Rice. 2.1. The formation of molecules from atoms is accompanied by redistribution of electrons of valence orbitals and leads to gain in energy because the energy of molecules is less than the energy of non-interacting atoms. The figure shows a diagram of the formation of a non-polar covalent chemical bond between hydrogen atoms.

§2 Chemical bond

Under normal conditions, the molecular state is more stable than the atomic state. (fig.2.1). The formation of molecules from atoms is accompanied by a redistribution of electrons in valence orbitals and leads to a gain in energy, since the energy of molecules is less than the energy of non-interacting atoms(Appendix 3). The forces that hold atoms in molecules have received a generalized name chemical bond.

The chemical bond between atoms is carried out by valence electrons and has an electrical nature . There are four main types of chemical bonding: covalent,ionic,metal And hydrogen.

1 Covalent bond

A chemical bond carried out by electron pairs is called atomic, or covalent. . Compounds with covalent bonds are called atomic, or covalent. .

When a covalent bond occurs, an overlap of electron clouds of interacting atoms occurs, accompanied by energy release (Fig. 2.1). In this case, a cloud with an increased negative charge density arises between positively charged atomic nuclei. Due to the action of the Coulomb forces of attraction between opposite charges, an increase in the negative charge density favors the approach of the nuclei.

A covalent bond is formed by unpaired electrons in the outer shells of atoms . In this case, electrons with opposite spins form electron pair(Fig. 2.2), common to interacting atoms. If one covalent bond has arisen between atoms (one common electron pair), then it is called single, two-double, etc.

Energy is a measure of the strength of a chemical bond. E sv spent on the destruction of the bond (gain in energy during the formation of a compound from individual atoms). Usually this energy is measured per 1 mol substances and are expressed in kilojoules per mol (kJ ∙ mol -1). The energy of a single covalent bond is in the range of 200–2000 kJmol–1.

Rice. 2.2. A covalent bond is the most general type of chemical bond that occurs due to the socialization of an electron pair through an exchange mechanism. (A), when each of the interacting atoms supplies one electron, or through the donor-acceptor mechanism (b) when an electron pair is shared by one atom (donor) to another atom (acceptor).

A covalent bond has properties satiety and focus . The saturation of a covalent bond is understood as the ability of atoms to form a limited number of bonds with their neighbors, determined by the number of their unpaired valence electrons. The directionality of a covalent bond reflects the fact that the forces that hold atoms near each other are directed along the straight line connecting the atomic nuclei. Besides, covalent bond can be polar or non-polar .

When non-polar In a covalent bond, an electron cloud formed by a common pair of electrons is distributed in space symmetrically with respect to the nuclei of both atoms. A non-polar covalent bond is formed between atoms of simple substances, for example, between identical atoms of gases that form diatomic molecules (O 2, H 2, N 2, Cl 2, etc.).

When polar covalent bond electron cloud bond is shifted to one of the atoms. The formation of a polar covalent bond between atoms is characteristic of complex substances. Molecules of volatile inorganic compounds can serve as an example: HCl, H 2 O, NH 3, etc.

The degree of displacement of the common electron cloud to one of the atoms during the formation of a covalent bond (degree of polarity of a bond ) determined mainly by the charge of atomic nuclei and the radius of interacting atoms .

The greater the charge of the atomic nucleus, the stronger it attracts a cloud of electrons. At the same time, the larger the atomic radius, the weaker the outer electrons are held near the atomic nucleus. The cumulative effect of these two factors is expressed in the different ability of different atoms to "pull" the cloud of covalent bonds towards themselves.

The ability of an atom in a molecule to attract electrons to itself is called electronegativity. . Thus, electronegativity characterizes the ability of an atom to polarize a covalent bond: the greater the electronegativity of an atom, the more the electron cloud of a covalent bond is shifted towards it .

A number of methods have been proposed to quantify electronegativity. At the same time, the method proposed by the American chemist Robert S. Mulliken, who determined the electronegativity  an atom as half the sum of its energy E e electron and energy affinities E i atom ionization:

. (2.1)

Ionization energy of an atom is called the energy that needs to be expended in order to “tear off” an electron from it and remove it to an infinite distance. The ionization energy is determined by photoionization of atoms or by bombarding atoms with electrons accelerated in an electric field. That smallest value of the energy of photons or electrons, which becomes sufficient for the ionization of atoms, is called their ionization energy E i. Usually this energy is expressed in electron volts (eV): 1 eV = 1.610 -19 J.

Atoms are the most willing to give away their outer electrons. metals, which contain a small number of unpaired electrons (1, 2 or 3) on the outer shell. These atoms have the lowest ionization energy. Thus, the value of the ionization energy can serve as a measure of the greater or lesser "metallicity" of the element: the lower the ionization energy, the stronger must be expressed metalproperties element.

In the same subgroup of the periodic system of elements of D.I. Mendeleev, with an increase in the ordinal number of the element, its ionization energy decreases (Table 2.1), which is associated with an increase in the atomic radius (Table 1.2), and, consequently, with a weakening of the bond of external electrons with a core. For elements of the same period, the ionization energy increases with increasing serial number. This is due to a decrease in the atomic radius and an increase in the nuclear charge.

Energy E e, which is released when an electron is attached to a free atom, is called electron affinity(expressed also in eV). The release (rather than absorption) of energy when a charged electron is attached to some neutral atoms is explained by the fact that atoms with filled outer shells are the most stable in nature. Therefore, for those atoms in which these shells are “slightly unfilled” (i.e., 1, 2, or 3 electrons are missing before filling), it is energetically beneficial to attach electrons to themselves, turning into negatively charged ions 1 . Such atoms include, for example, halogen atoms (Table 2.1) - elements of the seventh group (main subgroup) of the periodic system of D.I. Mendeleev. The electron affinity of metal atoms is usually zero or negative, i.e. it is energetically unfavorable for them to attach additional electrons, additional energy is required to keep them inside atoms. The electron affinity of non-metal atoms is always positive and the greater, the closer to the noble (inert) gas the non-metal is located in the periodic system. This indicates an increase non-metallic properties as we approach the end of the period.

From all that has been said, it is clear that the electronegativity (2.1) of atoms increases in the direction from left to right for elements of each period and decreases in the direction from top to bottom for elements of the same group of the Mendeleev periodic system. It is not difficult, however, to understand that to characterize the degree of polarity of a covalent bond between atoms, it is not the absolute value of the electronegativity that is important, but the ratio of the electronegativity of the atoms forming the bond. That's why in practice, they use the relative values ​​of electronegativity(Table 2.1), taking the electronegativity of lithium as a unit.

To characterize the polarity of a covalent chemical bond, the difference in the relative electronegativity of atoms is used. Usually the bond between atoms A and B is considered purely covalent, if |  A – B|0.5.

The idea of ​​the formation of a chemical bond with the help of a pair of electrons belonging to both connecting atoms was put forward in 1916 by the American physical chemist J. Lewis.

A covalent bond exists between atoms both in molecules and in crystals. It occurs both between identical atoms (for example, in H 2, Cl 2, O 2 molecules, in a diamond crystal), and between different atoms (for example, in H 2 O and NH 3 molecules, in SiC crystals). Almost all bonds in the molecules of organic compounds are covalent (C-C, C-H, C-N, etc.).

There are two mechanisms for the formation of a covalent bond:

1) exchange;

2) donor-acceptor.

Exchange mechanism for the formation of a covalent bondis that each of the connecting atoms provides for the formation of a common electron pair (bond) by one unpaired electron. The electrons of the interacting atoms must have opposite spins.

Consider, for example, the formation of a covalent bond in a hydrogen molecule. When hydrogen atoms approach each other, their electron clouds penetrate each other, which is called the overlap of electron clouds (Fig. 3.2), the electron density between the nuclei increases. The nuclei are attracted to each other. As a result, the energy of the system decreases. With a very strong approach of atoms, the repulsion of nuclei increases. Therefore, there is an optimal distance between the nuclei (bond length l) at which the system has a minimum energy. In this state, energy is released, called the binding energy E St.

Rice. 3.2. Scheme of overlapping electron clouds during the formation of a hydrogen molecule

Schematically, the formation of a hydrogen molecule from atoms can be represented as follows (a dot means an electron, a bar means a pair of electrons):

H + H→H: H or H + H→H - H.

In general terms, for AB molecules of other substances:

A + B = A: B.

Donor-acceptor mechanism of covalent bond formationconsists in the fact that one particle - the donor - presents an electron pair for the formation of a bond, and the second - the acceptor - a free orbital:

A: + B = A: B.

donor acceptor

Consider the mechanisms of formation of chemical bonds in the ammonia molecule and the ammonium ion.

1. Education

The nitrogen atom has two paired and three unpaired electrons in its outer energy level:

The hydrogen atom on the s - sublevel has one unpaired electron.

In the ammonia molecule, the unpaired 2p electrons of the nitrogen atom form three electron pairs with the electrons of 3 hydrogen atoms:

.

In the NH 3 molecule, 3 covalent bonds are formed by the exchange mechanism.

2. The formation of a complex ion - an ammonium ion.

NH 3 + HCl = NH 4 Cl or NH 3 + H + = NH 4 +

The nitrogen atom has a lone pair of electrons, i.e. two electrons with antiparallel spins in the same atomic orbital. The atomic orbital of the hydrogen ion does not contain electrons (a vacant orbital). When an ammonia molecule and a hydrogen ion approach each other, the lone pair of electrons of the nitrogen atom and the vacant orbital of the hydrogen ion interact. The unshared pair of electrons becomes common for nitrogen and hydrogen atoms, a chemical bond arises according to the donor-acceptor mechanism. The nitrogen atom of the ammonia molecule is the donor, and the hydrogen ion is the acceptor:

.

It should be noted that in the NH 4 + ion all four bonds are equivalent and indistinguishable, therefore, in the ion the charge is delocalized (dispersed) over the entire complex.

The considered examples show that the ability of an atom to form covalent bonds is determined not only by one-electron, but also by 2-electron clouds or by the presence of free orbitals.

According to the donor-acceptor mechanism, bonds are formed in complex compounds: - ; 2+ ; 2- etc.

A covalent bond has the following properties:

- satiety;

- orientation;

- polarity and polarizability.

Covalent, ionic, and metallic are the three main types of chemical bonds.

Let's get to know more about covalent chemical bond. Let's consider the mechanism of its occurrence. Let's take the formation of a hydrogen molecule as an example:

A spherically symmetric cloud formed by a 1s electron surrounds the nucleus of a free hydrogen atom. When atoms approach each other up to a certain distance, their orbitals partially overlap (see Fig.), as a result, a molecular two-electron cloud appears between the centers of both nuclei, which has a maximum electron density in the space between the nuclei. With an increase in the density of the negative charge, there is a strong increase in the forces of attraction between the molecular cloud and the nuclei.

So, we see that a covalent bond is formed by overlapping electron clouds of atoms, which is accompanied by the release of energy. If the distance between the nuclei of the atoms approaching to touch is 0.106 nm, then after the overlap of the electron clouds it will be 0.074 nm. The greater the overlap of electron orbitals, the stronger the chemical bond.

covalent called chemical bonding carried out by electron pairs. Compounds with a covalent bond are called homeopolar or atomic.

Exist two types of covalent bond: polar And non-polar.

With non-polar covalent bond formed by a common pair of electrons, the electron cloud is distributed symmetrically with respect to the nuclei of both atoms. An example can be diatomic molecules that consist of one element: Cl 2, N 2, H 2, F 2, O 2 and others, in which the electron pair belongs to both atoms equally.

At polar In a covalent bond, the electron cloud is displaced towards the atom with a higher relative electronegativity. For example, molecules of volatile inorganic compounds such as H 2 S, HCl, H 2 O and others.

The formation of the HCl molecule can be represented as follows:

Because the relative electronegativity of the chlorine atom (2.83) is greater than that of the hydrogen atom (2.1), the electron pair shifts towards the chlorine atom.

In addition to the exchange mechanism for the formation of a covalent bond - due to overlap, there is also donor-acceptor the mechanism of its formation. This is a mechanism in which the formation of a covalent bond occurs due to a two-electron cloud of one atom (donor) and a free orbital of another atom (acceptor). Let's look at an example of the mechanism for the formation of ammonium NH 4 +. In the ammonia molecule, the nitrogen atom has a two-electron cloud:

The hydrogen ion has a free 1s orbital, let's denote it as .

In the process of ammonium ion formation, the two-electron cloud of nitrogen becomes common for nitrogen and hydrogen atoms, which means it is converted into a molecular electron cloud. Therefore, a fourth covalent bond appears. The process of ammonium formation can be represented as follows:

The charge of the hydrogen ion is dispersed among all atoms, and the two-electron cloud that belongs to nitrogen becomes common with hydrogen.

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When two atoms of the same non-metal element interact, a covalent chemical bond is formed between them using common electron pairs. This covalent bond is called non-polar, since the common electron pairs belong to both atoms to the same extent and neither of them will have an excess or lack of negative charge carried by electrons.

However, if a covalent bond is formed between atoms of different non-metal elements, then the picture will be somewhat different. Consider, for example, the formation of a molecule of hydrogen chloride HC1 from hydrogen and chlorine atoms.

1. The hydrogen atom has one electron at a single level, and before its completion it lacks one more electron. The chlorine atom has seven electrons at the outer level, and it also lacks one electron to complete.

2. Hydrogen and chlorine atoms combine their unpaired electrons and form one common electron pair, i.e., a covalent bond occurs:

Structural formula of the hydrogen chloride molecule H-C1.

3. Since a covalent bond is formed between atoms of different non-metal elements, the common electron pair will no longer belong to the interacting atoms equally. In order to qualitatively determine which of these atoms the common electron pair will belong to a greater extent, the concept of electronegativity is used.

EO can be characterized as a measure of the non-metallicity of chemical elements. In order of decreasing EO, the chemical elements are arranged in the following row:

The most electronegative element in the table of D. I. Mendeleev is fluorine. This is, so to speak, the “gold medalist” of electronegativity. The "silver medal" is oxygen, and the "bronze" is nitrogen.

The value of the EC of an element depends on its position in the Mendeleev table: in each period, it usually increases with an increase in the ordinal number of the element, and in each subgroup it decreases.

Using a number of EOs, it is possible to determine where the common electron pairs are shifted. They are always displaced towards the atoms of the element with the highest EC. For example, in the HC1 molecule of hydrogen chloride, the common electron pair is shifted to the chlorine atom, since its EO is greater than that of hydrogen. As a result, partial charges are formed on the atoms , two poles appear in the molecule - positive and negative. Therefore, such a covalent bond is called polar.

The displacement of common electron pairs in the case of a covalent polar bond is sometimes indicated by arrows, and the partial charge by the Greek letter δ ("delta"):.

In the formulas of compounds, the chemical sign of the less electronegative element is written first. Since a covalent polar bond is a kind of covalent bond, the reasoning algorithm for its schematic representation is the same as for a covalent non-polar bond (see § 11), only in this case one more step will be added - the fourth: we will determine the more electronegative element and reflect the polarity of the bond in the structural formula with an arrow and the designation of partial charges.

For example, consider an algorithm for schematic representation of bond formation for the compound OF 2 - oxygen fluoride.

1. Oxygen is an element of the main subgroup of group VI (VIA group) of the Periodic Table of D. I. Mendeleev. Its atoms have six electrons in the outermost electron layer. There will be unpaired electrons: 8-6 = 2.

Fluorine is an element of the main subgroup of group VII (VIIA group) of the Periodic Table of D. I. Mendeleev. Its atoms contain seven electrons in the outer electron layer. One electron is unpaired.

2. Let's write down the signs of chemical elements with the designation of external electrons:

3. Let's write down the electronic and structural formulas of the formed molecules:

4. According to the EO series, we determine that the common electron pairs will be shifted from oxygen to fluorine, as to a more electronegative element, i.e., the bond will be covalent polar: .

Similarly, water molecules are formed:

In fact, the water molecule is not linear, but angular (∠HOH = 104°27"). The structure of the water molecule can be depicted in various ways (Fig. 40).

Rice. 40.
Different models of water molecule

The hydrogen atom forms only one covalent bond with other atoms. Therefore, hydrogen is said to be monovalent. The oxygen atom is connected to other atoms by two chemical bonds - it is divalent. In the formation of molecules, atoms are connected in such a way that all their valences are involved. It is clear that divalent oxygen must combine with two monovalent hydrogen atoms. If we designate valence with a dash, then the scheme for the formation of a water molecule can be represented as follows:

Similarly, trivalent nitrogen combines with three monovalent hydrogen atoms to form an ammonia molecule.

Formulas in which the valencies of the elements are indicated by dashes, as you know, are called structural.

The structural formula of methane CH 4 - a compound of tetravalent carbon with hydrogen - will be as follows:

And how are the atoms of tetravalent carbon and divalent oxygen combined into a molecule of carbon dioxide CO 2? Obviously, this method can only reflect the following structural formula:

Is valence constant? It turns out that this statement is true for hydrogen and oxygen, but not for nitrogen and carbon, since these elements can also exhibit other valency values. For example, nitrogen can be one-, two-, three-, four-valent. Its compounds with oxygen will have a different composition. Therefore, one distinguishes:

• elements with constant valence (for example, monovalent: H, F; divalent: O, Be; trivalent: B, A1);
• elements with variable valence (for example, S exhibits valences II, IV, VI; C1 - valences I, III, V and VII).

Let's learn how to derive formulas for two-element compounds by valency.

To derive the formula for the compound of phosphorus with oxygen, in which phosphorus is pentavalent, the procedure is as follows:

Similarly, we derive the formula for the compound of nitrogen with oxygen, in which nitrogen is tetravalent.

Index 1 is not written in formulas.

Knowledge of the valency of chemical elements is necessary in order to correctly write down the formula of a substance. However, the opposite is also true: the valence of one of the elements can be determined from the formula of a substance if the valency of the other is known. For example, let's determine the valency of sulfur in a compound whose formula is SO 3:

Laboratory experiment No. 4
Making models of molecules of binary compounds

Using ball and pin kits, assemble the molecular models of the following substances:

• option 1 - hydrogen chloride HC1, carbon tetrachloride CC1 4;
• option 2 - sulfur dioxide SO 2, aluminum chloride AlCl 3.

Keywords and phrases

1. Covalent non-polar and covalent polar chemical bonds.
2. Electronegativity.
3. partial charge.
4. Valence.
5. Drawing up formulas of covalent compounds by valence.
6. Determination of valence by formulas.

Work with computer

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Questions and tasks

1. Hydrogen and phosphorus atoms have almost identical EO values. What is the type of chemical bond in the phosphine PH 3 molecule?
2. Determine the type of chemical bond and write down the scheme of its formation for substances with the formulas: a) S 2 , K 2 O and H 2 S; b) N 2 , Li 3 N and C1 3 N.
3. In which of the molecules - hydrogen chloride HC1 or hydrogen fluoride HF - is the covalent chemical bond more polar?
4. In the following sentences, fill in the missing words and expressions: "A covalent chemical bond is formed due to .... According to the number of common electron pairs, it happens .... According to EO, a covalent bond is divided into ... and ...".
5. Determine the valencies of elements in compounds with the formulas: PbS, PbO 2, FeS 2, Fe 2 S 3, SF 6.
6. Write down the formulas of chlorides - compounds of elements with monovalent chlorine: iron (III), copper (I), copper (II), manganese (IV), phosphorus (V).