Substances with what type of chemical bond. The main types of chemical bonds. Prerequisites for the creation of chemical bond theories

m determination of the chemical bond;

m types chemical bonds;

m method of valence bonds;

m main features covalent bond;

m mechanisms for the formation of a covalent bond;

m complex compounds;

m method of molecular orbitals;

m intermolecular interactions.

CHEMICAL BOND DETERMINATION

chemical bond called the interaction between atoms, leading to the formation of molecules or ions and the strong holding of atoms near each other.

The chemical bond has an electronic nature, that is, it is carried out due to the interaction of valence electrons. Depending on the distribution of valence electrons in a molecule, the following types of bonds are distinguished: ionic, covalent, metallic, etc. An ionic bond can be considered as the limiting case of a covalent bond between atoms that differ sharply in nature.

TYPES OF CHEMICAL BOND

Ionic bond.

The main provisions of the modern theory of ionic bonding.

1.) An ionic bond is formed during the interaction of elements that differ sharply from each other in properties, that is, between metals and non-metals.

2.) The formation of a chemical bond is explained by the desire of atoms to achieve a stable eight-electron outer shell (s 2 p 6).

Ca: 1s 2 2s 2p 6 3s 2p 6 4s 2

Ca 2+ : 1s 2 2s 2 p 6 3s 2 p 6

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

Cl–: 1s 2 2s 2 p 6 3s 2 p 6

3.) The formed oppositely charged ions are held near each other due to electrostatic attraction.

4.) Ionic bond is not directed.

5.) A purely ionic bond does not exist. Since the ionization energy is greater than the energy of electron affinity, the complete transition of electrons does not occur even in the case of a pair of atoms with a large difference in electronegativity. Therefore, we can talk about the share of ionicity of the bond. The highest bond ionicity occurs in fluorides and chlorides of s-elements. Thus, in RbCl, KCl, NaCl, and NaF crystals, it is 99, 98, 90, and 97%, respectively.

covalent bond.

The main provisions of the modern theory of covalent bonds.

1.) A covalent bond is formed between elements that are similar in properties, that is, non-metals.

2.) Each element provides 1 electron for the formation of bonds, and the spins of the electrons must be antiparallel.

3.) If a covalent bond is formed by atoms of the same element, then this bond is not polar, that is, the common electron pair is not shifted to any of the atoms. If the covalent bond is formed by two different atoms, then the common electron pair is shifted to the most electronegative atom, this polar covalent bond.

4.) When a covalent bond is formed, the electron clouds of the interacting atoms overlap, as a result, a zone of increased electron density appears in the space between the atoms, attracting the positively charged nuclei of the interacting atoms and holding them near each other. As a result, the energy of the system decreases (Fig. 14). However, with a very strong approach of atoms, the repulsion of the nuclei increases. Therefore, there is an optimal distance between the nuclei ( bond length, l at which the system has the minimum energy. In this state, energy is released, called binding energy - E St.

Rice. Fig. 14. Dependence of the energy of systems of two hydrogen atoms with parallel (1) and antiparallel (2) spins on the distance between the nuclei (E is the energy of the system, Eb is the binding energy, r is the distance between the nuclei, l is the bond length).

Two methods are used to describe a covalent bond: the valence bond method (BC) and the molecular orbital method (MMO).

VALENCE BOND METHOD.

The VS method is based on the following provisions:

1. A covalent chemical bond is formed by two electrons with oppositely directed spins, and this electron pair belongs to two atoms. Combinations of such two-electron two-center bonds, reflecting the electronic structure of the molecule, are called valent schemes.

2. The stronger the covalent bond, the more the interacting electron clouds overlap.

For a visual representation of valence schemes, the following method is usually used: electrons located in the outer electronic layer are denoted by dots located around the chemical symbol of the atom. Electrons common to two atoms are shown by dots placed between them. chemical symbols; a double or triple bond is denoted respectively by two or three pairs of common dots:

N:1s2 2s 2 p 3;

C:1s2 2s 2 p 4

From the above diagrams, it can be seen that each pair of electrons that binds two atoms corresponds to one dash depicting a covalent bond in structural formulas:

The number of common electron pairs that bind an atom of a given element with other atoms, or, in other words, the number of covalent bonds formed by an atom, is called covalence according to the VS method. So, the covalence of hydrogen is 1, nitrogen - 3.

According to the way the electron clouds overlap, there are two types of bonds: s - bond and p - bond.

s - bond occurs when two electron clouds overlap along the axis connecting the nuclei of atoms.

Rice. 15. Scheme of formation of s - links.

p - bond is formed when electron clouds overlap on both sides of the line connecting the nuclei of interacting atoms.

Rice. 16. Scheme of formation of p-bonds.

MAIN CHARACTERISTICS OF COVALENT BOND.

1. Bond length, ℓ. This is the minimum distance between the nuclei of interacting atoms, which corresponds to the most stable state of the system.

2. Bond energy, E min - this is the amount of energy that must be spent to break the chemical bond and to remove atoms from the interaction.

3. Dipole moment of connection, , m=qℓ. The dipole moment serves as a quantitative measure of the polarity of a molecule. For nonpolar molecules, the dipole moment is 0, for nonpolar molecules it is not 0. The dipole moment of a polyatomic molecule is equal to the vector sum of the dipoles of individual bonds:

4. A covalent bond is characterized by orientation. The orientation of a covalent bond is determined by the need for maximum overlap in space of electron clouds of interacting atoms, which lead to the formation of the strongest bonds.

Since these s-bonds are strictly oriented in space, depending on the composition of the molecule, they can be at a certain angle to each other - such an angle is called a valence angle.

Diatomic molecules have a linear structure. Polyatomic molecules have a more complex configuration. Let us consider the geometry of various molecules using the example of the formation of hydrides.

1. VI group, main subgroup (except oxygen), H 2 S, H 2 Se, H 2 Te.

S 1s 2 2s 2 p 6 3s 2 p 4

For hydrogen, an electron with s-AO participates in the formation of a bond, for sulfur, 3p y and 3p z. The H 2 S molecule has a planar structure with an angle between bonds of 90 0 . .

Fig 17. The structure of the H 2 E molecule

2. Hydrides of elements of group V, the main subgroup: PH 3, AsH 3, SbH 3.

P 1s 2 2s 2 p 6 3s 2 p 3.

In the formation of bonds take part: in hydrogen s-AO, in phosphorus - p y, p x and p z AO.

The PH 3 molecule has the shape of a trigonal pyramid (at the base is a triangle).

Figure 18. The structure of the EN 3 molecule

5. Saturability covalent bond is the number of covalent bonds that an atom can form. It is limited, because An element has a limited number of valence electrons. The maximum number of covalent bonds that a given atom can form in the ground or excited state is called its covalence.

Example: hydrogen is monovalent, oxygen is bivalent, nitrogen is trivalent, etc.

Some atoms can increase their covalence in an excited state due to the separation of paired electrons.

Example. Be 0 1s 2 2s 2

A beryllium atom in an excited state has one valence electron on the 2p-AO and one electron on the 2s-AO, that is, the covalence Be 0 = 0 and the covalence Be * = 2. During the interaction, hybridization of the orbitals occurs.

Hybridization- this is the alignment of the energy of various AO as a result of mixing before chemical interaction. Hybridization is a conditional technique that makes it possible to predict the structure of a molecule using a combination of AOs. Those AOs whose energies are close can take part in hybridization.

Each type of hybridization corresponds to a certain geometric shape of the molecules.

In the case of hydrides of elements of group II of the main subgroup, two identical sp-hybrid orbitals participate in the formation of the bond. This type of bond is called sp hybridization.

Figure 19. Ven 2 molecule. sp hybridization.

sp-hybrid orbitals have an asymmetric shape, elongated parts of the AO with a bond angle of 180 o are directed towards hydrogen. Therefore, the BeH 2 molecule has a linear structure (Fig.).

The structure of the molecules of hydrides of elements Group III consider the main subgroup on the example of the formation of the BH 3 molecule.

B 0 1s 2 2s 2 p 1

Covalence B 0 = 1, covalence B* = 3.

Three sp-hybrid orbitals take part in the formation of bonds, which are formed as a result of the redistribution of the electron densities of s-AO and two p-AO. This type of connection is called sp 2 - hybridization. The bond angle at sp 2 - hybridization is equal to 120 0, therefore, the BH 3 molecule has a flat triangular structure.

Fig.20. BH 3 molecule. sp 2 -Hybridization.

Using the example of the formation of a CH 4 molecule, let us consider the structure of the hydride molecules of the elements of group IV of the main subgroup.

C 0 1s 2 2s 2 p 2

Covalence C 0 \u003d 2, covalence C * \u003d 4.

In carbon, four sp-hybrid orbitals are involved in the formation of a chemical bond, formed as a result of the redistribution of electron densities between s-AO and three p-AO. The shape of the CH 4 molecule is a tetrahedron, the bond angle is 109 o 28`.

Rice. 21. CH 4 molecule. sp 3 -Hybridization.

Exceptions to general rule are H 2 O and NH 3 molecules.

In a water molecule, the angles between bonds are 104.5 o. Unlike hydrides of other elements of this group, water has special properties, it is polar, diamagnetic. All this is explained by the fact that in the water molecule the bond type is sp 3 . That is, four sp - hybrid orbitals are involved in the formation of a chemical bond. Two orbitals contain one electron each, these orbitals interact with hydrogen, the other two orbitals contain a pair of electrons. The presence of these two orbitals explains the unique properties of water.

In the ammonia molecule, the angles between the bonds are approximately 107.3 o, that is, the shape of the ammonia molecule is a tetrahedron, the type of bond is sp 3 . Four hybrid sp 3 orbitals take part in the formation of a bond in a nitrogen molecule. Three orbitals contain one electron each, these orbitals are associated with hydrogen, the fourth AO contains an unshared pair of electrons, which determines the uniqueness of the ammonia molecule.

MECHANISMS OF COVALENT BOND FORMATION.

MVS makes it possible to distinguish three mechanisms for the formation of a covalent bond: exchange, donor-acceptor, and dative.

exchange mechanism. It includes those cases of the formation of a chemical bond, when each of the two bonded atoms allocates one electron for socialization, as if exchanging them. To bind the nuclei of two atoms, the electrons must be in the space between the nuclei. This area in the molecule is called the binding area (the area where the electron pair is most likely to stay in the molecule). In order for the exchange of unpaired electrons in atoms to occur, the overlap of atomic orbitals is necessary (Fig. 10.11). This is the action of the exchange mechanism for the formation of a covalent chemical bond. Atomic orbitals can overlap only if they have the same symmetry properties about the internuclear axis (Fig. 10, 11, 22).

Rice. 22. AO overlap that does not lead to the formation of a chemical bond.

Donor-acceptor and dative mechanisms.

The donor-acceptor mechanism is associated with the transfer of a lone pair of electrons from one atom to a vacant atomic orbital of another atom. For example, the formation of an ion -:

The vacant p-AO in the boron atom in the BF 3 molecule accepts a pair of electrons from the fluoride ion (donor). The resulting anion has four covalent connections B-F equal in length and energy. In the original molecule, all three B-F bonds were formed by the exchange mechanism.

Atoms, the outer shell of which consists only of s- or p-electrons, can be either donors or acceptors of the lone pair of electrons. Atoms that have valence electrons on the d-AO can simultaneously act as both donors and acceptors. To distinguish between these two mechanisms, the concepts of the dative mechanism of bond formation were introduced.

The simplest example manifestations of the dative mechanism - the interaction of two chlorine atoms.

Two chlorine atoms in a chlorine molecule form an exchange covalent bond by combining their unpaired 3p electrons. In addition, the Cl - 1 atom transfers the lone pair of electrons 3p 5 - AO to the Cl - 2 atom to the vacant 3d-AO, and the Cl - 2 atom transfers the same pair of electrons to the vacant 3d -AO of the Cl - 1 atom. Each atom simultaneously performs the functions of an acceptor and a donor. This is the dative mechanism. The action of the dative mechanism increases the strength of the bond, so the chlorine molecule is stronger than the fluorine molecule.

COMPLEX CONNECTIONS.

According to the principle of the donor-acceptor mechanism, a huge class of complex chemical compounds is formed - complex compounds.

Complex compounds are compounds that have in their composition complex ions capable of existing both in crystalline form and in solution, including a central ion or atom associated with negatively charged ions or neutral molecules by covalent bonds formed by the donor-acceptor mechanism.

The structure of complex compounds according to Werner.

Complex compounds consist of an inner sphere (complex ion) and an outer sphere. The connection between the ions of the inner sphere is carried out according to the donor-acceptor mechanism. Acceptors are called complexing agents, they can often be positive metal ions (except for metals of group IA) that have vacant orbitals. The ability to complex formation increases with an increase in the charge of the ion and a decrease in its size.

Donors of an electron pair are called ligands or addends. Ligands are neutral molecules or negatively charged ions. The number of ligands is determined by the coordination number of the complexing agent, which, as a rule, is equal to twice the valency of the complexing ion. Ligands are either monodentate or polydentate. The dentancy of a ligand is determined by the number of coordination sites that the ligand occupies in the coordination sphere of the complexing agent. For example, F - - monodentate ligand, S 2 O 3 2- - bidentate ligand. The charge of the inner sphere is equal to the algebraic sum of the charges of its constituent ions. If the inner sphere has a negative charge, it is an anionic complex; if it is positive, it is a cationic complex. Cationic complexes are called by the name of the complexing ion in Russian, in anionic complexes the complexing agent is called in Latin with the addition of the suffix - at. The connection between the outer and inner spheres in a complex compound is ionic.

Example: K 2 - potassium tetrahydroxozincate, an anionic complex.

1. 2- - inner sphere

2. 2K+ - outer sphere

3. Zn 2+ - complexing agent

4. OH - - ligands

5. coordination number - 4

6. The connection between the outer and inner spheres is ionic:

K 2 \u003d 2K + + 2-.

7. The bond between the Zn 2+ ion and hydroxyl groups is covalent, formed by the donor-acceptor mechanism: OH - - donors, Zn 2+ - acceptor.

Zn 0: … 3d 10 4s 2

Zn 2+ : … 3d 10 4s 0 p 0 d 0

Types of complex compounds:

1. Ammonia - ligands of the ammonia molecule.

Cl 2 - tetraamminecopper (II) chloride. Ammonia is obtained by the action of ammonia on compounds containing a complexing agent.

2. Hydroxo compounds - OH - ligands.

Na is sodium tetrahydroxoaluminate. Hydroxo complexes are obtained by the action of an excess of alkali on metal hydroxides, which have amphoteric properties.

3. Aquacomplexes - ligands of the water molecule.

Cl 3 is hexaaquachromium (III) chloride. Aquacomplexes are obtained by the interaction of anhydrous salts with water.

4. Acido complexes - ligands anions of acids - Cl -, F -, CN -, SO 3 2-, I -, NO 2 -, C 2 O 4 - and others.

K 4 - potassium hexacyanoferrate (II). Obtained by the interaction of an excess of a salt containing a ligand on a salt containing a complexing agent.

MOLECULAR ORBITAL METHOD.

MVS quite well explains the formation and structure of many molecules, but this method is not universal. For example, the method of valence bonds does not give a satisfactory explanation for the existence of an ion, although even in late XIX century, the existence of a fairly strong molecular hydrogen ion was established: the bond breaking energy here is 2.65 eV. However, no electron pair can be formed in this case, since only one electron is included in the composition of the ion.

The molecular orbital method (MMO) makes it possible to explain a number of contradictions that cannot be explained using the valence bond method.

Basic provisions of the IMO.

1. When two atomic orbitals interact, two molecular orbitals are formed. Accordingly, when interacting n-atomic orbitals, n-molecular orbitals are formed.

2. Electrons in a molecule equally belong to all nuclei of the molecule.

3. Of the two formed molecular orbitals, one has a lower energy than the original, is the bonding molecular orbital, the other has a higher energy than the original, it is antibonding molecular orbital.

4. MMOs use energy diagrams without scale.

5. When filling energy sublevels with electrons, the same rules are used as for atomic orbitals:

1) the principle of minimum energy, i.e. sublevels with lower energy are filled first;

2) the Pauli principle: at each energy sublevel there cannot be more than two electrons with antiparallel spins;

3) Hund's rule: the energy sublevels are filled in such a way that the total spin is maximum.

6. Multiplicity of communication. Communication multiplicity in IMO is determined by the formula:

when Kp=0, no bond is formed.

Examples.

1. Can an H 2 molecule exist?

Rice. 23. Scheme of the formation of the hydrogen molecule H 2 .

Conclusion: the H 2 molecule will exist, since the multiplicity of the bond Kp\u003e 0.

2. Can a He 2 molecule exist?

Rice. 24. Scheme of formation of the helium molecule He 2 .

Conclusion: the He 2 molecule will not exist, since the bond multiplicity Kp = 0.

3. Can a particle H 2 + exist?

Rice. 25. Scheme of the formation of the H 2 + particle.

The H 2 + particle can exist, since the multiplicity of the bond Kp > 0.

4. Can an O 2 molecule exist?

Rice. 26. Scheme of the formation of the O 2 molecule.

The O 2 molecule exists. It follows from Fig. 26 that the oxygen molecule has two unpaired electrons. Due to these two electrons, the oxygen molecule is paramagnetic.

Thus the molecular orbital method explains magnetic properties molecules.

INTERMOLECULAR INTERACTION.

All intermolecular interactions can be divided into two groups: universal and specific. Universal ones appear in all molecules without exception. These interactions are often called connection or van der Waals forces. Although these forces are weak (the energy does not exceed eight kJ/mol), they are the cause of the transition of most substances from the gaseous state to the liquid state, the adsorption of gases by the surfaces of solids, and other phenomena. The nature of these forces is electrostatic.

The main forces of interaction:

1). Dipole - dipole (orientation) interaction exists between polar molecules.

The orientational interaction is the greater, the larger the dipole moments, the smaller the distance between the molecules, and the lower the temperature. Therefore, the greater the energy of this interaction, the higher the temperature to which the substance must be heated in order for it to boil.

2). Inductive interaction occurs when there is contact between polar and non-polar molecules in a substance. A dipole is induced in a nonpolar molecule as a result of interaction with a polar molecule.

Cl d + - Cl d - ... Al d + Cl d - 3

The energy of this interaction increases with an increase in the polarizability of molecules, that is, the ability of molecules to form a dipole under the influence of electric field. The energy of the inductive interaction is much less than the energy of the dipole-dipole interaction.

3). Dispersion interaction- this is the interaction of non-polar molecules due to instantaneous dipoles that arise due to fluctuations in the electron density in atoms.

In a series of substances of the same type, the dispersion interaction increases with an increase in the size of the atoms that make up the molecules of these substances.

4) repulsive forces are due to the interaction of electron clouds of molecules and appear when they are further approached.

Specific intermolecular interactions include all types of donor-acceptor interactions, that is, those associated with the transfer of electrons from one molecule to another. The resulting intermolecular bond has all characteristic features covalent bond: saturation and directionality.

A chemical bond formed by a positively polarized hydrogen that is part of a polar group or molecule and an electronegative atom of another or the same molecule is called a hydrogen bond. For example, water molecules can be represented as follows:

Solid lines are polar covalent bonds inside water molecules between hydrogen and oxygen atoms; dots indicate hydrogen bonds. The reason for the formation of hydrogen bonds is that hydrogen atoms are practically devoid of electron shells: their only electrons are displaced to the oxygen atoms of their molecules. This allows protons, unlike other cations, to approach the nuclei of oxygen atoms of neighboring molecules without experiencing repulsion from the electron shells of oxygen atoms.

The hydrogen bond is characterized by a binding energy of 10 to 40 kJ/mol. However, this energy is sufficient to cause association of molecules those. their association into dimers or polymers, which in some cases exist not only in the liquid state of a substance, but are also preserved when it passes into vapor.

For example, hydrogen fluoride in the gas phase exists as a dimer.

In complex organic molecules, there are both intermolecular hydrogen bonds and intramolecular hydrogen bonds.

Molecules with intramolecular hydrogen bonds cannot enter into intermolecular hydrogen bonds. Therefore, substances with such bonds do not form associates, are more volatile, have lower viscosities, melting and boiling points than their isomers capable of forming intermolecular hydrogen bonds.

Covalent chemical bond, its varieties and formation mechanisms. Characteristics of a covalent bond (polarity and bond energy). Ionic bond. Metal connection. hydrogen bond

The doctrine of the chemical bond is the basis of all theoretical chemistry.

A chemical bond is such an interaction of atoms that binds them into molecules, ions, radicals, crystals.

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

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

An ionic bond can be considered as the limiting case of a covalent polar bond.

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

In substances, there are often no limiting cases of chemical bonding (or pure chemical bonds).

For example, lithium fluoride $LiF$ is classified as an ionic compound. In fact, the bond in it is $80%$ ionic and $20%$ covalent. Therefore, it is obviously more correct to speak of the degree of polarity (ionicity) of a chemical bond.

In the series of hydrogen halides $HF—HCl—HBr—HI—HAt$, the degree of polarity of the bond decreases, because the difference in the electronegativity values ​​of the halogen and hydrogen atoms decreases, and in astatine the bond becomes almost nonpolar $(EO(H) = 2.1; EO(At) = 2.2)$.

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

1. in bases: between the oxygen and hydrogen atoms in the hydroxo groups, the bond is polar covalent, and between the metal and the hydroxo group is ionic;
2. in salts of oxygen-containing acids: between the non-metal atom and the oxygen of the acid residue - covalent polar, and between the metal and the acid residue - ionic;
3. in salts of ammonium, methylammonium, etc.: between nitrogen and hydrogen atoms - covalent polar, and between ammonium or methylammonium ions and an acid residue - ionic;
4. in metal peroxides (for example, $Na_2O_2$), the bond between oxygen atoms is covalent non-polar, and between metal and oxygen it is ionic, and so on.

Different types of connections can pass one into another:

- during electrolytic dissociation in water of covalent compounds, a covalent polar bond passes into an ionic one;

- during the evaporation of metals, the metallic bond turns into a covalent non-polar, 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 an electron-nuclear interaction of atoms, accompanied by the release of energy.

Methods for the formation of a covalent bond. Characteristics of a covalent bond: bond length and energy

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

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

I. exchange mechanism acts when atoms form common electron pairs by combining unpaired electrons.

1) $H_2$ - hydrogen:

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

2) $HCl$ - hydrogen chloride:

The bond arises 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) $N_2$: three common electron pairs are formed between atoms in a nitrogen molecule:

II. Donor-acceptor mechanism Let us consider the formation of a covalent bond using the example of the ammonium ion $NH_4^+$.

The donor has an electron pair, the acceptor has an empty 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 by the exchange mechanism, one - by the donor-acceptor mechanism.

Covalent bonds can be classified by the way the electron orbitals overlap, as well as by their displacement to one of the bonded atoms.

Chemical bonds formed as a result of the overlap of electron orbitals along the bond line are called $σ$ -bonds (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" overlapping of electron orbitals outside the communication line, i.e. in two regions are called $π$ -bonds (pi-bonds).

By degree of bias common electron pairs to one of the atoms they bond, a covalent bond can be polar and non-polar.

A covalent chemical bond formed between atoms with the same electronegativity is called non-polar. Electron pairs are not shifted to any of the atoms, because atoms have the same EC - the property of pulling valence electrons towards themselves from other atoms. For example:

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

The length and energy of a covalent bond.

characteristic covalent bond properties is its length and energy. Link length is the distance between the nuclei of atoms. A chemical bond is stronger the shorter its length. However, the measure of bond strength is binding 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 $H_2, Cl_2$, and $N_2$ molecules are $0.074, 0.198$, and $0.109$ nm, respectively, and the binding energies are $436, 242$, and $946$ kJ/mol, respectively.

Ions. Ionic bond

Imagine that two atoms "meet": a metal atom of group I and a non-metal atom of group VII. A metal atom has a single electron in its outer energy level, while a non-metal atom lacks just one electron to complete its outer level.

The first atom will easily give up to the second its electron, which is far from the nucleus and weakly bound to it, and the second will give it free space on its outer electronic level.

Then an 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 received electron. Such particles are called ions.

The chemical bond that occurs between ions is called ionic.

Consider the formation of this bond using the well-known sodium chloride compound (table salt) as an example:

The process of transformation of atoms into ions is shown in the diagram:

Such a transformation of atoms into ions always occurs during the interaction of atoms of typical metals and typical non-metals.

Consider the algorithm (sequence) of reasoning when recording the formation of an ionic bond, for example, between calcium and chlorine atoms:

Numbers showing the number of atoms or molecules are called coefficients, and the numbers showing the number of atoms or ions in a molecule are called indexes.

metal connection

Let's get acquainted with how the atoms of metal elements interact with each other. Metals do not usually exist in the form of isolated atoms, but in the form of a piece, ingot, or metal product. What holds metal atoms together?

The atoms of most metals at the outer level contain a small number of electrons - $1, 2, 3$. These electrons are easily detached, and the atoms are converted 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, and so on. Consequently, in the volume of a metal, atoms are continuously converted into ions and vice versa.

The bond in metals between ions by means of socialized electrons is called metallic.

The figure schematically shows the structure of a sodium metal fragment.

In this case, a small number of socialized electrons binds a large number of ions and atoms.

The metallic bond bears some resemblance to the covalent bond, since it is based on the sharing of outer electrons. However, in a covalent bond, the outer unpaired electrons of only two neighboring atoms are socialized, while in a metallic bond, all atoms take part in the socialization of these electrons. That is why crystals with a covalent bond are brittle, while those with a metal bond are, as a rule, plastic, electrically conductive, and have a metallic sheen.

The metallic bond is characteristic of both pure metals and mixtures of various metals - alloys that are in solid and liquid states.

hydrogen bond

A chemical bond between positively polarized hydrogen atoms of one molecule (or part of it) and negatively polarized atoms of strongly electronegative elements having lone electron pairs ($F, O, N$ and less often $S$ and $Cl$), another molecule (or its parts) is called hydrogen.

The mechanism of hydrogen bond formation is partly electrostatic, partly donor-acceptor.

Examples of intermolecular hydrogen bonding:

In the presence of such a bond, even low molecular weight substances can under normal conditions be liquids (alcohol, water) or easily liquefying gases (ammonia, hydrogen fluoride).

Substances with a hydrogen bond have molecular crystal lattices.

Substances of molecular and non-molecular structure. Type of crystal lattice. The dependence of the properties of substances on their composition and structure

Molecular and non-molecular structure of substances

not enter into chemical interactions individual atoms or molecules, but substances. A substance under given conditions can be in one of three states of aggregation: solid, liquid or gaseous. The properties of a substance also depend on the nature of the chemical bond between the particles that form it - molecules, atoms or ions. According to the type of bond, substances of molecular and non-molecular structure are distinguished.

Substances made up of molecules are called molecular substances. The bonds between molecules in such substances are very weak, much weaker than between atoms inside a molecule, and already at relatively low temperatures they break - the substance turns into a liquid and then into a gas (iodine sublimation). The melting and boiling points of substances consisting of molecules increase with increasing molecular weight.

Molecular substances include substances with an atomic structure ($C, Si, Li, Na, K, Cu, Fe, W$), among them there are metals and non-metals.

Consider the physical properties alkali metals. The relatively low bond strength between atoms causes low mechanical strength: alkali metals are soft and can be easily cut with a knife.

The large sizes of atoms lead to a low density of alkali metals: lithium, sodium and potassium are even lighter than water. In the group of alkali metals, the boiling and melting points decrease with an increase in the ordinal number of the element, because. the size of the atoms increases and the bonds weaken.

To substances non-molecular structures include ionic compounds. Most compounds of metals with non-metals have this structure: all salts ($NaCl, K_2SO_4$), some hydrides ($LiH$) and oxides ($CaO, MgO, FeO$), bases ($NaOH, KOH$). Ionic (non-molecular) substances have high melting and boiling points.

Crystal lattices

A substance, as we know, can exist in three states of aggregation: gaseous, liquid and solid.

Solids: amorphous and crystalline.

Consider how the features of chemical bonds affect the properties of solids. Solids are divided into crystalline and amorphous.

Amorphous substances do not have a clear melting point - when heated, they gradually soften and become fluid. In the amorphous state, for example, are plasticine and various resins.

Crystalline substances are characterized by the correct arrangement of the particles of which they are composed: atoms, molecules and ions - at strictly defined points in space. When these points are connected by straight lines, a spatial frame is formed, called the crystal lattice. The points at which crystal particles are located are called lattice nodes.

Depending on the type of particles located at the nodes of the crystal lattice, and the nature of the connection between them, four types of crystal lattices are distinguished: ionic, atomic, molecular and metal.

Ionic crystal lattices.

Ionic called crystal lattices, in the nodes of which there are ions. They are formed by substances with an ionic bond, which can bind both simple ions $Na^(+), Cl^(-)$, and complex $SO_4^(2−), OH^-$. Consequently, salts, some oxides and hydroxides of metals have ionic crystal lattices. For example, a sodium chloride crystal consists of alternating $Na^+$ positive ions and $Cl^-$ negative ions, forming a cube-shaped lattice. The bonds between ions in such a crystal are very stable. Therefore, substances with an ionic lattice are characterized by relatively high hardness and strength, they are refractory and non-volatile.

Atomic crystal lattices.

nuclear called crystal lattices, in the nodes of which there are individual atoms. In such lattices, the atoms are interconnected by very strong covalent bonds. An example of substances with this type of crystal lattice is diamond, one of the allotropic modifications of carbon.

Most substances with an atomic crystal lattice have very high melting points (for example, for diamond it is above $3500°C$), they are strong and hard, practically insoluble.

Molecular crystal lattices.

Molecular called crystal lattices, at the nodes of which molecules are located. Chemical bonds in these molecules can be either polar ($HCl, H_2O$) or nonpolar ($N_2, O_2$). Despite the fact that the atoms within the molecules are bound by very strong covalent bonds, there are weak forces of intermolecular attraction between the molecules themselves. Therefore, substances with molecular crystal lattices have low hardness, low melting points, and are volatile. Most solid organic compounds have molecular crystal lattices (naphthalene, glucose, sugar).

Metallic crystal lattices.

Substances with a metallic bond have metallic crystal lattices. At the nodes of such lattices there are atoms and ions (either atoms or ions, into which metal atoms easily turn, giving their outer electrons “for common use”). Such internal structure metals determines their characteristic physical properties: ductility, ductility, electrical and thermal conductivity, characteristic metallic luster.

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 variety 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 features of the distribution of electrons, and hence their own types of bonds.

Basic concepts

chemical bond called a set of interactions that lead to the binding of atoms to form 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.

Valency accepted name the ability of an atom to form a certain number of bonds with other atoms. In ionic compounds, the number of given or attached electrons is taken as the value of valency. In covalent compounds, it is equal to the number of common electron pairs.

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

The multiplicity of the connection is called the number of shared electron pairs between the considered atoms.

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

Basic communication characteristics

By bond energy is the energy required to break all the bonds in a molecule. It is also the energy released during bond formation.

Communication length called such a distance between neighboring nuclei of atoms in a molecule, at which the forces of attraction and repulsion are balanced.

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

Valence angle It is customary to call the angle between the represented lines passing in the direction of the bond through the nuclei of atoms.

Relationship Description Methods

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

Method of molecular orbitals. He considers a 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 along these orbits. Also, this method is called MO LCAO, which stands for "molecular orbital - linear combination

The method of valence bonds. 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 with opposite spins, which are located between the two considered atoms. The formed electron pair belongs to two atoms equally.
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 take part 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 common electron pairs (electron cloud), which leads to bond polarization.

There are various ways to determine the values ​​of electronegativity 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 the electronegativity of atoms, the more pronounced its ionicity. On the contrary, equal or close electronegativity values ​​indicate the covalent nature of the bond. In other words, it is possible to determine which chemical bond is observed in a particular molecule mathematically. To do this, you need to calculate ΔX - the difference in the electronegativity of atoms according to the formula: ΔX=|X 1 -X 2 |.

• If a ΔX>1.7, then the bond is ionic.
• If a 0.5≤ΔХ≤1.7, the covalent bond is polar.
• If a ΔX=0 or close to it, then the bond is covalent non-polar.

Ionic bond

An ionic bond is such 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 bonding is carried out by the forces of electrostatic attraction.

Ions are charged particles formed from atoms as a result of the addition or release of electrons. When an atom accepts electrons, it acquires a negative charge and becomes an anion. If an atom donates 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 of this process is the aspiration 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 donate an electron pulled 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 referred to 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 size.

covalent chemical bond

The bond formed when the electron clouds of non-metal atoms overlap, that is, carried out by a common electron pair, is called a covalent bond. The number of shared pairs of electrons determines the multiplicity of the bond. So, hydrogen atoms are connected by a single H··H bond, and oxygen atoms form a double bond O::O.

There are two mechanisms for its formation:

• Exchange - each atom represents for the formation of a common pair of one electron: A + B = A: B, while the connection involves external atomic orbitals, on which one electron is located.
• 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 \u003d A: B.

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

1. Direct. The cloud overlap region lies on a straight imaginary line connecting the nuclei of the considered atoms. In this case, σ-bonds are formed. The type of chemical bond that occurs in this case depends on the type of electron clouds undergoing overlap: s-s, s-p, p-p, s-d or p-d σ-bonds. In a particle (molecule or ion), only one σ-bond can occur 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. Apart from the σ-bond, the π-bond is never formed; it can be in molecules containing multiple (double and triple) bonds.

Properties of a covalent bond

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

Orientation connections are due to the features of the molecular structure of substances and the geometric shape of their molecules. Its essence lies in the fact that the best overlap of electron clouds is possible with a certain orientation in space. The options for the formation of σ- and π-bonds have already been considered above.

Under satiety 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. It determines the uniformity of the distribution of electrons between the nuclei of atoms. The covalent bond on this basis can be polar or non-polar.

• If a common electron pair equally belongs to each of the atoms and is located at the same distance from their nuclei, then the covalent bond is non-polar.
• If the common pair of electrons is displaced to 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 action 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 forms during the implementation of a chemical bond. This is necessary to achieve the most effective overlap. There are the following types of hybridization:

• 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 feature of the structure of metal atoms is a 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 easily broken.

metal a bond is such an interaction between metal atoms-ions, which is carried out with the help of delocalized electrons.

In metal particles, valence electrons can easily leave outer orbitals, as well as occupy vacant places on them. Thus, at different times, the same particle can be an atom and an ion. The electrons torn off 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. As well as for ionic, ions are necessary for the existence of a metallic bond. But if for the implementation of electrostatic interaction in the first case, cations and anions are needed, then in the second, the role of negatively charged particles is played by electrons. If we compare a metallic bond with a covalent bond, then the formation of both requires common electrons. However, unlike a polar chemical bond, they are not localized between two atoms, but belong to all metal particles in the crystal lattice.

The metallic bond is responsible for the special properties of almost all metals:

• plasticity, present due to the possibility of displacement of layers of atoms in the crystal lattice held by the 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 along it);
• electrical conductivity, which is carried out by a stream 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 referred to as intermediate between covalent and intermolecular interactions. If a hydrogen atom has a bond with one of the strongly electronegative elements (such as phosphorus, oxygen, chlorine, nitrogen), then it is able to form an additional bond, called hydrogen.

It is much weaker than all the types of bonds considered above (the energy is not more than 40 kJ/mol), but it cannot be neglected. That is why the hydrogen chemical bond in the diagram looks like a dotted line.

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

Hydrogen bonding can occur both between neighboring molecules, for example, water, carboxylic acids, alcohols, ammonia, and inside the molecule, for example, salicylic acid.

The presence of a hydrogen bond between water molecules explains a number of its unique features. physical properties:

• The values ​​of its heat capacity, dielectric constant, boiling and melting points, in accordance with the calculations, should be much lower than the real ones, which is explained by the bonding of molecules and the need to expend energy to break intermolecular hydrogen bonds.
• Unlike other substances, as the temperature decreases, the volume of water increases. This is 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 bond plays a special role for living organisms, since its presence in protein molecules determines their special structure, and hence their properties. In addition, nucleic acids, making up the double helix of DNA, are also connected precisely by hydrogen bonds.

Bonds in crystals

The vast majority of solids have a crystal lattice - a special mutual arrangement 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 bonds between them, all crystal structures are divided into atomic, molecular, ionic and metallic.

At the nodes of the ionic crystal lattice are 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, as they require a lot of energy to break down.

At the nodes of the molecular crystal lattice, there are molecules of substances formed by a covalent bond (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 valence values. They are connected by strong covalent bonds, which means that the substances have high boiling and melting points and high hardness. An example is a diamond.

Thus, all types of connections available in chemicals, 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.

Crystals.

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

Ionic chemical bond

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

As you know, the most stable is such an electronic configuration of atoms, in which at the external electronic level, like atoms noble gases, there will be 8 electrons (or for the first energy level - 2). In chemical interactions, atoms tend 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 (reduction process), or as a result of giving up their valence electrons (oxidation process). Atoms that have attached "foreign" electrons turn into negative ions, or anions. Atoms that donate their electrons turn into positive ions, or cations. It is clear that electrostatic attraction forces arise between anions and cations, which will keep them near each other, thereby carrying out 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 typical for compounds of typical metals (elements of the main subgroups of groups I and II, except for 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 their ability to interact with oppositely charged ions, as a result of which compounds with an 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) transition of electrons does not occur. An ionic bond is an extreme case of a covalent polar bond.

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

An ionic bond also exists in ammonium salts, where there are no metal atoms (their role is played by the ammonium cation).

covalent chemical bond

There is no unified theory of chemical bonding; chemical bonding is conditionally divided into covalent (universal type of bond), ionic (a special case of 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 exchange mechanism the formation of a covalent bond occurs due to the socialization of common electron pairs. In this case, each atom tends to acquire an inert gas shell, i.e. get the completed outer energy level. The formation of an exchange-type chemical bond 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 the structure of the atom and quantum mechanics, the formation of a covalent bond is represented as an 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 its energy. A covalent bond can be formed by overlapping different orbitals. As a result of overlapping s-s, s-p orbitals, as well as d-d, p-p, d-p orbitals, the formation of bonds occurs with the side lobes. Perpendicular to the line connecting the nuclei of 2 atoms, a bond is formed. One - and one - bonds are able to form 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).

The formation of a covalent bond donor-acceptor mechanism consider 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 the formation of a covalent bond 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 covalent bond characteristics 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); directivity (this value is associated with the geometry of the molecule and the concept of "valence angle" - the angle between bonds).

Ionic bond

There are no compounds with a pure ionic bond, although this is understood as such a chemically bound state of atoms in which a stable electronic environment of the atom is created with the complete transition of the total electron density to an 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 called electrically charged particles formed by detaching or attaching an electron to an atom.

When transferring an electron, the atoms of metals and non-metals tend to form a stable configuration around their nucleus. electron shell. 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 an ionic bond exists in its pure form are found in the vapor state of a substance. The ionic bond is very strong, in connection with this, substances with this bond have a high melting point. Unlike covalent bonds, ionic bonds are not characterized by directivity and saturation, since the electric field created by ions acts equally on all ions due to spherical symmetry.

metal bond

A metallic bond is realized only in metals - this is an interaction that holds metal atoms in a single lattice. Only the valence electrons of the metal atoms, which belong to its entire volume, participate in the formation of the bond. In metals, electrons are constantly detached from atoms, which move throughout the mass of the metal. Metal atoms, devoid of electrons, turn into positively charged ions, which tend to take moving electrons towards them. 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 an "electron gas" gives metals malleability and ductility.

hydrogen bond

The hydrogen bond is a specific intermolecular interaction, because its occurrence and strength depend on chemical nature substances. 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 introduced 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 by the donor-acceptor mechanism.