Definition

A covalent bond is a chemical bond formed due to the socialization of atoms of their valence electrons. An obligatory condition for the formation of a covalent bond is the overlap of atomic orbitals (AO), on which valence electrons are located. In the simplest case, the overlap of two AOs leads to the formation of two molecular orbitals(MO): binding MO and anti-bonding (loosening) MO. Shared electrons are located on a lower energy binding MO:

Communication education

Covalent bond (atomic bond, homeopolar bond) - a bond between two atoms due to the socialization (electron sharing) of two electrons - one from each atom:

A. + B. -> A: B

For this reason, the homeopolar relationship has a directional character. A pair of electrons making a bond belongs simultaneously to both bonding atoms, for example:

	..		..						..
:	Cl	:	Cl	:			H	:	O	:	H
	..		..						..

Types of covalent bond

There are three types of covalent chemical bonds that differ in the mechanism of their formation:

1. Simple covalent bond . For its formation, each of the atoms provides one unpaired electron. When a simple covalent bond is formed, the formal charges of the atoms remain unchanged. If the atoms forming a simple covalent bond are the same, then the true charges of the atoms in the molecule are also the same, since the atoms forming the bond equally own a socialized electron pair, such a bond is called a non-polar covalent bond. If the atoms are different, then the degree of ownership of a socialized pair of electrons is determined by the difference in the electronegativity of the atoms, an atom with a greater electronegativity has a pair of bond electrons to a greater extent, and therefore its true charge has a negative sign, an atom with a lower electronegativity acquires, respectively, the same charge, but with a positive sign.

Sigma (σ)-, pi (π)-bonds - an approximate description of the types of covalent bonds in molecules organic compounds, σ-bond is characterized by the fact that the density of the electron cloud is maximum along the axis connecting the nuclei of atoms. When a π-bond is formed, the so-called lateral overlap of electron clouds occurs, and the density of the electron cloud is maximum "above" and "below" the plane of the σ-bond. For example, take ethylene, acetylene and benzene.

In the ethylene molecule C 2 H 4 there is a double bond CH 2 \u003d CH 2, its electronic formula: N:S::S:N. The nuclei of all ethylene atoms are located in the same plane. Three electron clouds of each carbon atom form three covalent bonds with other atoms in the same plane (with angles between them of about 120°). The cloud of the fourth valence electron of the carbon atom is located above and below the plane of the molecule. Such electron clouds of both carbon atoms, partially overlapping above and below the plane of the molecule, form a second bond between carbon atoms. The first, stronger covalent bond between carbon atoms is called a σ-bond; the second, less strong covalent bond is called a π-bond.

In a linear acetylene molecule

H-S≡S-N (N: S::: S: N)

there are σ-bonds between carbon and hydrogen atoms, one σ-bond between two carbon atoms, and two π-bonds between the same carbon atoms. Two π-bonds are located above the sphere of action of the σ-bond in two mutually perpendicular planes.

All six carbon atoms of the C 6 H 6 cyclic benzene molecule lie in the same plane. σ-bonds act between carbon atoms in the plane of the ring; the same bonds exist for each carbon atom with hydrogen atoms. Each carbon atom spends three electrons to make these bonds. Clouds of the fourth valence electrons of carbon atoms, having the shape of eights, are located perpendicular to the plane of the benzene molecule. Each such cloud overlaps equally with the electron clouds of neighboring carbon atoms. In the benzene molecule, not three separate π-bonds are formed, but a single π-electron system of six electrons, common to all carbon atoms. The bonds between the carbon atoms in the benzene molecule are exactly the same.

A covalent bond is formed as a result of the socialization of electrons (with the formation of common electron pairs), which occurs during the overlap of electron clouds. Electron clouds of two atoms participate in the formation of a covalent bond. There are two main types of covalent bonds:

• A covalent non-polar bond is formed between non-metal atoms of the same chemical element. Simple substances have such a bond, for example, O 2; N 2 ; C 12 .
• A covalent polar bond is formed between atoms of different non-metals.

see also

Literature

• "Chemical encyclopedic Dictionary", M., " Soviet Encyclopedia", 1983, p.264.

Organic chemistry
List of organic compounds

Structural chemistry
Chemical bond:	Aromaticity | covalent bond| Ionic bond | Metal connection | Hydrogen bond | Donor-acceptor bond | Tautomerism
Structure display:	Functional group | Structural formula | Chemical formula | ligand
Electronic properties:	Electronegativity | Electron affinity | Ionization energy | Dipole | Octet Rule
Stereochemistry:	Asymmetric atom | Isomerism | Configuration | Chirality | Conformation

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A chemical bond is the interaction of particles (ions or atoms), which is carried out in the process of exchanging electrons located at the last electronic level. There are several types of such a bond: covalent (it is divided into non-polar and polar) and ionic. In this article, we will dwell in more detail on the first type of chemical bonds - covalent. And to be more precise, in its polar form.

A covalent polar bond is a chemical bond between the valence electron clouds of neighboring atoms. The prefix "ko-" - means in this case "together", and the basis of "valence" is translated as strength or ability. Those two electrons that bond with each other are called an electron pair.

Story

For the first time this term was used in a scientific context by the laureate Nobel Prize chemist Irving Lengryum. It happened in 1919. In his work, the scientist explained that the bond in which electrons common to two atoms are observed differs from metallic or ionic. So, it requires a separate name.

Later, already in 1927, F. London and W. Heitler, taking as an example the hydrogen molecule as the chemically and physically simplest model, described a covalent bond. They got down to business from the other end, and substantiated their observations using quantum mechanics.

The essence of the reaction

The process of converting atomic hydrogen into molecular hydrogen is a typical chemical reaction, the qualitative feature of which is a large release of heat when two electrons combine. It looks something like this: two helium atoms are approaching each other, having one electron in their orbit. Then these two clouds approach each other and form a new one, similar to a helium shell, in which two electrons already rotate.

Completed electron shells are more stable than incomplete ones, so their energy is significantly lower than that of two separate atoms. During the formation of a molecule, excess heat is dissipated in the environment.

Classification

In chemistry, there are two types of covalent bonds:

1. A non-polar covalent bond formed between two atoms of the same non-metallic element, such as oxygen, hydrogen, nitrogen, carbon.
2. A covalent polar bond occurs between atoms of different non-metals. A good example is the hydrogen chloride molecule. When atoms of two elements combine with each other, the unpaired electron from hydrogen partially passes to the last electronic level of the chlorine atom. Thus, a positive charge is formed on the hydrogen atom, and a negative charge on the chlorine atom.

Donor-acceptor bond is also a type of covalent bond. It consists in the fact that one atom from a pair provides both electrons, becoming a donor, and the atom accepting them, respectively, is considered an acceptor. When a bond is formed between atoms, the charge of the donor increases by one, and the charge of the acceptor decreases.

Semipolar bond - e It can be considered a subspecies of donor-acceptor. Only in this case, atoms unite, one of which has a complete electron orbital (halogens, phosphorus, nitrogen), and the second has two unpaired electrons (oxygen). Communication is formed in two stages:

• first, one electron is removed from the lone pair and joined to the unpaired ones;
• the union of the remaining unpaired electrodes, that is, a covalent polar bond is formed.

Properties

A polar covalent bond has its own physicochemical characteristics such as directivity, saturation, polarity, polarizability. They determine the characteristics of the resulting molecules.

The direction of the bond depends on the future molecular structure of the resulting substance, namely on geometric shape, which is formed by two atoms upon attachment.

Saturation shows how many covalent bonds one atom of a substance can form. This number is limited by the number of outer atomic orbitals.

The polarity of the molecule arises because the electron cloud, formed from two different electrons, is uneven along its entire circumference. This is due to the difference in negative charge in each of them. It is this property that determines whether a bond is polar or non-polar. When two atoms of the same element combine, the electron cloud is symmetrical, which means that the bond is covalent non-polar. And if atoms of different elements combine, then an asymmetric electron cloud is formed, the so-called dipole moment of the molecule.

Polarizability reflects how actively the electrons in a molecule are displaced under the action of external physical or chemical agents, such as electrical or magnetic field, other particles.

The last two properties of the resulting molecule determine its ability to react with other polar reagents.

Sigma bond and pi bond

The formation of these bonds depends on the distribution density of electrons in the electron cloud during the formation of the molecule.

The sigma bond is characterized by the presence of a dense accumulation of electrons along the axis connecting the nuclei of atoms, that is, in the horizontal plane.

The pi bond is characterized by the compaction of electron clouds at the point of their intersection, that is, above and below the nucleus of an atom.

Visualizing Relationships in a Formula Entry

Let's take the chlorine atom as an example. Its outer electronic level contains seven electrons. In the formula, they are arranged in three pairs and one unpaired electron around the designation of the element in the form of dots.

If the chlorine molecule is written in the same way, it will be seen that two unpaired electrons have formed a pair common to two atoms, it is called shared. In addition, each of them received eight electrons.

Octet-Doublet Rule

The chemist Lewis, who proposed how a polar covalent bond is formed, was the first of his colleagues to formulate a rule explaining the stability of atoms when they are combined into molecules. Its essence lies in the fact that chemical bonds between atoms are formed when a sufficient number of electrons are socialized to obtain an electronic configuration that repeats similar to the atoms of noble elements.

That is, when molecules are formed, for their stabilization it is necessary that all atoms have a complete external electronic level. For example, hydrogen atoms, uniting into a molecule, repeat the electron shell of helium, chlorine atoms, acquire similarity at the electronic level with the argon atom.

Link length

A covalent polar bond, among other things, is characterized by a certain distance between the nuclei of the atoms that form the molecule. They are located at such a distance from each other at which the energy of the molecule is minimal. In order to achieve this, it is necessary that the electron clouds of atoms overlap each other as much as possible. There is a directly proportional pattern between the size of the atoms and the long bond. The larger the atom, the longer the bond between the nuclei.

A variant is possible when an atom forms not one, but several covalent polar bonds. Then the so-called valence angles are formed between the nuclei. They can be from ninety to one hundred and eighty degrees. They define geometric formula molecules.

covalent bond formed by the interaction of non-metals. Atoms of non-metals have a high electronegativity and tend to fill the outer electron layer at the expense of foreign electrons. Two such atoms can go into a stable state if they combine their electrons .

Consider the emergence of a covalent bond in simple substances.

1.The formation of a hydrogen molecule.

Every atom hydrogen has one electron. It needs one more electron to reach the stable state.

When two atoms approach each other, the electron clouds overlap. A shared electron pair is formed, which binds the hydrogen atoms into a molecule.

In the space between two nuclei, common electrons are more common than in other places. There is formed an area with increased electron density and negative charge. Positively charged nuclei are attracted to it, and a molecule is formed.

In this case, each atom receives a completed two-electron external level and passes into a stable state.

A covalent bond due to the formation of one common electron pair is called single.

Shared electron pairs (covalent bonds) are formed due to unpaired electrons , located at the outer energy levels of the interacting atoms.

Hydrogen has one unpaired electron. For other elements, their number is 8 - group number.

non-metals VII And groups (halogens) have one unpaired electron on the outer layer.

Non-metals VI A groups (oxygen, sulfur) there are two such electrons.

Non-metals V And groups (nitrogen, phosphorus) - three unpaired electrons.

2.The formation of a fluorine molecule.

Atom fluorine It has seven electrons in the outer level. Six of them form pairs, and the seventh is unpaired.

When atoms combine, one common electron pair is formed, that is, one covalent bond arises. Each atom receives a completed eight-electron outer layer. The bond in the fluorine molecule is also single. The same single bonds exist in molecules chlorine, bromine and iodine .

If atoms have several unpaired electrons, then two or three common pairs are formed.

3.The formation of an oxygen molecule.

At the atom oxygen the outer level has two unpaired electrons.

When two atoms interact oxygen there are two common electron pairs. Each atom fills its outer level with up to eight electrons. The bond in the oxygen molecule is double.

For the first time about such a concept as covalent bond chemical scientists started talking after the discovery of Gilbert Newton Lewis, who described it as the socialization of two electrons. Later studies made it possible to describe the very principle of covalent bonding. Word covalent can be considered within the framework of chemistry as the ability of an atom to form bonds with other atoms.

Let's explain with an example:

There are two atoms with slight differences in electronegativity (C and CL, C and H). As a rule, these are which are as close as possible to the structure of the electron shell of noble gases.

When these conditions are met, the nuclei of these atoms are attracted to the electron pair common to them. In this case, the electron clouds do not simply overlap each other, as in the case of a covalent bond, which ensures a reliable connection of two atoms due to the fact that the electron density is redistributed and the energy of the system changes, which is caused by the "drawing" of one atom of the electron cloud of another into the internuclear space. The more extensive the mutual overlap of electron clouds, the stronger the connection is considered.

From here, covalent bond- this is a formation that has arisen by the mutual socialization of two electrons belonging to two atoms.

As a rule, substances with a molecular crystal lattice are formed through a covalent bond. Characteristics are melting and boiling at low temperatures, poor solubility in water and low electrical conductivity. From this we can conclude: the basis of the structure of such elements as germanium, silicon, chlorine, hydrogen is a covalent bond.

Properties characteristic of this type of connection:

1. Saturability. This property is usually understood as the maximum number of bonds that they can establish specific atoms. This quantity is determined total number those orbitals in the atom that can participate in the formation of chemical bonds. The valency of an atom, on the other hand, can be determined by the number of orbitals already used for this purpose.
2. Orientation. All atoms tend to form the strongest possible bonds. The greatest strength is achieved in the case of the coincidence of the spatial orientation of the electron clouds of two atoms, since they overlap each other. In addition, it is precisely such a property of a covalent bond as directionality that affects the spatial arrangement of molecules, that is, is responsible for their "geometric shape".
3. Polarizability. This position is based on the idea that there are two types of covalent bonds:

• polar or asymmetrical. A bond of this type can only be formed by atoms of different types, i.e. those whose electronegativity differs significantly, or in cases where the shared electron pair is not symmetrically separated.
• arises between atoms, the electronegativity of which is almost equal, and the distribution of electron density is uniform.

In addition, there are certain quantitative:

• Bond energy. This parameter characterizes the polar bond in terms of its strength. Energy is understood as the amount of heat that was necessary to break the bond of two atoms, as well as the amount of heat that was released when they were combined.
• Under bond length and in molecular chemistry, the length of a straight line between the nuclei of two atoms is understood. This parameter also characterizes the bond strength.
• Dipole moment- a value that characterizes the polarity of the valence bond.

chemical bond - electrostatic interaction between electrons and nuclei, leading to the formation of molecules.

A chemical bond is formed by valence electrons. For s- and p-elements, the electrons of the outer layer are valence, for d-elements, the s-electrons of the outer layer and the d-electrons of the pre-outer layer. When a chemical bond is formed, the atoms complete their outer electron shell to the shell of the corresponding noble gas.

Link length is the average distance between the nuclei of two chemically bonded atoms.

Chemical bond energy- the amount of energy required to break the bond and throw the fragments of the molecule to an infinitely long distance.

Valence angle is the angle between lines connecting chemically bonded atoms.

The following main types of chemical bond are known: covalent (polar and non-polar), ionic, metallic and hydrogen.

covalent called a chemical bond formed by the formation of a common electron pair.

If the bond is formed by a pair of common electrons, equally belonging to both connecting atoms, then it is called covalent non-polar bond. This bond exists, for example, in the molecules H 2 , N 2 , O 2 , F 2 , Cl 2 , Br 2 , I 2 . A covalent non-polar bond occurs between identical atoms, and the electron cloud connecting them is evenly distributed between them.

In molecules between two atoms, a different number of covalent bonds can form (for example, one in the halogen molecules F 2, Cl 2, Br 2, I 2, three in the nitrogen molecule N 2).

covalent polar bond occurs between atoms with different electronegativity. The electron pair that forms it shifts towards the more electronegative atom, but remains bound to both nuclei. Examples of compounds with a covalent polar bond: HBr, HI, H 2 S, N 2 O, etc.

Ionic called the limiting case polar bond, at which the electron pair completely passes from one atom to another and the bound particles turn into ions.

Strictly speaking, only compounds for which the difference in electronegativity is greater than 3 can be classified as ionic compounds, but very few such compounds are known. These include alkali fluorides and alkaline earth metals. It is conventionally believed that an ionic bond occurs between atoms of elements whose electronegativity difference is greater than 1.7 on the Pauling scale. Examples of compounds with an ionic bond: NaCl, KBr, Na 2 O. More details about the Pauling scale will be discussed in the next lesson.

metal called the chemical bond between positive ions in metal crystals, which is carried out as a result of the attraction of electrons freely moving through the metal crystal.

Metal atoms turn into cations, forming a metallic crystal lattice. In this lattice, they are held by electrons common to the entire metal (electron gas).

Training tasks

1. Each of the substances is formed by a covalent non-polar bond, the formulas of which are

1) O 2, H 2, N 2
2) Al, O 3 , H 2 SO 4
3) Na, H 2 , NaBr
4) H 2 O, O 3, Li 2 SO 4

2. Each of the substances is formed by a covalent polar bond, the formulas of which are

1) O 2, H 2 SO 4, N 2
2) H 2 SO 4, H 2 O, HNO 3
3) NaBr, H 3 PO 4, HCl
4) H 2 O, O 3, Li 2 SO 4

3. Each of the substances is formed only by ionic bond, the formulas of which

1) CaO, H 2 SO 4, N 2
2) BaSO 4 , BaCl 2 , BaNO 3
3) NaBr, K 3 PO 4, HCl
4) RbCl, Na 2 S, LiF

4. metal connection specific to list items

1) Ba, Rb, Se
2) Cr, Ba, Si
3) Na, P, Mg
4) Rb, Na, Cs

5. Compounds with only ionic and only covalent polar bonds are, respectively,

1) HCl and Na 2 S
2) Cr and Al (OH) 3
3) NaBr and P 2 O 5
4) P 2 O 5 and CO 2

6. Ionic bond formed between elements

1) chlorine and bromine
2) bromine and sulfur
3) cesium and bromine
4) phosphorus and oxygen

7. A polar covalent bond is formed between elements

1) oxygen and potassium
2) sulfur and fluorine
3) bromine and calcium
4) rubidium and chlorine

8. In volatile hydrogen compounds elements of VA group of the 3rd period chemical bond

1) covalent polar
2) covalent non-polar
3) ionic
4) metal

9. In higher oxides of elements of the 3rd period, the type of chemical bond changes with an increase in the ordinal number of the element

1) from ionic bond to covalent polar bond
2) from metallic to covalent non-polar
3) from covalent polar bond to ionic bond
4) from a covalent polar bond to a metallic bond

10. The length of the chemical bond E–N increases in a number of substances

1) HI - PH 3 - HCl
2) PH 3 - HCl - H 2 S
3) HI - HCl - H 2 S
4) HCl - H 2 S - PH 3

11. The length of the chemical bond E–N decreases in a number of substances

1) NH 3 - H 2 O - HF
2) PH 3 - HCl - H 2 S
3) HF - H 2 O - HCl
4) HCl - H 2 S - HBr

12. The number of electrons that participate in the formation of chemical bonds in the hydrogen chloride molecule is

1) 4
2) 2
3) 6
4) 8

13. The number of electrons that participate in the formation of chemical bonds in the P 2 O 5 molecule, -

1) 4
2) 20
3) 6
4) 12

14. In phosphorus(V) chloride, the chemical bond

1) ionic
2) covalent polar
3) covalent non-polar
4) metal

15. The most polar chemical bond in a molecule

1) hydrogen fluoride
2) hydrogen chloride
3) water
4) hydrogen sulfide

16. Least polar chemical bond in a molecule

1) hydrogen chloride
2) hydrogen bromide
3) water
4) hydrogen sulfide

17. Due to the common electron pair, a bond is formed in a substance

1) Mg
2) H2
3) NaCl
4) CaCl2

18. A covalent bond is formed between elements whose serial numbers

1) 3 and 9
2) 11 and 35
3) 16 and 17
4) 20 and 9

19. An ionic bond is formed between elements whose serial numbers

1) 13 and 9
2) 18 and 8
3) 6 and 8
4) 7 and 17

20. In the list of substances whose formulas are compounds with only ionic bonds, these are

1) NaF, CaF2
2) NaNO 3 , N 2
3) O2, SO3
4) Ca(NO 3) 2, AlCl 3