equal 1,74756 .

5 ion polarization

Besides charge magnitude And radius important characteristic and she are his polarization properties. Let's consider this issue in a little more detail. U non-polar particles (atoms, ions, molecules) the centers of gravity of positive and negative charges coincide. In an electric field, electron shells shift in the direction of a positively charged plate, and nuclei - towards the negatively charged plate. Due to particle deformation arises in it dipole, she becomes polar .

Source electric field in compounds with an ionic type of bond are the ions themselves. Therefore, speaking about polarization properties of the ion , necessary distinguish polarizing effect of a given ion And its ability to polarize in an electric field .

Polarizing effect of the ion will be the one big, how more of his force field, i.e. than greater charge and smaller radius of the ion. Therefore in within subgroups in the Periodic Table of Elements the polarizing effect of ions decreases from top to bottom, since in subgroups, with a constant charge of the ion, its radius increases from top to bottom .

That's why the polarizing effect of alkali metal ions, for example, increases from cesium to lithium, and in the row halide ions - from I to F. In periods the polarizing effect of ions increases from left to right together with increasing ion charge And decreasing its radius .

Ion polarizability, his ability to deformations increase with decreasing force field, i.e. with decreasing the amount of charge And increasing radius . Polarizability of anions usually higher, how cations and in a row halides increases from F to I .

On polarization properties of cations provides influence of the nature of their outer electron shell . Polarization properties of cations how in active, and in passive sense at same charge and close radii increase upon transition from cations with a filled shell to cations with an incomplete outer shell and then to cations with an 18-electron shell.

For example, in the series of cations Mg 2+, Ni 2+, Zn 2+ polarization properties are intensifying. This pattern is consistent with the change in the radius of the ion and the structure of its electron shell given in the series:

For anions polarization properties deteriorate in this sequence:

I - , Br - , Cl - , CN - , OH - , NO 3 - , F - , ClO 4 - .

The result polarization interaction of ions is deformation of their electronic shells and, as a consequence of this, reduction of interionic distances And incomplete separation of the negative And positive charges between ions.

For example, in a crystal sodium chloride amount of charge on sodium ion amounts to +0,9 , and on chlorine ion - 0.9 instead of expected unit. In a molecule KCl located in vapor state, value charges on potassium ions And chlorine is 0.83 charge units, and in the molecule hydrogen chloride- only 0,17 units of charge.

Ion polarization provides noticeable effect on the properties of compounds with ionic bonds , lowering their melting and boiling points , reducing electrolytic dissociation in solutions and melts, etc. .

Ionic compounds are formed when interaction of elements , significantly different in chemical properties. The more elements removed from each other in the periodic table, those in ionic bonding is more pronounced in their compounds . Against, in molecules, formed by identical atoms or atoms of elements similar in chemical properties, arise other types of communication. That's why ionic bond theory It has limited use .

6 The influence of ion polarization on the properties of substances and the properties of ionic bonds and ionic compounds

Ideas about Ion polarizations help explain differences in the properties of many similar substances. For example, comparison sodium chlorides And potassium with silver chloride shows that when close ionic radii

polarizability of the Ag+ cation having 18-electron outer shell , higher, What leads to an increase in the strength of the metal-chlorine bond And less solubility of silver chloride in water .

Mutual polarization of ions facilitates the destruction of crystals, that leads to lowering the melting point of substances. For this reason melting temperature TlF (327 oС) significantly lower than RbF (798 oC). The decomposition temperature of substances also decreases with increasing mutual polarization of ions. That's why iodides usually decompose at lower temperatures, how other halides, A lithium compounds - less thermally stable , than compounds of other alkaline elements .

Deformability of electron shells also affects the optical properties of substances. How the particle is more polarized , the lower the energy of electronic transitions. If polarization is low , excitation of electrons requires higher energy what answers ultraviolet part of the spectrum. Such substances are usually colorless. In the case of strong polarization of ions, electrons are excited by absorption of electromagnetic radiation in the visible region of the spectrum. That's why some substances, educated colorless ions, colored .

Characteristics ionic compounds serves good solubility in polar solvents (water, acids, etc.). This is due to charge of parts of the molecule. Wherein solvent dipoles are attracted to the charged ends of the molecule, and as a result Brownian motion , « are being taken away» molecule substances into parts and surround them , not allowing us to connect again. The result is surrounded ions solvent dipoles .

When dissolving such compounds, as a rule, energy is released, since the total energy of the formed bonds solvent-ion is greater than the anion-cation binding energy. There are many exceptions nitric acid salts (nitrates), which absorb heat when dissolved (solutions are cooled). The latter fact is explained on the basis of laws that considered in physical chemistry .

7 Crystalline grid

Ionic compounds(for example, sodium chloride NaCl) - hard And refractory because of between the charges of their ions(“+” and “–”) exist powerful forces of electrostatic attraction .

The negatively charged chlorine ion attracts Not only " mine " Na+ ion, but also other sodium ions around you. This leads to, What near any of the ions there is more than one ion with the opposite sign , and a few(Fig. 1).

Rice. 1. Crystal structure table salt NaCl .

In fact, about every chlorine ion contains 6 sodium ions, and about each sodium ion - 6 chlorine ions .

This ordered packing of ions is called ionic crystal. If you isolate a separate chlorine atom, then among surrounding sodium atoms already impossible to find one, which chlorine reacted. Attracted to each other electrostatic forces , ions are extremely reluctant to change their location under the influence of external force or temperature rise. But if the temperature is very high (approximately 1500°C), That NaCl evaporates, forming diatomic molecules. This suggests that covalent bonding forces never turn off completely .

Ionic crystals differ high melting temperatures, usually significant band gap, have ionic conductivity at high temperatures And a number of specific optical properties(For example, transparency in the near-IR spectrum). They can be built from either monatomic, and from polyatomic ions. Example ionic crystals of the first type - alkaline halide crystals And alkaline earth metals ; anions are arranged according to the law of closest spherical packing or dense ball masonry , cations occupy the corresponding voids. Most characteristic structures of this type are NaCl, CsCl, CaF2. Ionic crystals of the second type built from monoatomic cations of the same metals and finite or infinite anion fragments . Final anions(acid residues) - NO3-, SO42-, СО32-, etc. . Acidic residues can form endless chains , layers or form a three-dimensional frame, in the cavities of which cations are located, as, for example, in crystal structures of silicates. For ionic crystals you can calculate the energy of the crystal structure U(see table), approximately equal enthalpy of sublimation; results are in good agreement with experimental data. According to the equation Born-Maier, For crystal, consisting of formally singly charged ions :

U = -A/R + Be-R/r - C/R6 - D/R8 + E0

(R - shortest interionic distance , A - Madelung constant , dependent from structure geometry , IN And r - options , describing repulsion between particles , C/R6 And D/R8 characterize the relevant Dipole-dipole and dipole-quadrupole interaction of ions , E 0 - zero-point energy , e - electron charge). WITH As the cation becomes larger, the contribution of dipole-dipole interactions increases .