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 to themselves from the valence shells of other particles. For quantitative determination, the scale of relative electronegativity proposed by L. Polling (dimensionless value) is used. The ability to attract electrons from fluorine atoms is more pronounced than other elements, its EO is 4. In the Polling scale, oxygen, nitrogen, and chlorine immediately follow fluorine. The EO values of hydrogen and other typical non-metals 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 bond ionicity on the difference between the EO of chemical elements. The larger it is, the higher the probability that an ionic bond will occur. This type of interaction is more common when the difference EO=1.7 and higher. If the value is less, then the compounds are polar covalent.
Ionization energy
Ionization energy (EI) is required for detachment of external electrons weakly bound to the nucleus. The unit of change of this physical quantity is 1 electron volt. There are patterns of change in EI in the rows and columns of the periodic system, depending on the increase in the charge of the nucleus. In periods from left to right, the ionization energy increases and acquires the highest values for non-metals. In groups, it decreases from top to bottom. The main reason is the 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 judge whether an ionic bond occurs. The properties also depend on the ionization energy. For example, alkali and alkaline earth metals have low EI values. They have pronounced reducing (metallic) properties. Inert gases are chemically inactive due to their high ionization energy.
electron affinity
In chemical interactions, atoms can attach 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 the 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 the atoms of the elements - 7 electrons, only one is missing up to an octet. The electron affinity of halogens is high, they have strong oxidizing (non-metallic) properties.
Interactions of atoms in the formation of an ionic bond
Atoms that have an incomplete external 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). For them, this type of interaction is more suitable as an ionic chemical bond, in which the unifying molecular orbital (common electron pair) moves. It is believed that during the formation of ions, metals completely transfer electrons to non-metals. The strength of the resulting bond depends on the work required to destroy the molecules that make up 1 mol of the substance under study. This physical quantity is known as the binding energy. For ionic compounds, its values range from several tens to hundreds of kJ/mol.
Ion formation
An atom that gives up its electrons during chemical interactions turns into a cation (+). The receiving particle is an anion (-). To find out how atoms will behave, whether ions will appear, it is necessary to establish the difference between their EC. 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 up 1 electron than to attach 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 are external. One is missing to achieve an octet and a stable structure. The chemical properties support the assumption that the sodium atom donates and chlorine accepts electrons. There are ions: positive (sodium cation) and negative (chlorine anion).
Ionic bond
Losing an electron, sodium acquires a positive charge and a stable shell of an atom of the inert gas neon (1s 2 2s 2 2p 6). Chlorine, as a result of interaction with sodium, 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 electric charge is called the charge of the ion. For example, Na + , Ca 2+ , Cl - , F - . Ions can contain atoms of several elements: NH 4 + , SO 4 2- . Inside such complex ions, the particles are linked by a donor-acceptor or covalent mechanism. Electrostatic attraction occurs between oppositely charged particles. Its value in the case of an ionic bond is proportional to the charges, and with increasing distance between 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;
- There is an electrostatic attraction between oppositely charged particles.
Crystal lattices of ionic compounds
In chemical reactions, metals of the 1st, 2nd and 3rd groups of the periodic system usually lose electrons. One-, two- and three-charged positive ions are formed. Nonmetals of the 6th and 7th groups usually add electrons (with the exception of reactions with fluorine). There are singly and doubly charged negative ions. The energy costs for these processes, as a rule, are compensated when a substance crystal is created. Ionic compounds are usually in a solid state, forming structures consisting of oppositely charged cations and anions. These particles are attracted 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 an ionic bond
Ionic crystalline substances are characterized by high boiling and melting points. Typically, these compounds are heat resistant. The following feature can be found when such substances are dissolved in a polar solvent (water). Crystals are easily destroyed, and ions pass into a solution that has electrical conductivity. Ionic compounds are also destroyed when melted. Free charged particles appear, which means that the melt conducts electric current. Substances with an ionic bond 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 are widely used in science, technology, chemical production, metallurgy.
Ionic (electrovalent) chemical bond- a bond due to the formation of electron pairs due to the transition of valence electrons from one atom to another. It is typical for metal compounds with the most typical non-metals, for example:
Na + + Cl - = Na + Cl
The mechanism of formation of an ionic bond can be considered using the example of the reaction between sodium and chlorine. An alkali metal atom easily loses an electron, while a halogen atom gains one. As a result, a sodium cation and a chloride ion are formed. They form a connection due to the electrostatic attraction between them.
The interaction between cations and anions does not depend on the direction, so the ionic bond is said to be non-directional. Each cation can attract any number of anions, and vice versa. This is why an ionic bond is unsaturated. The number of interactions between ions in the solid state is limited only by the size of the crystal. Therefore, the "molecule" of an ionic compound should be considered the entire crystal.
An ideal ionic bond practically does not exist. Even in those compounds that are usually referred to as ionic, there is no complete transfer of electrons from one atom to another; electrons partially remain in common use. Thus, the bond in lithium fluoride is 80% ionic, and 20% covalent. Therefore, it is more correct to speak of degree of ionicity(polarity) of a covalent chemical bond. It is believed that with a difference in the electronegativity of the elements of 2.1, the bond is 50% ionic. If the difference is greater, the compound can be considered ionic.
The ionic model of chemical bonding is widely used to describe the properties of many substances, primarily compounds of alkali and alkaline earth metals with nonmetals. This is due to the simplicity of describing such compounds: they are considered to be built from incompressible charged spheres corresponding to 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.
hydrogen bond
The hydrogen bond is a special type of chemical bond. It is known that hydrogen compounds with highly electronegative non-metals such as F, O, N have abnormally high boiling points. If in the series Н 2 Тe–H 2 Se–H 2 S the boiling point naturally decreases, then when passing from H 2 S to Н 2 О, a sharp jump to an increase in this temperature is observed. The same picture is observed in the series of hydrohalic acids. This indicates the presence of a specific interaction between H 2 O molecules and HF molecules. Such an interaction should hinder the separation of molecules from each other, i.e. reduce their volatility, and, consequently, increase the boiling point of the corresponding substances. Due to the large difference in ER, the chemical bonds H–F, H–O, H–N are strongly polarized. Therefore, the hydrogen atom has a positive effective charge (δ +), and the F, O and N atoms have an excess of electron density, and they are negatively charged ( -). Due to Coulomb attraction, a positively charged hydrogen atom of one molecule interacts with an electronegative atom of another molecule. Due to this, the molecules are attracted to each other (bold dots indicate hydrogen bonds).
Hydrogen called such a bond, which is formed by means of a hydrogen atom, which is part of one of the two connected particles (molecules or ions). Hydrogen bond energy ( 21–29 kJ/mol or 5–7 kcal/mol) approximately 10 times less the energy of an ordinary chemical bond. And yet, the hydrogen bond causes the existence in pairs of dimeric molecules (H 2 O) 2, (HF) 2 and formic acid.
In a series of combinations of atoms HF, HO, HN, HCl, HS, the hydrogen bond energy decreases. It also decreases with increasing temperature, so substances in the vapor state exhibit hydrogen bonding only to a small extent; it is characteristic of substances in liquid and solid states. Substances such as water, ice, liquid ammonia, organic acids, alcohols, and phenols are associated into dimers, trimers, and polymers. In the liquid state, dimers are the most stable.
An ionic chemical bond is a bond that forms between atoms of chemical elements (positively or negatively charged ions). So what is an ionic bond, and how does it form?
General characteristics of the ionic chemical bond
Ions are charged particles that atoms become when they donate or accept electrons. They are attracted to each other quite strongly, it is for this reason that substances with this type of bond have high boiling and melting points.
Rice. 1. Ions.
An ionic bond is a chemical bond between dissimilar ions due to their electrostatic attraction. It can be considered the limiting case of a covalent bond, when the difference between the electronegativity of the bound atoms is so great that complete separation of charges occurs.
Rice. 2. Ionic chemical bond.
It is usually believed that the bond acquires an electronic character if EC > 1.7.
The difference in the value of electronegativity is greater, the further the elements are located from each other in the periodic system by period. This connection is characteristic of metals and non-metals, especially those located in the most remote groups, for example, I and VII.
Example: table salt, sodium chloride NaCl:
Rice. 3. Scheme of the ionic chemical bond of sodium chloride.
The ionic bond exists in crystals, it has strength, length, but is not saturated and not directed. Ionic bonding is characteristic only for complex substances, such as salts, alkalis, and some metal oxides. In the gaseous state, such substances exist in the form of ionic molecules.
An ionic chemical bond is formed between typical metals and non-metals. Electrons without fail pass from the metal to the non-metal, forming ions. As a result, an electrostatic attraction is formed, which is called an ionic bond.
In fact, a completely ionic bond does not occur. The so-called ionic bond is partly ionic, partly covalent. However, the bond of complex molecular ions can be considered ionic.
Examples of ionic bond formation
There are several examples of the formation of an ionic bond:
- interaction of calcium and fluorine
Ca 0 (atom) -2e \u003d Ca 2 + (ion)
It is easier for calcium to donate two electrons than to receive the missing ones.
F 0 (atom) + 1e \u003d F- (ion)
- Fluorine, on the contrary, is easier to accept one electron than to give seven electrons.
Let us find the least common multiple between the charges of the formed ions. It is equal to 2. Let's determine the number of fluorine atoms that will accept two electrons from a calcium atom: 2: 1 = 2. 4.
Let's make a formula for an ionic chemical bond:
Ca 0 +2F 0 →Ca 2 +F−2.
- interaction of sodium and oxygen
The theory of chemical bonding occupies an important place in modern chemistry. It explains why atoms combine into chemical particles and allows comparison of the stability of these particles. Using chemical bond theory, the composition and structure of various compounds can be predicted. The concept of the breaking of some chemical bonds and the formation of others underlies modern ideas about the transformations of substances in the course of chemical reactions.
A chemical bond is the interaction of atoms that determines the stability of a chemical particle or crystal as a whole. A chemical bond is formed due to the electrostatic interaction between charged particles: cations and anions, nuclei and electrons. When atoms approach each other, 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. At 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 in comparison with free atoms can occur. In the limiting case, this leads to the formation of charged particles - ions (from the Greek "ion" - going).
Interaction of ions
If an atom loses one or more electrons, then it turns into a positive ion - a cation (translated from Greek - "going down). This is how hydrogen cations H +, lithium Li +, barium Ba 2+ are formed. 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 are able to attract each other. In this case, a chemical bond occurs, and chemical compounds are formed. This type of chemical bond is called an ionic bond:
Ionic bond is a chemical bond formed by electrostatic attraction between cations and anions.
The mechanism of formation of an ionic bond can be considered using the example of the reaction between sodium and chlorine. An alkali metal atom easily loses an electron, while a halogen atom gains one. As a result, a sodium cation and a chloride ion are formed. They form a connection due to the electrostatic attraction between them.
The interaction between cations and anions does not depend on the direction, so the ionic bond is said to be non-directional. Each cation can attract any number of anions, and vice versa. This is why an ionic bond is unsaturated. The number of interactions between ions in the solid state is limited only by the size of the crystal. Therefore, the "molecule" of an ionic compound should be considered the entire crystal.
For an ionic bond to occur, it is necessary that the sum of the values of the ionization energy E i(for cation formation) and electron affinity A e(for anion formation) should be energetically favorable. This limits the formation of ionic bonds by atoms of active metals (elements of the IA- and IIA-groups, some elements of the IIIA-group and some transition elements) and active non-metals (halogens, chalcogens, nitrogen).
An ideal ionic bond practically does not exist. Even in those compounds that are usually referred to as ionic, there is no complete transfer of electrons from one atom to another; electrons partially remain in common use. Thus, the bond in lithium fluoride is 80% ionic, and 20% covalent. Therefore, it is more correct to speak of degree of ionicity(polarity) of a covalent chemical bond. It is believed that with a difference in the electronegativity of the elements of 2.1, the bond is 50% ionic. If the difference is greater, the compound can be considered ionic.
The ionic model of chemical bonding is widely used to describe the properties of many substances, primarily compounds of alkali and alkaline earth metals with nonmetals. This is due to the simplicity of describing such compounds: they are considered to be built from incompressible charged spheres corresponding to 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 radii
A simple electrostatic model of ionic bonding uses the concept of ionic radii. The sum of the radii of the neighboring cation and anion must be equal to the corresponding internuclear distance:
r 0 = r + + r −
In this case, it remains unclear where the boundary between the cation and anion should be drawn. Today it is known that a purely ionic bond does not exist, since there is always some overlap of electron clouds. To calculate the radii of ions, research methods are used that allow you to determine the electron density between two atoms. The internuclear distance is divided at the point where the electron density is minimum.
The size of an ion depends on many factors. With a constant charge of the ion, with an increase in the serial number (and, consequently, the charge of the nucleus), the ionic radius decreases. This is especially noticeable in the lanthanide series, where the ionic radii change monotonically from 117 pm for (La 3+) to 100 pm (Lu 3+) at 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 the ionic radius can occur (for example, from 73 pm for Zr 4+ to 72 pm for Hf 4+ at a coordination number of 4).
In the period, there is a noticeable decrease in the ionic radius, associated with an increase in the 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 ion charge leads to 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 of ionic radii can only be done at the same coordination number, since it affects the size of the ion due to the repulsive forces between the counterions. This is clearly seen in the example of the Ag + ion; its ionic radius is 81, 114, and 129 pm for coordination numbers 2, 4, and 6, respectively.
The structure of an ideal ionic compound, due to the maximum attraction between unlike ions and the minimum repulsion of like ions, is largely determined by the ratio of the ionic radii of cations and anions. This can be shown by simple geometric constructions.
| Attitude r + : r − | Cation coordination number | Environment | Example |
| 0,225−0,414 | 4 | tetrahedral | ZnS |
| 0,414−0,732 | 6 | Octahedral | NaCl |
| 0,732−1,000 | 8 | cubic | CsCl |
| >1,000 | 12 | dodecahedral | Not found in ionic crystals |
Ionic bond energy
The binding energy for an ionic compound is the energy that is 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, repulsion of electron shells).
When an ionic bond occurs between two free ions, the energy of their attraction is determined by Coulomb's law:
E(adj.) = q + q− / (4π r ε),
where q+ and q− - charges of interacting ions, r is the distance between them, ε is the permittivity of the medium.
Since one of the charges is negative, the energy value will also be negative.
According to Coulomb's law, at infinitesimal distances, the energy of attraction must become infinitely large. However, this does not happen, since ions are not point charges. When ions approach each other, repulsive forces arise between them, due to the interaction of electron clouds. The repulsive energy of ions is described by the Born equation:
E(ott.) = AT / rn,
where AT- some constant, n can take values from 5 to 12 (depending on the size of the ions). The total energy is determined by the sum of the energies of attraction and repulsion:
E = E(adv.) + E(ott.)
Its value passes through a minimum. The coordinates of the minimum point correspond to the equilibrium distance r 0 and equilibrium energy of interaction between ions E 0:
E 0 = q + q − (1 - 1 / n) / (4π r 0 ε)
There are always more interactions in a crystal lattice than between a pair of ions. This number is determined primarily by the type of crystal lattice. To take into account all interactions (weakening with increasing distance), the so-called Madelung constant is introduced into the expression for the energy of the ionic crystal lattice BUT:
E(adj.) = A q + q− / (4π r ε)
The value of the Madelung constant is determined only by the geometry of the lattice and does not depend on the radius and charge of the ions. For example, for sodium chloride it is 1.74756.
Ionic bond
Chemical bond theory takes important place in modern chemistry. She is explains why atoms combine to form chemical particles, and makes it possible to compare the stability of these particles. Using chemical bond theory, can predict the composition and structure of various compounds. The concept of the breaking of some chemical bonds and the formation of others underlies modern ideas about the transformations of substances in the course of chemical reactions .
chemical bond- this is interaction of atoms , determining the stability of a chemical particle or crystal as a whole . chemical bond formed through electrostatic interaction between charged particles : cations and anions, nuclei and electrons. When atoms approach each other, 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 the 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 compared to free atoms.
In the limiting case, this leads to the formation of charged particles - ions (from the Greek "ion" - going).
1 Interaction of ions
If a atom loses one or few electrons, then he turns into a positive ion - cation(translated from Greek - " going down"). This is how cations hydrogen H +, lithium Li +, barium Ba 2+ . 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. This gives rise to chemical bond, and chemical compounds are formed. This type of chemical bond is called ionic bond :
2 Ionic bond definition
Ionic bond is a chemical bond educated at the expense electrostatic attraction between cations and anions .
The mechanism of formation of an ionic bond can be considered on the example of the reaction between sodium and chlorine . An alkali metal atom easily loses an electron, a halogen atom - acquires. As a result of this, there sodium cation and chloride ion. They form a connection through electrostatic attraction between them .
Interaction between cations and anions does not depend on direction, that's why about ionic bond they talk about non-directional. Each 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 is limited only by the size of the crystal. That's why " molecule " ionic compound should be considered the whole crystal .
For the emergence ionic bond necessary, to sum of ionization energies Ei(to form a cation) and electron affinity Ae(for anion formation) must be energetically profitable. it limits the formation of ionic bonds by atoms of active metals(elements of IA- and IIA-groups, some elements of the IIIA-group and some transitional elements) and active non-metals(halogens, chalcogens, nitrogen).
Ideal ionic bond practically does not exist. Even in those compounds that are usually referred to as ionic , there is no complete transfer of electrons from one atom to another ; electrons partially remain in common use. Yes, the connection lithium fluoride by 80% ionic, and by 20% - covalent. Therefore, it is more correct to speak of degree of ionicity (polarity) covalent chemical bond. It is believed that with a difference electronegativity elements 2.1 communication is on 50% ionic. At greater difference compound can be considered ionic .
The ionic model of a chemical bond is widely used to describe the properties of many substances., in the first place, connections alkaline and alkaline earth metals with non-metals. This is due ease of description of such compounds: believe they are built from incompressible charged spheres, corresponding 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 a large difference (>1.7 on the Pauling scale) of electronegativity, with which the shared electron pair goes entirely to the atom with the greater 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 a covalent polar bond. Formed between typical metal and non-metal. In this case, the electrons in the metal completely transferred 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 shared electron pair is completely goes to an atom with a higher EC. This results in the formation of a compound oppositely charged ions :
Between the formed ions there is electrostatic attraction, which is called ionic bond. Rather, this view convenient. In practice ionic bond between the atoms in in its pure form is not realized anywhere or almost nowhere, usually in reality the connection is partially ionic , and partially covalent character. 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 non-directionality and unsaturation. That is why crystals formed by ionic bonding gravitate towards various close packings of the corresponding ions.
3 Ionic radii
In idle electrostatic model of ionic bond concept is used ionic radii . The sum of the radii of the neighboring cation and anion must be equal to the corresponding internuclear distance :
r 0 = r + + r −
At the same time, it remains obscure where to take boundary between cation and anion . Known today , that a purely ionic bond does not exist, as always there is some electron cloud overlap. For ion radii calculations use research methods, which allow you to determine the electron density between two atoms . The internuclear distance is divided at a point, where electron density is minimal .
Ion size depends on many factors. At constant charge of the ion with increasing serial number(and consequently, nuclear charge) ionic radius decreases. This is especially noticeable in the lanthanide series, where ionic radii change monotonically from 117 pm for (La 3+) to 100 pm (Lu 3+) at a coordination number of 6. This effect is called lanthanide compression .
AT element groups 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 the ionic radius can occur(for example, from 73 pm for Zr 4+ to 72 pm for Hf 4+ with a coordination number of 4).
In the period, there is a noticeable decrease in the ionic radius associated with an increase in the 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 leads to 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 carried out only with the same coordination number, because the it affects the size of the ion due to the repulsive forces between the 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 perfect ionic compound, due to maximum attraction between dissimilar ions and minimum repulsion between like ions, in many ways determined by the ratio of the ionic radii of cations and anions. It can be shown simple geometric constructions.
4 Ionic bond energy
Bond energy and for ionic compound- this is energy, which in is 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 includes also the contribution of non-electrostatic forces(for example, repulsion of electron shells).
When ionic bond between two free ion energy them attraction is determined by Coulomb's law :
E(adj.) = q+ q− / (4π r ε),
where q+ and q−- charges interacting ions , r - the distance between them , ε - medium permittivity .
Since one of the charges negative, then energy value also will be negative .
According to Coulomb's law, on the At infinitesimal distances, the energy of attraction must become infinitely large. However, this not happening, because ions are not point charges. At approach of ions there is a repulsive force between them, due to interaction of electron clouds . Ion repulsion energy described Born equation :
E (ott.) \u003d B / rn,
where AT - 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 \u003d E (adv.) + E (ott.)
Its meaning goes 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)
AT 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(adj.) = 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 to 1,74756 .
5 polarization of ions
Apart from charge magnitude and radius important characteristic and she are his polarization properties. Let's consider this question in more detail. At non-polar particles (atoms, ions, molecules) the centers of gravity of positive and negative charges coincide. In an electric field, the electron shells are displaced in the direction of a positively charged plate, and nuclei - in the direction of a 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 of polarization properties of the ion , necessary making a difference the polarizing effect of a given ion and the ability of itself to polarize in an electric field .
The polarizing effect of the ion will be the one big, how more of his force field, i.e. than more charge and less ion radius. Therefore, in within subgroups in the Periodic Table of the Elements the polarizing effect of ions decreases from top to bottom, because in subgroups with a constant value of the charge of the ion from top to bottom, its radius increases .
That's why the polarizing effect of alkali metal ions, for example, increases from cesium to lithium, and in a row halide ions - from I to F. In periods the polarizing effect of the ions increases from left to right together with an increase in the charge of the ion and decreasing its radius .
Ion polarizability, its ability to deformations increase with decreasing force field, i.e. with a decrease in the amount of charge and increase in radius . Anion polarizability usually above, how cations and in a row halides grows from F to I .
On the polarization properties of cations renders influence the nature of their outer electron shell . Polarization properties of cations how in active, as well as in passive sense at the same charge and a close radius increase on passing from cations with a filled shell to cations with an unfinished outer shell and further to cations with an 18-electron shell.
For example, in the series of cations Mg 2+ , Ni 2+ , Zn 2+ polarization properties intensify. This pattern is consistent with the change in the ion radius and the structure of its electron shell given in the series:
for anions polarization properties deteriorate in this order:
I - , Br - , Cl - , CN - , OH - , NO 3 - , F - , ClO 4 - .
result polarization interaction of ions is deformation of their electron shells and, as a consequence of this, shortening of interionic distances and incomplete separation of the negative and positive charges between ions.
For example, in a crystal sodium chloride the amount of charge on sodium ion is +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 renders noticeable effect on the properties of compounds with ionic bond , lowering their melting and boiling points , reducing electrolytic dissociation in solutions and melts, etc. .
Ionic compounds formed when interaction of elements , significantly different in chemical properties. The more the distance between elements in the periodic table, topics in ionic bond is more pronounced in their compounds . Against, in molecules, formed by the same atoms or atoms of elements that are similar in chemical properties, arise other types of communication. That's why ionic bond theory It has limited use .
6 Effect of ion polarization on the properties of substances and 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 chloride and potassium with silver chloride shows that when close ionic radii
polarizability of the Ag+ cation having 18-electron outer shell , above, what leads to an increase in the metal-chlorine bond strength and lower solubility of silver chloride in water .
Mutual polarization of ions facilitates the destruction of crystals, that leads to lowering the melting points of substances. For this reason melting temperature TlF (327 oС) significantly lower than RbF (798 oC). The decomposition temperature of substances will also decrease with an increase in the mutual polarization of ions. That's why iodides generally decompose at lower temperatures, how other halides, a lithium compounds - thermally less stable , than compounds of other alkaline elements .
Deformability of electron shells affects the optical properties of substances. How more polarized particle , the lower the energy of electronic transitions. If a polarization is low , excitation of electrons requires higher energy, which answers ultraviolet part of the spectrum. Such substances are usually colorless. In the case of strong polarization of ions, the excitation of electrons occurs upon absorption of electromagnetic radiation in the visible region of the spectrum. That's why some substances, formed colorless ions, colored .
characteristic ionic compounds serves good solubility in polar solvents (water, acids, etc.). This is due to the charge of the parts of the molecule. Wherein solvent dipoles are attracted to the charged ends of the molecule, and as a result brownian motion , « take away» molecule substances into parts and surround them , preventing reconnection. The result is ions surrounded by solvent dipoles .
When such compounds are dissolved, as a rule, energy is released, since the total energy of the formed bonds solvent-ion has more anion-cation bond energy. Exceptions are many salts of nitric acid (nitrates), which absorb heat when dissolved (solutions are cooled). The latter fact is explained on the basis of the laws that considered in physical chemistry .
7 Crystal lattice
Ionic compounds(e.g. sodium chloride NaCl) - solid and refractory because of between the charges of their ions("+" and "-") exist powerful forces of electrostatic attraction .
The negatively charged chloride ion attracts Not only " mine " Na+ ion, but also other sodium ions around. it leads to, what near any of the ions there is more than one ion with the opposite sign , but a few(Fig. 1).
Rice. one. Crystal structure common salt NaCl .
In fact, about every chloride ion is located 6 sodium ions, and about each sodium ion - 6 chloride ions .
This ordered packing of ions is called ionic crystal. If we single out a separate chlorine atom, then among surrounding sodium atoms already impossible to find one, which chlorine reacted.. Drawn to each other electrostatic forces , ions are extremely reluctant to change their location under the influence of an external force or temperature increase. But if the temperature is very high (approx. 1500°C), then NaCl evaporates, forming diatomic molecules. This suggests that covalent bonding forces never turn off completely .
Ionic crystals different high melting points, usually significant band gap, possess 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 monatomic, and from polyatomic ions. Example ionic crystals of the first type - alkali 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 monatomic cations of the same metals and finite or infinite anionic fragments . Terminal anions(acid residues) - NO3-, SO42-, CO32- and others . 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 it is possible to calculate the energy of the crystal structure U(see table), approximately equal to enthalpy of sublimation; results are in good agreement with the experimental data. According to the equation Born-Meyer, for crystal, consisting of formally singly charged ions :
U \u003d -A / R + Be-R / r - C / R6 - D / R8 + E0
(R - shortest inter-ion distance , BUT - Madelung constant , dependent from structure geometry , AT and r - options , describing repulsion between particles , C/R6 and D/R8 characterize the respective dipole-dipole and dipole-quadrupole interaction of ions , E 0 - zero point energy , e - electron charge). FROM as the cation grows larger, the contribution of dipole-dipole interactions increases .