Substances of molecular structure are formed with the help of special type relationships. Covalent bond in a molecule, polar and non-polar, also called atomic. This name comes from the Latin “co” - “together” and “vales” - “having force”. In this method of forming compounds, a pair of electrons is shared between two atoms.

What are polar and nonpolar covalent bonds? If a new compound is formed in this way, thensocialization of electron pairs. Typically, such substances have a molecular structure: H 2, O 3, HCl, HF, CH 4.

There are also non-molecular substances in which the atoms are connected in this way. These are the so-called atomic crystals: diamond, silicon dioxide, silicon carbide. In them, each particle is connected to four others, resulting in a very strong crystal. Crystals with a molecular structure are usually not very strong.

Properties of this method of forming compounds:

• multiplicity;
• direction;
• degree of polarity;
• polarizability;
• pairing.

Multiplicity is the number of electron pairs shared. There can be from one to three. Oxygen does not have enough electrons to fill its shell, so it will be double. In nitrogen molecule N2 it is triple.

Polarizability - the possibility of forming a covalent polar bond and a non-polar one. Moreover, it can be more or less polar, closer to ionic or vice versa - this is the property of the degree of polarity.

Directionality means that atoms tend to connect in such a way that there remains as much electron density between them as possible. It makes sense to talk about directionality when p or d orbitals are connected. S-orbitals are spherically symmetrical, for them all directions are equivalent. In p-orbitals, the nonpolar or polar covalent bond is directed along their axis, so that the two “eights” overlap at the vertices. This is a σ bond. There are also less strong π bonds. In the case of p-orbitals, the “eight” orbitals are overlapped by the lateral sides outside the axis of the molecule. In the double or triple case, the p orbitals form one σ bond, and the rest will be of the π type.

Conjugation is the alternation of primes and multiples, making the molecule more stable. This property is characteristic of complex organic compounds.

Types and methods of formation of chemical bonds

Polarity

Important! How to determine whether substances with a non-polar covalent or polar bond are in front of us? It is very simple: the first always occurs between identical atoms, and the second - between different atoms that have unequal electronegativity.

Examples of covalent nonpolar bonds - simple substances:

• hydrogen H 2;
• nitrogen N2;
• oxygen O 2;
• chlorine Cl2.

The formation scheme of a covalent nonpolar bond shows that by combining an electron pair, atoms tend to complement the outer shell to 8 or 2 electrons. For example, fluorine is one electron short of an eight-electron shell. After the formation of the shared electron pair, it will be filled. A common formula for a substance with covalent non-polar bond- diatomic molecule.

Polar usually only connect:

• H 2 O;
• CH4.

But there are exceptions, such as AlCl 3. Aluminum has the property of amphotericity, that is, in some compounds it behaves like a metal, and in others it behaves like a non-metal. The difference in electronegativity in this compound is small, so aluminum combines with chlorine in this way, and not according to the ionic type.

In this case, the molecule is formed by different elements, but the difference in electronegativity is not so great that the electron is completely transferred from one atom to another, as in substances with an ionic structure.

Schemes for the formation of this type of covalent structure show that the electron density shifts to a more electronegative atom, that is, the shared electron pair is closer to one of them than to the second. The parts of the molecule acquire a charge, which is denoted by the Greek letter delta. In hydrogen chloride, for example, the chlorine becomes more negatively charged and the hydrogen more positively charged. The charge will be partial, and not whole, like with ions.

Important! Bond polarity should not be confused with molecular polarity. In methane CH4, for example, the atoms are bonded polarly, but the molecule itself is nonpolar.

Useful video: polar and non-polar covalent bonds

Education mechanism

The formation of new substances can occur through an exchange or donor-acceptor mechanism. In this case, atomic orbitals are combined. One or more molecular orbitals arise. They differ in that they span both atoms. Like an atomic electron, it can contain no more than two electrons, and their spins must also be in different directions.

How to determine which mechanism is involved? This can be done by the number of electrons in the outer orbitals.

Exchange

In this case, an electron pair in a molecular orbital is formed from two unpaired electrons, each of which belongs to its own atom. Each of them strives to fill its outer electron shell and make it stable eight- or two-electron. This is how substances with a non-polar structure are usually formed.

For example, consider hydrochloric acid HCl. Hydrogen has external level one electron. Chlorine has seven. Having drawn diagrams of the formation of a covalent structure for it, we will see that each of them lacks one electron to fill the outer shell. By sharing an electron pair among themselves, they will be able to complete the outer shell. The same principle is used to form diatomic molecules of simple substances, for example, hydrogen, oxygen, chlorine, nitrogen and other non-metals.

Education mechanism

Donor-acceptor

In the second case, both electrons are a lone pair and belong to the same atom (donor). The other (acceptor) has an empty orbital.

The formula of a substance with a covalent polar bond formed in this way, for example, ammonium ion NH 4 +. It is formed from a hydrogen ion, which has an empty orbital, and ammonia NH3, which contains one “extra” electron. The electron pair from ammonia is socialized.

Hybridization

When an electron pair is shared between orbitals various shapes, for example, s and p, a hybrid electron cloud sp is formed. Such orbitals overlap more, so they bind more tightly.

This is how the molecules of methane and ammonia are structured. In the CH 4 methane molecule, three bonds should have been formed in p-orbitals and one in s. Instead, the orbital hybridizes with three p orbitals, resulting in three sp3 hybrid orbitals in the shape of elongated droplets. This happens because the 2s and 2p electrons have similar energies, they interact with each other when they combine with another atom. Then a hybrid orbital can be formed. The resulting molecule has the shape of a tetrahedron, with hydrogen located at its vertices.

Other examples of substances with hybridization:

• acetylene;
• benzene;
• diamond;
• water.

Carbon is characterized by sp3 hybridization, so it is often found in organic compounds.

Useful video: polar covalent bond

Conclusion

A covalent bond, polar or nonpolar, is characteristic of substances with a molecular structure. Atoms of one element are nonpolarly bonded, while atoms of different elements are polarly bonded, but with slightly different electronegativity. Usually non-metallic elements are connected in this way, but there are exceptions, such as aluminum.

Rice. 2.1. The formation of molecules from atoms is accompanied by redistribution of electrons of valence orbitals and leads to gain in energy, since the energy of molecules turns out to be less than the energy of non-interacting atoms. The figure shows a diagram of the formation of a nonpolar 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 are collectively called chemical bond.

The chemical bond between atoms is carried out by valence electrons and is electrical in nature . There are four main types of chemical bonds: 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 the release of energy (Fig. 2.1). In this case, a cloud with an increased density of negative charge appears between the positively charged atomic nuclei. Due to the action of Coulomb forces of attraction between unlike charges, an increase in the density of the negative charge favors the bringing together of 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 (one common electron pair) has arisen between atoms, then it is called single, double, double, etc.

Energy is a measure of the strength of a chemical bond. E sv spent on breaking the bond (gain in energy when forming a compound from individual atoms). This energy is usually measured per 1 mole. substances and are expressed in kilojoules per mole (kJ∙mol –1). The energy of a single covalent bond lies in the range of 200–2000 kJmol –1.

Rice. 2.2. Covalent bond is the most general form chemical bond arising due to the sharing of an electron pair through an exchange mechanism (A), when each of the interacting atoms supplies one electron, or through a donor-acceptor mechanism (b) when an electron pair is shared by one atom (donor) to another atom (acceptor).

A covalent bond has the properties saturation 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 holding 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, the electron cloud formed by a common pair of electrons is distributed in space symmetrically relative to the nuclei of both atoms. A nonpolar 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 In a covalent bond, the electron cloud of the bond is shifted toward one of the atoms. The formation of polar covalent bonds between atoms is characteristic of complex substances. An example is the molecules of volatile inorganic compounds: HCl, H 2 O, NH 3, etc.

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

The greater the charge of an atomic nucleus, the more strongly it attracts a cloud of electrons. At the same time, the larger the radius of the atom, the weaker the outer electrons are held near the atomic nucleus. The combined 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 strongly the electron cloud of the covalent bond is shifted towards it .

A number of methods have been proposed to quantify electronegativity. In this case, the clearest physical meaning has the method proposed by the American chemist Robert S. Mulliken, who determined electronegativity  of an atom as half the sum of its energy E e electron affinity and energy E i ionization of atom:

. (2.1)

Ionization energy of an atom is the energy that must be expended to “tear off” an electron from it and remove it to an infinite distance. Ionization energy is determined by photoionization of atoms or by bombarding atoms with electrons accelerated in an electric field. The smallest value of photon or electron energy that becomes sufficient to ionize atoms is called their ionization energy E i. This energy is usually expressed in electron volts (eV): 1 eV = 1.610 –19 J.

Atoms are most willing to give up 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 magnitude of the ionization energy can serve as a measure of the greater or lesser “metallicity” of an element: the lower the ionization energy, the more pronounced the metalproperties element.

In the same subgroup of the periodic system of elements of D.I. Mendeleev, with an increase in the atomic number of an 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, ionization energy increases with increasing atomic number. This is due to a decrease in atomic radius and an increase in nuclear charge.

Energy E e, which is released when an electron is added to a free atom, is called electron affinity(also expressed in eV). The release (rather than absorption) of energy when a charged electron attaches to some neutral atoms is explained by the fact that the most stable atoms in nature are those with filled outer shells. Therefore, for those atoms in which these shells are “a little unfilled” (i.e., 1, 2 or 3 electrons are missing before filling), it is energetically favorable 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 D.I. Mendeleev’s periodic system. 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 the atoms. The electron affinity of nonmetal atoms is always positive and the greater, the closer the nonmetal is located to a noble (inert) gas in the periodic table. 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 electronegativity that is important, but the ratio of the electronegativities of the atoms forming the bond. That's why in practice they use relative electronegativity values(Table 2.1), taking the electronegativity of lithium as unity.

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

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 come close to a certain distance, their orbitals partially overlap (see figure), 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, a strong increase in the forces of attraction between the molecular cloud and the nuclei occurs.

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 atoms approaching before touching is 0.106 nm, then after the electron clouds overlap it will be 0.074 nm. The greater the overlap of electron orbitals, the stronger the chemical bond.

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

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

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

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

The formation of an 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 is shifted to the chlorine atom.

In addition to the exchange mechanism of covalent bond formation - due to overlap, there is also donor-acceptor mechanism of its formation. This is a mechanism in which the formation of a covalent bond occurs due to the two-electron cloud of one atom (donor) and the 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 this as .

During the formation of the ammonium ion, the two-electron cloud of nitrogen becomes common to the nitrogen and hydrogen atoms, which means it is converted into a molecular electron cloud. Consequently, a fourth covalent bond appears. You can imagine the process of ammonium formation with the following diagram:

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

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Data on ionization energy (IE), PEI and the composition of stable molecules - their actual values ​​and comparisons - both of free atoms and of atoms bound into molecules, allow us to understand how atoms form molecules through the mechanism of covalent bonding.

COVALENT BOND- (from the Latin “co” together and “vales” having force) (homeopolar bond), a chemical bond between two atoms that arises when the electrons belonging to these atoms are shared. Atoms in the molecules of simple gases are connected by covalent bonds. A bond in which there is one shared pair of electrons is called a single bond; There are also double and triple bonds.

Let's look at a few examples to see how we can use our rules to determine the number of covalent chemical bonds an atom can form if we know the number of electrons in a given atom's outer shell and the charge on its nucleus. The charge of the nucleus and the number of electrons in the outer shell are determined experimentally and are included in the table of elements.

Calculation of the possible number of covalent bonds

For example, let's count the number of covalent bonds that can form sodium ( Na), aluminum (Al), phosphorus (P), and chlorine ( Cl). Sodium ( Na) and aluminum ( Al) have, respectively, 1 and 3 electrons in the outer shell, and, according to the first rule (for the mechanism of covalent bond formation, one electron in the outer shell is used), they can form: sodium (Na)- 1 and aluminum ( Al)- 3 covalent bonds. After bond formation, the number of electrons in the outer shells of sodium ( Na) and aluminum ( Al) equal to 2 and 6, respectively; i.e., less maximum quantity(8) for these atoms. Phosphorus ( P) and chlorine ( Cl) have, respectively, 5 and 7 electrons on the outer shell and, according to the second of the above-mentioned laws, they could form 5 and 7 covalent bonds. In accordance with the fourth law, the formation of a covalent bond, the number of electrons on the outer shell of these atoms increases by 1. According to the sixth law, when a covalent bond is formed, the number of electrons on the outer shell of the bonded atoms cannot be more than 8. That is, phosphorus ( P) can only form 3 bonds (8-5 = 3), while chlorine ( Cl) can form only one (8-7 = 1).

Example: Based on the analysis, we discovered that a certain substance consists of sodium atoms (Na) and chlorine ( Cl). Knowing the regularities of the mechanism of formation of covalent bonds, we can say that sodium ( Na) can form only 1 covalent bond. Thus, we can assume that each sodium atom ( Na) bonded to the chlorine atom ( Cl) through a covalent bond in this substance, and that this substance is composed of molecules of an atom NaCl. The structural formula for this molecule is: Na-Cl. Here the dash (-) denotes a covalent bond. The electronic formula of this molecule can be shown as follows:
. .
Na:Cl:
. .
In accordance with the electronic formula, on the outer shell of the sodium atom ( Na) V NaCl there are 2 electrons, and on the outer shell of the chlorine atom ( Cl) there are 8 electrons. In this formula, electrons (dots) between sodium atoms ( Na) And chlorine (Cl) are bonding electrons. Since the PEI of chlorine ( Cl) is equal to 13 eV, and for sodium (Na) it is equal to 5.14 eV, the bonding pair of electrons is much closer to the atom Cl than to an atom Na. If the ionization energies of the atoms forming the molecule are very different, then the bond formed will be polar covalent bond.

Let's consider another case. Based on the analysis, we discovered that a certain substance consists of aluminum atoms ( Al) and chlorine atoms ( Cl). In aluminum ( Al) there are 3 electrons in the outer shell; thus, it can form 3 covalent chemical bonds while chlorine (Cl), as in the previous case, can form only 1 bond. This substance is presented as AlCl3, and its electronic formula can be illustrated as follows:

Figure 3.1. Electronic formulaAlCl 3

whose formula of structure is:
Cl - Al - Cl
Cl

This electronic formula shows that AlCl3 on the outer shell of chlorine atoms ( Cl) there are 8 electrons, while the outer shell of the aluminum atom ( Al) there are 6 of them. According to the mechanism of formation of a covalent bond, both bonding electrons (one from each atom) go to the outer shells of the bonded atoms.

Multiple covalent bonds

Atoms that have more than one electron in their outer shell can form not one, but several covalent bonds with each other. Such connections are called multiple (more often multiples) connections. Examples of such bonds are the bonds of nitrogen molecules ( N= N) and oxygen ( O=O).

The bond formed when single atoms join together is called homoatomic covalent bond, e If the atoms are different, then the bond is called heteroatomic covalent bond[Greek prefixes "homo" and "hetero" respectively mean same and different].

Let's imagine what a molecule with paired atoms actually looks like. The simplest molecule with paired atoms is the hydrogen molecule.

Lecture outline:

1. The concept of covalent bond.

2. Electronegativity.

3. Polar and non-polar covalent bonds.

A covalent bond is formed due to shared electron pairs that appear in the shells of the bonded atoms.

It can be formed by atoms of the same element and then it is non-polar; for example, such a covalent bond exists in the molecules of single-element gases H 2, O 2, N 2, Cl 2, etc.

A covalent bond can be formed by atoms different elements, similar in chemical character, and then it is polar; for example, such a covalent bond exists in the molecules H 2 O, NF 3, CO 2.

It is necessary to introduce the concept of electronegativity.

Electronegativity is the ability of atoms chemical element attract common electron pairs involved in the formation of a chemical bond.

electronegativity series

Elements with greater electronegativity will draw shared electrons from elements with less electronegativity.

For a visual representation of the covalent bond in chemical formulas dots are used (each dot corresponds to a valence electron, and a bar also corresponds to a common electron pair).

Example.The bonds in the Cl 2 molecule can be depicted as follows:

Such formulas are equivalent. Covalent bonds have a spatial direction. As a result of the covalent bonding of atoms, either molecules or atomic crystal lattices with a strictly defined geometric arrangement of atoms are formed. Each substance has its own structure.

From the perspective of Bohr's theory, the formation of a covalent bond is explained by the tendency of atoms to convert their outer layer into an octet (full filling of up to 8 electrons). Both atoms present one unpaired electron each to form a covalent bond, and both electrons become shared.
Example. Formation of a chlorine molecule.

The dots represent electrons. When arranging, you should follow the rule: electrons are placed in a certain sequence - left, top, right, bottom, one at a time, then add one at a time, unpaired electrons and take part in the formation of a bond.

A new electron pair, arising from two unpaired electrons, becomes common to two chlorine atoms. There are several ways to form covalent bonds by overlapping electron clouds.

σ bond is much stronger than a π bond, and a π bond can only be with a σ bond. Due to this bond, double and triple multiple bonds are formed.

Polar covalent bonds form between atoms with different electronegativity.

Due to the displacement of electrons from hydrogen to chlorine, the chlorine atom is charged partially negatively, and the hydrogen atom partially positively.

Polar and non-polar covalent bond

If a diatomic molecule consists of atoms of one element, then the electron cloud is distributed in space symmetrically relative to the atomic nuclei. Such a covalent bond is called nonpolar. If a covalent bond is formed between atoms of different elements, then the common electron cloud is shifted towards one of the atoms. In this case, the covalent bond is polar. Electronegativity is used to assess the ability of an atom to attract a shared electron pair.

As a result of the formation of a polar covalent bond, the more electronegative atom acquires a partial negative charge, and the atom with less electronegativity acquires a partial positive charge. These charges are usually called the effective charges of the atoms in the molecule. They may have a fractional value. For example, in an HCl molecule the effective charge is 0.17e (where e is the charge of an electron. The charge of an electron is 1.602.10 -19 C):

A system of two equal in magnitude but opposite in sign charges located at a certain distance from each other is called an electric dipole. Obviously, a polar molecule is a microscopic dipole. Although the total charge of the dipole is zero, there is an electric field in the space surrounding it, the strength of which is proportional to the dipole moment m:

In the SI system, the dipole moment is measured in Cm, but usually for polar molecules the Debye is used as a unit of measurement (the unit is named after P. Debye):

1 D = 3.33×10 –30 C×m

The dipole moment serves as a quantitative measure of the polarity of a molecule. For polyatomic molecules, the dipole moment is the vector sum of the dipole moments of chemical bonds. Therefore, if a molecule is symmetrical, then it can be nonpolar, even if each of its bonds has a significant dipole moment. For example, in a flat BF 3 molecule or in a linear BeCl 2 molecule, the sum of the dipole moments of the bonds is zero:

Similarly, tetrahedral molecules CH 4 and CBr 4 have zero dipole moment. However, violation of symmetry, for example in the BF 2 Cl molecule, causes a dipole moment that is different from zero.

The limiting case of a polar covalent bond is ionic bond. It is formed by atoms whose electronegativity differs significantly. When an ionic bond is formed, almost full transition bonding electron pair to one of the atoms, and positive and negative ions are formed, held close to each other by electrostatic forces. Since the electrostatic attraction to a given ion acts on any ions of the opposite sign, regardless of direction, an ionic bond, unlike a covalent bond, is characterized by lack of direction And unsaturation. Molecules with the most pronounced ionic bonds are formed from atoms of typical metals and typical non-metals (NaCl, CsF, etc.), i.e. when the difference in electronegativity of the atoms is large.