The concept of a chemical bond is of no small importance in various fields of chemistry as a science. This is due to the fact that it is with its help that individual atoms are able to combine into molecules, forming all kinds of substances, which, in turn, are the subject of chemical research.

The diversity of atoms and molecules is associated with the emergence of various types of bonds between them. Different classes of molecules are characterized by their own characteristics of electron distribution, and therefore their own types of bonds.

Basic Concepts

Chemical bond called a set of interactions that lead to the bonding of atoms with the formation of stable particles of a more complex structure (molecules, ions, radicals), as well as aggregates (crystals, glasses, etc.). The nature of these interactions is electrical in nature, and they arise during the distribution of valence electrons in approaching atoms.

Valence accepted name the ability of an atom to form a certain number of bonds with other atoms. In ionic compounds, the number of electrons given up or gained is taken as the valence value. In covalent compounds it is equal to the number of shared electron pairs.

Under the degree of oxidation is understood as a conditional the charge that could be on an atom if all polar covalent bonds were ionic in nature.

The multiplicity of a connection is called the number of shared electron pairs between the atoms under consideration.

The bonds considered in various branches of chemistry can be divided into two types of chemical bonds: those that lead to the formation of new substances (intramolecular) , And those that occur between molecules (intermolecular).

Basic communication characteristics

Energy of communication is the energy required to break all existing bonds in a molecule. It is also the energy released during bond formation.

Link length is the distance between neighboring nuclei of atoms in a molecule at which the forces of attraction and repulsion are balanced.

These two characteristics of a chemical bond between atoms are a measure of its strength: the shorter the length and the greater the energy, the stronger the bond.

Bond angle it is customary to call the angle between the represented lines passing in the direction of communication through the nuclei of atoms.

Methods for describing connections

The most common two approaches to explaining chemical bonding, borrowed from quantum mechanics:

Molecular orbital method. He views the molecule as a collection of electrons and atomic nuclei, with each individual electron moving in the field of action of all other electrons and nuclei. The molecule has an orbital structure, and all its electrons are distributed in these orbits. This method is also called MO LCAO, which stands for “molecular orbital - linear combination

Valence bond method. Represents a molecule as a system of two central molecular orbitals. Moreover, each of them corresponds to one bond between two adjacent atoms in the molecule. The method is based on the following provisions:

1. The formation of a chemical bond is carried out by a pair of electrons having opposite spins, which are located between the two atoms in question. The electron pair formed belongs equally to the two atoms.
2. The number of bonds formed by one or another atom is equal to the number of unpaired electrons in the ground and excited states.
3. If electron pairs do not participate in the formation of a bond, then they are called lone pairs.

Electronegativity

The type of chemical bond in substances can be determined based on the difference in the electronegativity values ​​of its constituent atoms. Under electronegativity understand the ability of atoms to attract shared electron pairs (electron cloud), which leads to bond polarization.

There are various ways to determine the electronegativity values ​​of chemical elements. However, the most used is the scale based on thermodynamic data, which was proposed back in 1932 by L. Pauling.

The greater the difference in electronegativity of atoms, the more pronounced its ionicity. On the contrary, equal or similar electronegativity values ​​indicate the covalent nature of the bond. In other words, it is possible to determine mathematically what chemical bond is observed in a particular molecule. To do this, you need to calculate ΔХ - the difference in electronegativity of atoms using the formula: ΔХ=|Х 1 -X 2 |.

• If ΔХ>1.7, then the bond is ionic.
• If 0.5≤ΔХ≤1.7, then the covalent bond is polar.
• If ΔХ=0 or close to it, then the bond is classified as covalent nonpolar.

Ionic bond

An ionic bond is a bond that appears between ions or due to the complete withdrawal of a common electron pair by one of the atoms. In substances, this type of chemical bond is carried out by forces of electrostatic attraction.

Ions are charged particles formed from atoms by gaining or losing electrons. If an atom accepts electrons, it acquires a negative charge and becomes an anion. If an atom gives up valence electrons, it becomes a positively charged particle called a cation.

It is characteristic of compounds formed by the interaction of atoms of typical metals with atoms of typical non-metals. The main reason for this process is the desire of atoms to acquire stable electronic configurations. And for this, typical metals and non-metals need to give or accept only 1-2 electrons, which they do with ease.

The mechanism of formation of an ionic chemical bond in a molecule is traditionally considered using the example of the interaction of sodium and chlorine. Alkali metal atoms easily give up an electron, drawn by a halogen atom. As a result, the Na + cation and the Cl - anion are formed, which are held together by electrostatic attraction.

There is no ideal ionic bond. Even in such compounds, which are often classified as ionic, the final transfer of electrons from atom to atom does not occur. The formed electron pair still remains in common use. Therefore, they talk about the degree of ionicity of a covalent bond.

An ionic bond is characterized by two main properties related to each other:

• non-directionality, i.e. the electric field around the ion has the shape of a sphere;
• unsaturation, i.e., the number of oppositely charged ions that can be placed around any ion, is determined by their sizes.

Covalent chemical bond

A bond formed by overlapping electron clouds of nonmetal atoms, that is, carried out by a common electron pair, is called a covalent bond. The number of shared electron pairs determines the multiplicity of the bond. Thus, hydrogen atoms are connected by a single H··H bond, and oxygen atoms form an O::O double bond.

There are two mechanisms for its formation:

• Exchange - each atom represents one electron to form a common pair: A· + ·B = A:B, while external atomic orbitals, on which one electron is located, participate in the bonding.
• Donor-acceptor - to form a bond, one of the atoms (donor) provides a pair of electrons, and the second (acceptor) provides a free orbital for its placement: A + : B = A: B.

The ways in which electron clouds overlap during the formation of a covalent chemical bond are also different.

1. Direct. The region of cloud overlap lies on a straight imaginary line connecting the nuclei of the atoms in question. In this case, σ bonds are formed. The type of chemical bond that occurs in this case depends on the type of electron clouds that overlap: s-s, s-p, p-p, s-d or p-d σ bonds. In a particle (molecule or ion), only one σ bond is possible between two neighboring atoms.
2. Lateral. It is carried out on both sides of the line connecting the nuclei of atoms. This is how a π bond is formed, and its varieties are also possible: p-p, p-d, d-d. A π bond is never formed separately from a σ bond; it can occur in molecules containing multiple (double and triple) bonds.

Properties of covalent bonds

They determine the chemical and physical properties of compounds. The main properties of any chemical bond in substances are its directionality, polarity and polarizability, as well as saturation.

Focus connections are determined by the features of the molecular structure of substances and the geometric shape of their molecules. Its essence is that the best overlap of electron clouds is possible at a certain orientation in space. The options for the formation of σ- and π-bonds have already been discussed above.

Under saturation understand the ability of atoms to form a certain number of chemical bonds in a molecule. The number of covalent bonds for each atom is limited by the number of outer orbitals.

Polarity bond depends on the difference in the electronegativity values ​​of the atoms. The uniformity of the distribution of electrons between the nuclei of atoms depends on it. According to this characteristic, a covalent bond can be polar or nonpolar.

• If the common electron pair belongs equally to each of the atoms and is located at the same distance from their nuclei, then the covalent bond is non-polar.
• If a common pair of electrons is displaced towards the nucleus of one of the atoms, then a covalent polar chemical bond is formed.

Polarizability is expressed by the displacement of bond electrons under the influence of an external electric field, which may belong to another particle, neighboring bonds in the same molecule, or come from external sources of electromagnetic fields. Thus, a covalent bond under their influence can change its polarity.

Hybridization of orbitals is understood as a change in their shapes during a chemical bond. This is necessary to achieve the most effective overlap. The following types of hybridization exist:

• sp3. One s and three p orbitals form four “hybrid” orbitals of the same shape. Outwardly it resembles a tetrahedron with an angle between the axes of 109°.
• sp2. One s- and two p-orbitals form a flat triangle with an angle between the axes of 120°.
• sp. One s- and one p-orbital form two “hybrid” orbitals with an angle between their axes of 180°.

A special feature of the structure of metal atoms is their rather large radius and the presence of a small number of electrons in outer orbitals. As a result, in such chemical elements the bond between the nucleus and valence electrons is relatively weak and is easily broken.

Metal A bond is an interaction between metal atoms and ions that occurs with the help of delocalized electrons.

In metal particles, valence electrons can easily leave the outer orbitals, as well as occupy vacant positions on them. Thus, at different moments of time the same particle can be an atom and an ion. The electrons detached from them move freely throughout the entire volume of the crystal lattice and carry out a chemical bond.

This type of bond has similarities with ionic and covalent bonds. Just like ionic bonds, metallic bonds require ions to exist. But if cations and anions are needed to carry out electrostatic interaction in the first case, then in the second the role of negatively charged particles is played by electrons. When comparing a metallic bond with a covalent bond, both require shared electrons to form. However, unlike polar chemical bonds, they are not localized between two atoms, but belong to all metal particles in the crystal lattice.

Metallic bonding is responsible for the special properties of almost all metals:

• plasticity is present due to the possibility of displacement of layers of atoms in a crystal lattice held by an electron gas;
• metallic luster, which is observed due to the reflection of light rays from electrons (in the powder state there is no crystal lattice and, therefore, electrons moving through it);
• electrical conductivity, which is carried out by a flow of charged particles, and in this case small electrons move freely among large metal ions;
• thermal conductivity is observed due to the ability of electrons to transfer heat.

This type of chemical bond is sometimes called intermediate between covalent and intermolecular interactions. If a hydrogen atom has a bond with one of the highly electronegative elements (such as phosphorus, oxygen, chlorine, nitrogen), then it is capable of forming an additional bond, called a hydrogen bond.

It is much weaker than all the types of bonds discussed above (energy no more than 40 kJ/mol), but it cannot be neglected. This is why a hydrogen chemical bond appears as a dotted line in the diagram.

The occurrence of a hydrogen bond is possible due to the simultaneous donor-acceptor electrostatic interaction. A large difference in electronegativity values ​​leads to the appearance of excess electron density on the O, N, F and other atoms, as well as to its deficiency on the hydrogen atom. In the event that there is no existing chemical bond between such atoms, when they are close enough, attractive forces are activated. In this case, the proton is the acceptor of the electron pair, and the second atom is the donor.

Hydrogen bonds can occur both between neighboring molecules, for example, water, carboxylic acids, alcohols, ammonia, and within a molecule, for example, salicylic acid.

The presence of hydrogen bonds between water molecules explains a number of its unique physical properties:

• The values ​​of its heat capacity, dielectric constant, boiling and melting points, in accordance with calculations, should be significantly less than real ones, which is explained by the connectivity of molecules and the need to expend energy on breaking intermolecular hydrogen bonds.
• Unlike other substances, the volume of water increases as the temperature decreases. This occurs due to the fact that the molecules occupy a certain position in the crystal structure of ice and move away from each other by the length of the hydrogen bond.

This connection plays a special role for living organisms, since its presence in protein molecules determines their special structure, and therefore their properties. In addition, nucleic acids, making up the double helix of DNA, are also connected by hydrogen bonds.

Bonds in crystals

The vast majority of solids have a crystal lattice - a special relative arrangement of the particles that form them. In this case, three-dimensional periodicity is observed, and atoms, molecules or ions are located at the nodes, which are connected by imaginary lines. Depending on the nature of these particles and the connections between them, all crystalline structures are divided into atomic, molecular, ionic and metallic.

The nodes of the ionic crystal lattice contain cations and anions. Moreover, each of them is surrounded by a strictly defined number of ions with only the opposite charge. A typical example is sodium chloride (NaCl). They tend to have high melting points and hardness because they require a lot of energy to break down.

At the nodes of the molecular crystal lattice there are molecules of substances formed by covalent bonds (for example, I 2). They are connected to each other by a weak van der Waals interaction, and therefore such a structure is easy to destroy. Such compounds have low boiling and melting points.

The atomic crystal lattice is formed by atoms of chemical elements with high valency values. They are connected by strong covalent bonds, which means that the substances have high boiling and melting points and great hardness. An example is a diamond.

Thus, all types of bonds present in chemical substances have their own characteristics, which explain the subtleties of the interaction of particles in molecules and substances. The properties of the compounds depend on them. They determine all processes occurring in the environment.

Characteristics of chemical bonds

The doctrine of chemical bonding forms the basis of all theoretical chemistry. A chemical bond is understood as the interaction of atoms that binds them into molecules, ions, radicals, and crystals. There are four types of chemical bonds: ionic, covalent, metallic and hydrogen. Different types of bonds can be found in the same substances.

1. In bases: between the oxygen and hydrogen atoms in hydroxo groups the bond is polar covalent, and between the metal and the hydroxo group it is ionic.

2. In salts of oxygen-containing acids: between the non-metal atom and the oxygen of the acidic residue - covalent polar, and between the metal and the acidic residue - ionic.

3. In ammonium, methylammonium, etc. salts, between the nitrogen and hydrogen atoms there is a polar covalent, and between ammonium or methylammonium ions and the acid residue - ionic.

4. In metal peroxides (for example, Na 2 O 2), the bond between the oxygen atoms is covalent, nonpolar, and between the metal and oxygen is ionic, etc.

The reason for the unity of all types and types of chemical bonds is their identical chemical nature - electron-nuclear interaction. The formation of a chemical bond in any case is the result of electron-nuclear interaction of atoms, accompanied by the release of energy.

Methods for forming a covalent bond

Covalent chemical bond is a bond that arises between atoms due to the formation of shared electron pairs.

Covalent compounds are usually gases, liquids, or relatively low-melting solids. One of the rare exceptions is diamond, which melts above 3,500 °C. This is explained by the structure of diamond, which is a continuous lattice of covalently bonded carbon atoms, and not a collection of individual molecules. In fact, any diamond crystal, regardless of its size, is one huge molecule.

A covalent bond occurs when the electrons of two nonmetal atoms combine. The resulting structure is called a molecule.

The mechanism of formation of such a bond can be exchange or donor-acceptor.

In most cases, two covalently bonded atoms have different electronegativity and the shared electrons do not belong to the two atoms equally. Most of the time they are closer to one atom than to another. In a hydrogen chloride molecule, for example, the electrons that form a covalent bond are located closer to the chlorine atom because its electronegativity is higher than that of hydrogen. However, the difference in the ability to attract electrons is not large enough for complete electron transfer from the hydrogen atom to the chlorine atom to occur. Therefore, the bond between hydrogen and chlorine atoms can be considered as a cross between an ionic bond (complete electron transfer) and a non-polar covalent bond (a symmetrical arrangement of a pair of electrons between two atoms). The partial charge on atoms is denoted by the Greek letter δ. Such a bond is called a polar covalent bond, and the hydrogen chloride molecule is said to be polar, that is, it has a positively charged end (hydrogen atom) and a negatively charged end (chlorine atom).

1. The exchange mechanism operates when atoms form shared electron pairs by combining unpaired electrons.

1) H 2 - hydrogen.

The bond occurs due to the formation of a common electron pair by the s-electrons of hydrogen atoms (overlapping s-orbitals).

2) HCl - hydrogen chloride.

The bond occurs due to the formation of a common electron pair of s- and p-electrons (overlapping s-p orbitals).

3) Cl 2: In a chlorine molecule, a covalent bond is formed due to unpaired p-electrons (overlapping p-p orbitals).

4) N ​​2: In the nitrogen molecule, three common electron pairs are formed between the atoms.

Donor-acceptor mechanism of covalent bond formation

Donor has an electron pair acceptor- free orbital that this pair can occupy. In the ammonium ion, all four bonds with hydrogen atoms are covalent: three were formed due to the creation of common electron pairs by the nitrogen atom and hydrogen atoms according to the exchange mechanism, one - through the donor-acceptor mechanism. Covalent bonds are classified by the way the electron orbitals overlap, as well as by their displacement towards one of the bonded atoms. Chemical bonds formed as a result of overlapping electron orbitals along a bond line are called σ - connections(sigma bonds). The sigma bond is very strong.

p orbitals can overlap in two regions, forming a covalent bond through lateral overlap.

Chemical bonds formed as a result of the “lateral” overlap of electron orbitals outside the bond line, i.e., in two regions, are called pi bonds.

According to the degree of displacement of common electron pairs to one of the atoms they connect, a covalent bond can be polar or non-polar. A covalent chemical bond formed between atoms with the same electronegativity is called non-polar. Electron pairs are not displaced towards any of the atoms, since atoms have the same electronegativity - the property of attracting valence electrons from other atoms. For example,

that is, molecules of simple non-metal substances are formed through a covalent non-polar bond. A covalent chemical bond between atoms of elements whose electronegativity differs is called polar.

For example, NH 3 is ammonia. Nitrogen is a more electronegative element than hydrogen, so the shared electron pairs are shifted towards its atom.

Characteristics of a covalent bond: bond length and energy

The characteristic properties of a covalent bond are its length and energy. Bond length is the distance between atomic nuclei. The shorter the length of a chemical bond, the stronger it is. However, a measure of bond strength is bond energy, which is determined by the amount of energy required to break the bond. It is usually measured in kJ/mol. Thus, according to experimental data, the bond lengths of the H 2, Cl 2 and N 2 molecules, respectively, are 0.074, 0.198 and 0.109 nm, and the bond energies, respectively, are 436, 242 and 946 kJ/mol.

Ions. Ionic bond

There are two main possibilities for an atom to obey the octet rule. The first of these is the formation of ionic bonds. (The second is the formation of a covalent bond, which will be discussed below). When an ionic bond is formed, a metal atom loses electrons, and a non-metal atom gains electrons.

Let's imagine that two atoms “meet”: an atom of a group I metal and a non-metal atom of group VII. A metal atom has a single electron at its outer energy level, while a non-metal atom just lacks one electron for its outer level to be complete. The first atom will easily give the second its electron, which is far from the nucleus and weakly bound to it, and the second will provide it with a free place on its outer electronic level. Then the atom, deprived of one of its negative charges, will become a positively charged particle, and the second will turn into a negatively charged particle due to the resulting electron. Such particles are called ions.

This is a chemical bond that occurs between ions. Numbers showing the number of atoms or molecules are called coefficients, and numbers showing the number of atoms or ions in a molecule are called indices.

Metal connection

Metals have specific properties that differ from the properties of other substances. Such properties are relatively high melting temperatures, the ability to reflect light, and high thermal and electrical conductivity. These features are due to the existence of a special type of bond in metals - a metallic bond.

Metallic bonding is a bond between positive ions in metal crystals due to the attraction of electrons moving freely throughout the crystal. The atoms of most metals at the outer level contain a small number of electrons - 1, 2, 3. These electrons come off easily, and the atoms turn into positive ions. The detached electrons move from one ion to another, binding them into a single whole. Connecting with ions, these electrons temporarily form atoms, then break off again and combine with another ion, etc. A process occurs endlessly, which can be schematically depicted as follows:

Consequently, in the volume of the metal, atoms are continuously converted into ions and vice versa. The bond in metals between ions through shared electrons is called metallic. The metallic bond has some similarities with the covalent bond, since it is based on the sharing of external electrons. However, with a covalent bond, the outer unpaired electrons of only two neighboring atoms are shared, while with a metallic bond, all atoms take part in the sharing of these electrons. That is why crystals with a covalent bond are brittle, but with a metal bond, as a rule, they are ductile, electrically conductive and have a metallic luster.

Metallic bonding is characteristic of both pure metals and mixtures of various metals - alloys in solid and liquid states. However, in the vapor state, metal atoms are connected to each other by a covalent bond (for example, sodium vapor fills yellow light lamps to illuminate the streets of large cities). Metal pairs consist of individual molecules (monatomic and diatomic).

A metal bond also differs from a covalent bond in strength: its energy is 3-4 times less than the energy of a covalent bond.

Bond energy is the energy required to break a chemical bond in all molecules that make up one mole of a substance. The energies of covalent and ionic bonds are usually high and amount to values ​​of the order of 100-800 kJ/mol.

Hydrogen bond

Chemical bond between positively polarized hydrogen atoms of one molecule(or parts thereof) and negatively polarized atoms of highly electronegative elements having shared electron pairs (F, O, N and less often S and Cl), another molecule (or parts thereof) is called hydrogen. The mechanism of hydrogen bond formation is partly electrostatic, partly d honoror-acceptor character.

Examples of intermolecular hydrogen bonding:

In the presence of such a connection, even low-molecular substances can, under normal conditions, be liquids (alcohol, water) or easily liquefied gases (ammonia, hydrogen fluoride). In biopolymers - proteins (secondary structure) - there is an intramolecular hydrogen bond between carbonyl oxygen and the hydrogen of the amino group:

Polynucleotide molecules - DNA (deoxyribonucleic acid) - are double helices in which two chains of nucleotides are linked to each other by hydrogen bonds. In this case, the principle of complementarity operates, i.e., these bonds are formed between certain pairs consisting of purine and pyrimidine bases: the thymine (T) is located opposite the adenine nucleotide (A), and the cytosine (C) is located opposite the guanine (G).

Substances with hydrogen bonds have molecular crystal lattices.

There is no unified theory of chemical bonds; chemical bonds are conventionally divided into covalent (a universal type of bond), ionic (a special case of a covalent bond), metallic and hydrogen.

Covalent bond

The formation of a covalent bond is possible by three mechanisms: exchange, donor-acceptor and dative (Lewis).

According to metabolic mechanism The formation of a covalent bond occurs due to the sharing of common electron pairs. In this case, each atom tends to acquire a shell of an inert gas, i.e. obtain a completed external energy level. The formation of a chemical bond by exchange type is depicted using Lewis formulas, in which each valence electron of an atom is represented by dots (Fig. 1).

Rice. 1 Formation of a covalent bond in the HCl molecule by the exchange mechanism

With the development of the theory of atomic structure and quantum mechanics, the formation of a covalent bond is represented as the overlap of electronic orbitals (Fig. 2).

Rice. 2. Formation of a covalent bond due to the overlap of electron clouds

The greater the overlap of atomic orbitals, the stronger the bond, the shorter the bond length, and the greater the bond energy. A covalent bond can be formed by overlapping different orbitals. As a result of the overlap of s-s, s-p orbitals, as well as d-d, p-p, d-p orbitals with lateral lobes, the formation of bonds occurs. A bond is formed perpendicular to the line connecting the nuclei of 2 atoms. One and one bond are capable of forming a multiple (double) covalent bond, characteristic of organic substances of the class of alkenes, alkadienes, etc. One and two bonds form a multiple (triple) covalent bond, characteristic of organic substances of the class of alkynes (acetylenes).

Formation of a covalent bond by donor-acceptor mechanism Let's look at the example of the ammonium cation:

NH 3 + H + = NH 4 +

7 N 1s 2 2s 2 2p 3

The nitrogen atom has a free lone pair of electrons (electrons not involved in the formation of chemical bonds within the molecule), and the hydrogen cation has a free orbital, so they are an electron donor and acceptor, respectively.

Let us consider the dative mechanism of covalent bond formation using the example of a chlorine molecule.

17 Cl 1s 2 2s 2 2p 6 3s 2 3p 5

The chlorine atom has both a free lone pair of electrons and vacant orbitals, therefore, it can exhibit the properties of both a donor and an acceptor. Therefore, when a chlorine molecule is formed, one chlorine atom acts as a donor and the other as an acceptor.

Main characteristics of a covalent bond are: saturation (saturated bonds are formed when an atom attaches as many electrons to itself as its valence capabilities allow; unsaturated bonds are formed when the number of attached electrons is less than the valence capabilities of the atom); directionality (this value is related to the geometry of the molecule and the concept of “bond angle” - the angle between bonds).

Ionic bond

There are no compounds with a pure ionic bond, although this is understood as a chemically bonded state of atoms in which a stable electronic environment of the atom is created when the total electron density is completely transferred to the atom of a more electronegative element. Ionic bonding is possible only between atoms of electronegative and electropositive elements that are in the state of oppositely charged ions - cations and anions.

DEFINITION

Ion are electrically charged particles formed by the removal or addition of an electron to an atom.

When transferring an electron, metal and nonmetal atoms tend to form a stable electron shell configuration around their nucleus. A non-metal atom creates a shell of the subsequent inert gas around its core, and a metal atom creates a shell of the previous inert gas (Fig. 3).

Rice. 3. Formation of an ionic bond using the example of a sodium chloride molecule

Molecules in which ionic bonds exist in their pure form are found in the vapor state of the substance. The ionic bond is very strong, and therefore substances with this bond have a high melting point. Unlike covalent bonds, ionic bonds are not characterized by directionality and saturation, since the electric field created by ions acts equally on all ions due to spherical symmetry.

Metal connection

The metallic bond is realized only in metals - this is the interaction that holds metal atoms in a single lattice. Only the valence electrons of the metal atoms belonging to its entire volume participate in the formation of a bond. In metals, electrons are constantly stripped from atoms and move throughout the entire mass of the metal. Metal atoms, deprived of electrons, turn into positively charged ions, which tend to accept moving electrons. This continuous process forms the so-called “electron gas” inside the metal, which firmly binds all the metal atoms together (Fig. 4).

The metallic bond is strong, therefore metals are characterized by a high melting point, and the presence of “electron gas” gives metals malleability and ductility.

Hydrogen bond

A hydrogen bond is a specific intermolecular interaction, because its occurrence and strength depend on the chemical nature of the substance. It is formed between molecules in which a hydrogen atom is bonded to an atom with high electronegativity (O, N, S). The occurrence of a hydrogen bond depends on two reasons: firstly, the hydrogen atom associated with an electronegative atom does not have electrons and can easily be incorporated into the electron clouds of other atoms, and, secondly, having a valence s-orbital, the hydrogen atom is able to accept a lone pair electrons of an electronegative atom and form a bond with it through the donor-acceptor mechanism.

Crystals.

There are four types of chemical bonds: ionic, covalent, metallic and hydrogen.

Ionic chemical bond

Ionic chemical bond is a bond formed due to the electrostatic attraction of cations to anions.

As you know, the most stable electronic configuration of atoms is one in which the outer electronic level, like the atoms of noble gases, contains 8 electrons (or for the first energy level - 2). During chemical interactions, atoms strive to acquire just such a stable electronic configuration and often achieve this either as a result of the addition of valence electrons from other atoms (the reduction process), or as a result of the donation of their valence electrons (the oxidation process). Atoms that have acquired “foreign” electrons turn into negative ions, or anions. Atoms that donate their electrons become positive ions, or cations. It is clear that electrostatic attraction forces arise between anions and cations, which will hold them near each other, thereby realizing an ionic chemical bond.

Since cations form mainly metal atoms, and anions form non-metal atoms, it is logical to conclude that this type of bond is characteristic of compounds of typical metals (elements of the main subgroups of groups I and II, except magnesium and beryllium Be) with typical non-metals (elements of the main subgroup VII group). A classic example is the formation of alkali metal halides (fluorides, chlorides, etc.). For example, consider the scheme for the formation of an ionic bond in sodium chloride:

Two oppositely charged ions bound by attractive forces do not lose the ability to interact with oppositely charged ions, as a result of which compounds with the ionic crystal lattice are formed. Ionic compounds are solid, strong, refractory substances with a high melting point.

Solutions and melts of most ionic compounds are electrolytes. This type of bond is characteristic of hydroxides of typical metals and many salts of oxygen-containing acids. However, when an ionic bond is formed, an ideal (complete) transfer of electrons does not occur. An ionic bond is an extreme case of a polar covalent bond.

In an ionic compound, ions are presented as if in the form of electric charges with spherical symmetry of the electric field, which equally decreases with increasing distance from the Center of the charge (ion) in any direction. Therefore, the interaction of ions does not depend on direction, that is, an ionic bond, unlike a covalent bond, will be non-directional.

Ionic bonds also exist in ammonium salts, where there are no metal atoms (their role is played by the ammonium cation).

Covalent chemical bond

A covalent chemical bond is a bond that occurs between atoms due to the formation of shared electron pairs.

Its description is also based on the idea that atoms of chemical elements acquire an energetically favorable and stable electronic configuration of eight electrons (for the hydrogen atom, two). Atoms obtain this configuration not by donating or gaining electrons, as in the case of an ionic bond, but by forming shared electron pairs. The mechanism of formation of such a bond can be exchange or donor-acceptor.

The exchange mechanism operates when atoms form shared electron pairs by combining unpaired electrons. For example:

1) H2 - hydrogen:

The bond arises due to the formation of a common electron pair by the s-electrons of hydrogen atoms (overlapping s-orbitals):

The bond occurs due to the formation of a common electron pair of s- and p-electrons (overlapping s-p orbitals):

Let us consider the donor-acceptor mechanism of covalent bond formation using the classic example of the formation of ammonium ion NH4+:

The donor has an electron pair, the acceptor has a free orbital that this pair can occupy. In the ammonium ion, all four bonds with hydrogen atoms are covalent: three were formed due to the creation of common electron pairs by the nitrogen atom and hydrogen atoms according to the exchange mechanism, one was formed through the donor-acceptor mechanism. All four N-H bonds in the ammonium cation are equivalent.

Similarly, a donor-acceptor bond is formed in the methylammonium ion [CH3NH3] +.

Covalent bonds are classified not only by the mechanism of formation of common electron pairs connecting atoms, but also by the method of overlapping electronic orbitals, by the number of common electron pairs, as well as by their displacement to one of the bonded atoms.

Based on the method of overlapping electron orbitals, sigma and pi covalent bonds are distinguished.

In a nitrogen molecule, one common electron pair is formed due to a sigma bond (the electron density is in one region located on the line connecting the atomic nuclei; the bond is strong).

The other two shared electron pairs are formed through p-bonds, that is, the lateral overlap of p-orbitals in two regions; The pi bond is weaker than the sigma bond.

In the nitrogen molecule, there is one sigma bond and two pi bonds between the atoms, which are located in mutually perpendicular planes (since 3 unpaired p-electrons of each atom interact).

Therefore, o-bonds can be formed by overlapping electron orbitals:

and also due to the overlap of “pure” and hybrid orbitals:

sp 2 -sp 2 (C2H4), etc.

Based on the number of common electron pairs connecting atoms, that is, by multiplicity, they distinguish covalent bonds:

1) single:

2) double:
CO,

carbon(IV) monoxide

3) triple:
С2Н2
HC=-CH acetylene

According to the degree of displacement of common electron pairs to one of the atoms they connect, a covalent bond can be non-polar and polar. In a nonpolar covalent bond, the shared electron pairs are not shifted to any of the atoms, since these atoms have the same electronegativity (EO) - the property of attracting valence electrons from other atoms.

A covalent chemical bond formed between atoms with the same electronegativity is called non-polar.
Molecules of simple non-metal substances are formed through covalent non-polar bonds.

The relative electronegativity values ​​of phosphorus and hydrogen are almost the same: EO (H) = 2.1; EO (P) = 2.1, therefore, in the PH3 phosphine molecule, the bonds between the phosphorus atom and the hydrogen atoms are covalent nonpolar.

A covalent chemical bond between atoms of elements whose electronegativity differs is called polar

For example:

NH3
ammonia

Nitrogen is a more electronegative element than hydrogen, so shared electron pairs are shifted towards its atom.

It is necessary to distinguish between the polarity of the molecule and the polarity of the bond. The polarity of a bond depends on the electronegativity values ​​of the bonded atoms, and the polarity of a molecule depends on both the polarity of the bond and the geometry of the molecule. For example, the bonds in the CO2 carbon dioxide molecule will be polar, but the molecule will not be polar, since it has a linear structure.

The H20 water molecule is polar, as it is formed by two covalent polar bonds H-> 0 and has an angular shape. The bond angle of HOH is 104.5°, therefore, the oxygen atom with a partial negative charge of 6 and two lone electron pairs forms a negative pole of the molecule, and the hydrogen atoms with a charge of 6+ form a positive pole. A water molecule is a dipole.

Substances with covalent bonds are characterized by two types of crystal lattice:

atomic - very durable (diamond, graphite, quartz); molecular - under normal conditions these are gases, highly volatile liquids and solid, but fusible or sublimable substances (Cl2, H20, iodine I2, “dry ice” CO2, etc.).

The intramolecular covalent bond is strong, but the intermolecular interaction is very weak, as a result of which the molecular crystal lattice is fragile.

Metal connection

The bond in metals and alloys, which is performed by relatively free electrons between metal ions in a metal crystal lattice, is called metallic.

This bond is nondirectional, unsaturated, and is characterized by a small number of valence electrons and a large number of free orbitals, which is typical for metal atoms. Scheme of metal bond formation (M - metal):

_
M 0 - ne<->M n+

The presence of a metallic bond determines the physical properties of metals and alloys: hardness, electrical and thermal conductivity, malleability, ductility, metallic luster. Substances with a metallic bond have a metallic crystal lattice. Its nodes contain metal ions or atoms, between which electrons move freely (within the crystal) (“electron gas”).

Hydrogen bond

A chemical bond between positively polarized hydrogen atoms of one molecule (or part thereof) and negatively polarized atoms of strongly electronegative elements having lone electron pairs of another molecule (or part thereof) is called hydrogen bonding.

The mechanism of hydrogen bond formation is partly electrostatic, partly donor-acceptor in nature. In the presence of such a connection, even low-molecular substances can, under normal conditions, be liquids (alcohol, water) or easily liquefied gases (ammonia, hydrogen fluoride).

In biopolymers - proteins (secondary structure) there is an intramolecular hydrogen bond between carbonyl oxygen and the hydrogen of the amino group.

Polynucleotide molecules - DNA (deoxyribonucleic acid) are double helices in which two chains of nucleotides are linked to each other by hydrogen bonds. In this case, the principle of complementarity operates, that is, these bonds are formed between certain pairs consisting of purine and pyrimidine bases: the thymine (T) is located opposite the adenine nucleotide (A), and the cytosine (C) is located opposite the guanine (G).

Substances with hydrogen bonds have molecular crystal lattices.

The unified nature of the chemical bond

The division of chemical bonds into types is conditional, since they are all characterized by a certain unity.

An ionic bond can be considered as an extreme case of a polar covalent bond.

A metallic bond combines the covalent interaction of atoms using shared electrons and the electrostatic attraction between these electrons and metal ions.

Substances often lack extreme cases of chemical bonding (or “pure” chemical bonding).

For example, lithium fluoride 1lK is classified as an ionic compound. In fact, the bond in it is 80% ionic and 20% covalent. It is therefore more correct, obviously, to talk about the degree of polarity (ionicity) of a chemical bond.

In the series of hydrogen halides HF - HCl - HBr - HI - HAt, the degree of bond polarity decreases, because the difference in the electronegativity values ​​of the halogen and hydrogen atoms decreases, and in hydrogen atom the bond becomes almost non-polar (EO(H) = 2.1; EO(Ar) = 2.2).

Different types of bonds can be found in the same substances, for example:

1) in bases - between the oxygen and hydrogen atoms in hydroxo groups the bond is covalent polar, and between the metal and the hydroxo group it is ionic;

2) in salts of oxygen-containing acids - between the non-metal atoms and the oxygen of the acidic residue - covalent polar, and between the metal and the acidic residue - ionic;

3) in ammonium, methylammonium, etc. salts - between nitrogen and hydrogen atoms - polar covalent, and between ammonium or methylammonium ions and the acid residue - ionic;

4) in metal peroxides (for example, Na 2 O 2) - the bond between the oxygen atoms is covalent non-polar, and between the metal and oxygen - ionic, etc.

Different types of connections can transform into one another:

During electrolytic dissociation of covalent compounds in water, the covalent polar bond becomes ionic;

When metals evaporate, a metallic bond turns into a nonpolar covalent bond, etc.

The reason for the unity of all types and types of chemical bonds is their identical physical nature - electron-nuclear interaction. The formation of a chemical bond in any case is the result of electron-nuclear interaction of atoms, accompanied by the release of energy (Table 7).

Table 7 Types of chemical bond

1. The expression is often found: “The molecules of noble gases are monatomic.” How true is it?

2. Why, unlike most non-metal elements, their brightest representatives - halogens - do not form allotropic modifications?

3. Give the most complete description of the chemical bond in the nitrogen molecule, using the following characteristics: EO of bonded atoms, formation mechanism, method of overlapping electronic orbitals, bond multiplicity.

4. Determine the type of chemical bond and consider the schemes of its formation in substances with the formulas: Ca, CaF2, F2, ОF2.

5. Write the structural formulas of the substances: CO, CaC2, CS2, FeS2. Determine the oxidation states of elements and their valence (if possible) in these substances.

6. Prove that all types of chemical bonds have a common nature.

7. Why are the molecules N2, CO and C2H2 called isoelectronic?

Basic and additional textbooks

BC Leon is a leading online bookmaker in the gambling market. The company pays special attention to the uninterrupted operation of the service. The functionality of the portal is also constantly being improved. For the convenience of users, the Leon mirror has been created.

Go to mirror

What is a mirror Leon.

To gain access to the official portal of BC Leon, you need to use the mirror. The working mirror provides the user with many advantages such as:

• a diverse range of sporting events that have high odds;
• providing the opportunity to play in Live mode, watching matches will be an interesting experience;
• detailed material regarding the competitions held;
• a convenient interface that even an inexperienced user can quickly understand.

The working mirror is a copy of the official portal. It has identical functionality and a synchronous database. Due to this, your account information does not change. The developers have provided the ability to block the working mirror; in such cases, something else is provided. These exact copies are sent and controlled by BC Leon employees. If you use a functioning mirror, you can access the official portal of BC Leon.

The user will not have difficulty finding a mirror, since their list is subject to updating. With closed access, the site visitor is required to install the Leon mobile phone application on the computer. You also need to change your IP to another country using a VPN. To change the location of the user or provider, you need to use the TOP browser.

The developers have provided various possibilities for using the mirror. To do this, on the right side of the site there is the inscription “Access to the site”; the green “Bypass blocking” button allows the player to go to the submenu and add a universal bookmark to the browser.

The mobile application also provides convenience to the user. If you need to find out about the new address of the portal mirror, you can call the toll-free number. The channel @leonbets_official on Telegram allows you to access the mirror. The Leonacsess app for Windows allows you to always access the site. These methods allow the player to gain access to a working mirror.

Why was the main Leon website blocked?

This is due to the actions of the Roskomnadzor service. This is due to the lack of a license to conduct bookmaking activities. Blue Leon did not receive a license so that the player does not pay 13% on winnings.

How to register on the Leonbets mirror

Registering on this site is much easier than officially. The user does not need to register on two portals, which takes up to two days. If you give preference to a working mirror, then this procedure will be as simple as possible.

To do this, the user will only need to fill in the information regarding full name, contacts. You also need to decide on the currency, indicate your date of birth and home address. You also need to subscribe to the newsletter. This will allow you to quickly receive information from bookmakers. A registered user gets the opportunity to have access to his personal account, which allows him to place bets on matches and events. If difficulties arise, you can contact technical support.