For the first time about such a concept as covalent bond chemists 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 bonds. Word covalentcan be considered within the framework of chemistry as the ability of an atom to form bonds with other atoms.
Let us explain with an example:
There are two atoms with minor differences in electronegativity (C and CL, C and H). As a rule, these 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 the Covalent bond provides 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 "pulling" 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 bond is considered.
Hence, covalent bond - This is a formation that arose through the mutual socialization of two electrons belonging to two atoms.
As a rule, substances with a molecular crystal lattice are formed precisely through a covalent bond. Melting and boiling at low temperatures, poor water solubility and low electrical conductivity are characteristic. Hence, we can conclude that the structure of elements such as germanium, silicon, chlorine, hydrogen is based on a covalent bond.
Properties typical for this type of connection:
- Saturability.This property is usually understood as the maximum number of bonds that they can establish specific atoms. This number is determined by the total number of those orbitals in the atom that can participate in the formation of chemical bonds. The valence of an atom, on the other hand, can be determined by the number of orbitals already used for this purpose.
- Focus... All atoms strive to form the strongest bonds possible. The greatest strength is achieved when the spatial directionality of the electron clouds of two atoms coincides, 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, it is responsible for their "geometric shape".
- Polarizability.This provision is based on the idea that there are two types of covalent bond:
- polar or unbalanced. A bond of this type can be formed only by atoms of different types, i.e. those whose electronegativity differs significantly, or in cases where the common electron pair is asymmetrically separated.
- arises between atoms, the electronegativity of which is practically equal, and the distribution of the electron density is uniform.
In addition, there are certain quantitative ones:
- Communication 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 between two atoms, as well as the amount of heat that was released when they were combined.
- Under bond lengthand in molecular chemistry the length of the 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.
It is extremely rare that chemicals are composed of separate, unrelated atoms of chemical elements. Only a small number of gases called noble gases have such a structure under normal conditions: helium, neon, argon, krypton, xenon and radon. More often than not, chemical substances do not consist of scattered atoms, but of their combinations into various groups. Such associations of atoms can number several units, hundreds, thousands, or even more atoms. The force that keeps these atoms in the composition of such groups is called chemical bond.
In other words, we can say that a chemical bond is an interaction that provides a bond between individual atoms in more complex structures (molecules, ions, radicals, crystals, etc.).
The reason for the formation of a chemical bond is that the energy of more complex structures is less than the total energy of the individual atoms that form it.
So, in particular, if an XY molecule is formed during the interaction of atoms X and Y, this means that the internal energy of the molecules of this substance is lower than the internal energy of the individual atoms from which it was formed:
E (XY)< E(X) + E(Y)
For this reason, when chemical bonds form between individual atoms, energy is released.
The formation of chemical bonds is attended by the electrons of the outer electron layer with the lowest binding energy with the nucleus, called valence... For example, in boron, these are electrons of 2 energy levels - 2 electrons for 2 s-orbitals and 1 by 2 p-orbitals:
When a chemical bond is formed, each atom seeks to obtain an electronic configuration of atoms of noble gases, i.e. so that there are 8 electrons in its outer electron layer (2 for the elements of the first period). This phenomenon is called the octet rule.
Achievement of the electronic configuration of a noble gas by atoms is possible if initially single atoms make part of their valence electrons common to other atoms. In this case, common electron pairs are formed.
Depending on the degree of electron socialization, covalent, ionic and metallic bonds can be distinguished.
Covalent bond
A covalent bond occurs most often between the atoms of nonmetal elements. If the atoms of non-metals that form a covalent bond belong to different chemical elements, such a bond is called a covalent polar bond. The reason for this name lies in the fact that the atoms of different elements also have a different ability to attract a common electron pair. Obviously, this leads to a displacement of the common electron pair towards one of the atoms, as a result of which a partial negative charge is formed on it. In turn, a partial positive charge is formed on the other atom. For example, in a molecule of hydrogen chloride, an electron pair is shifted from a hydrogen atom to a chlorine atom:
Examples of substances with a covalent polar bond:
СCl 4, H 2 S, CO 2, NH 3, SiO 2, etc.
A covalent non-polar bond is formed between the atoms of non-metals of the same chemical element. Since the atoms are identical, their ability to pull off shared electrons is the same. In this regard, the displacement of the electron pair is not observed:
The above mechanism for the formation of a covalent bond, when both atoms provide electrons for the formation of common electron pairs, is called exchange.
There is also a donor-acceptor mechanism.
When a covalent bond is formed by the donor-acceptor mechanism, a common electron pair is formed due to the filled orbital of one atom (with two electrons) and the empty orbital of another atom. An atom providing a lone electron pair is called a donor, and an atom with a free orbital is called an acceptor. Atoms with paired electrons act as donors of electron pairs, for example, N, O, P, S.
For example, according to the donor-acceptor mechanism, the fourth covalent N-H bond is formed in the ammonium cation NH 4 +:
In addition to polarity, covalent bonds are also characterized by energy. Bond energy is the minimum energy required to break a bond between atoms.
The binding energy decreases with an increase in the radii of the bonded atoms. Since, as we know, atomic radii increase downward along subgroups, one can, for example, conclude that the strength of the halogen-hydrogen bond increases in the series:
HI< HBr < HCl < HF
Also, the bond energy depends on its multiplicity - the greater the bond multiplicity, the more its energy. The bond multiplicity refers to the number of common electron pairs between two atoms.
Ionic bond
The ionic bond can be considered as the limiting case of the covalent polar bond. If in a covalent-polar bond the total electron pair is partially displaced to one of the pair of atoms, then in the ionic it is almost completely "given" to one of the atoms. The atom that donated the electron (s) acquires a positive charge and becomes cation, and the atom, which took the electrons from it, acquires a negative charge and becomes anion.
Thus, an ionic bond is a bond formed by the electrostatic attraction of cations to anions.
The formation of this type of bond is characteristic of the interaction of atoms of typical metals and typical non-metals.
For example, potassium fluoride. The potassium cation is obtained as a result of the abstraction of one electron from the neutral atom, and the fluorine ion is formed when one electron is attached to the fluorine atom:
A force of electrostatic attraction arises between the resulting ions, as a result of which an ionic compound is formed.
During the formation of a chemical bond, the electrons from the sodium atom passed to the chlorine atom and oppositely charged ions were formed, which have a complete external energy level.
It was found that the electrons from the metal atom are not completely detached, but only shifted towards the chlorine atom, as in a covalent bond.
Most binary compounds that contain metal atoms are ionic. For example, oxides, halides, sulfides, nitrides.
An ionic bond also occurs between simple cations and simple anions (F -, Cl -, S 2-), as well as between simple cations and complex anions (NO 3 -, SO 4 2-, PO 4 3-, OH -). Therefore, the ionic compounds include salts and bases (Na 2 SO 4, Cu (NO 3) 2, (NH 4) 2 SO 4), Ca (OH) 2, NaOH)
Metal bond
This type of bond is formed in metals.
The atoms of all metals on the outer electron layer have electrons that have a low binding energy with the atomic nucleus. For most metals, the process of loss of external electrons is energetically favorable.
In view of such a weak interaction with the nucleus, these electrons in metals are very mobile and the following process continuously occurs in each metal crystal:
М 0 - ne - \u003d M n +,
where M 0 is a neutral metal atom, and M n + a cation of the same metal. The figure below shows an illustration of the ongoing processes.
That is, electrons "carry" along the metal crystal, detaching from one metal atom, forming a cation from it, joining another cation, forming a neutral atom. This phenomenon was called "electronic wind", and the set of free electrons in a crystal of a non-metal atom was called "electron gas". This type of interaction between metal atoms was called a metal bond.
Hydrogen bond
If a hydrogen atom in any substance is associated with an element with high electronegativity (nitrogen, oxygen or fluorine), such a substance is characterized by such a phenomenon as a hydrogen bond.
Since a hydrogen atom is bonded to an electronegative atom, a partial positive charge is formed on the hydrogen atom, and a partial negative charge is formed on the electronegative element. In this regard, it becomes possible electrostatic attraction between the partially positively charged hydrogen atom of one molecule and the electronegative atom of another. For example, a hydrogen bond is observed for water molecules:
It is the hydrogen bond that explains the abnormally high melting point of water. In addition to water, strong hydrogen bonds are also formed in substances such as hydrogen fluoride, ammonia, oxygen-containing acids, phenols, alcohols, and amines.
Chemical bond - electrostatic interaction between electrons and nuclei, leading to the formation of molecules.
The chemical bond is formed by valence electrons. For s- and p-elements, valence are electrons of the outer layer, for d-elements - s-electrons of the outer layer and d-electrons of the pre-outer layer. When a chemical bond is formed, 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 fragments of the molecule at an infinitely large distance.
Valence angle - the angle between the lines connecting chemically bonded atoms.
The following main types of chemical bonds are known: covalent (polar and non-polar), ionic, metallic and hydrogen.
Covalent called a chemical bond formed due to the formation of a common electron pair.
If a 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 arises between identical atoms, and the electron cloud connecting them is evenly distributed between them.
In molecules, a different number of covalent bonds can form between two atoms (for example, one in the molecules of halogens F 2, Cl 2, Br 2, I 2, three in the nitrogen molecule N 2).
Covalent polar bond arises between atoms with different electronegativity. The electron pair forming it is displaced towards the more electronegative atom, but remains associated with both nuclei. Examples of compounds with a covalent polar bond: HBr, HI, H 2 S, N 2 O, etc.
Ionic is called the limiting case of polar bond, in which an 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 compounds with an ionic bond, but very few such compounds are known. These include fluorides of alkali and alkaline earth metals. Conventionally, it is believed that the ionic bond occurs between the atoms of elements, the difference in electronegativity of which is greater than 1.7 on the Pauling scale... Examples of compounds with an ionic bond: NaCl, KBr, Na 2 O. More about the Pauling scale will be discussed in the next lesson.
Metal is 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
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
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. The metallic link is typical for 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 is formed between elements
1) chlorine and bromine
2) bromine and sulfur
3) cesium and bromine
4) phosphorus and oxygen
7. A covalent polar 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 of group VA elements of the 3rd period, the 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 covalent polar bond to metallic bond
10. The length of the E – N chemical bond 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 E – N chemical bond 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 involved in the formation of chemical bonds in the P 2 O 5 molecule is
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 the substance
1) Mg
2) H 2
3) NaCl
4) CaCl 2
18. A covalent bond is formed between elements whose ordinal numbers are
1) 3 and 9
2) 11 and 35
3) 16 and 17
4) 20 and 9
19. The ionic bond is formed between the elements, the serial numbers of which
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 only with an ionic bond, these are
1) NaF, CaF 2
2) NaNO 3, N 2
3) O 2, SO 3
4) Ca (NO 3) 2, AlCl 3
Definition
A covalent bond is a chemical bond formed due to the sharing of their valence electrons by atoms. A prerequisite for the formation of a covalent bond is the overlap of atomic orbitals (AO), on which the valence electrons are located. In the simplest case, the overlap of two AO leads to the formation of two molecular orbitals (MO): a bonding MO and an antibonding (antibonding) MO. The shared electrons are located at the bonding MO, which is lower in energy:
Communication formation
A covalent bond (atomic bond, homeopolar bond) is a bond between two atoms due to the electron sharing of two electrons - one from each atom:
A. + B. -\u003e A: B
For this reason, the homeopolar relationship is directional. The pair of electrons that make a bond belongs to both atoms being bonded simultaneously, for example:
| .. | .. | .. | |||||||||
| : | Cl | : | Cl | : | H | : | O | : | H | ||
| .. | .. | .. |
Types of covalent bonds
There are three types of covalent chemical bonds, which differ in the mechanism of its 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 the shared electron pair, such a bond is called a non-polar covalent bond. If the atoms are different, then the degree of ownership of the socialized pair of electrons is determined by the difference in the electronegativities 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 lesser electronegativity acquires the same charge, respectively, but with a positive sign.
Sigma (σ) -, pi (π) -bonds - an approximate description of the types of covalent bonds in molecules of 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. Let's take ethylene, acetylene and benzene as examples.
In the ethylene molecule C 2 H 4 there is a double bond CH 2 \u003d CH 2, its electronic formula: H: C :: C: H. 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 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 the carbon atoms. The first, stronger covalent bond between carbon atoms is called the σ-bond; the second, less strong covalent bond is called π -bond.
In a linear acetylene molecule
N-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 cyclic C 6 H 6 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. The carbon atoms spend three electrons to make these bonds. The eights-shaped clouds of the fourth valence electrons of carbon atoms 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 -electronic system of six electrons, common to all carbon atoms. The bonds between carbon atoms in a 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. The formation of a covalent bond involves electron clouds of two atoms. 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 the atoms of various 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 Bonding | Metal bond | 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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Covalent bond (from the Latin "co" together and "vales" having force) is carried out at the expense of an electron pair belonging to both atoms. Formed between atoms of non-metals.
The electronegativity of non-metals is quite large, so that during the chemical interaction of two atoms of non-metals, complete transfer of electrons from one to the other (as in the case) is impossible. In this case, it is necessary to combine the electrons to perform.
As an example, let us discuss the interaction of hydrogen and chlorine atoms:
H 1s 1 - one electron
Cl 1s 2 2s 2 2 p 6 3 s 2 3 p 5 - seven electrons at the outer level
Each of the two atoms lacks one electron in order to have a complete outer electron shell. And each of the atoms allocates "for general use" one electron. This enforces the octet rule. This is best portrayed using Lewis formulas:
Formation of a covalent bond
The shared electrons now belong to both atoms. The hydrogen atom has two electrons (its own and shared electron of the chlorine atom), and the chlorine atom has eight electrons (its own plus the shared electron of the hydrogen atom). These two shared electrons form a covalent bond between hydrogen and chlorine atoms. The particle formed by the bonding of two atoms is called molecule.
Non-polar covalent bond
A covalent bond can also form between two the same atoms. For example:
This diagram explains why hydrogen and chlorine exist as diatomic molecules. By pairing and sharing two electrons, the octet rule is fulfilled for both atoms.
In addition to single bonds, a double or triple covalent bond can be formed, as, for example, in the molecules of oxygen O 2 or nitrogen N 2. Nitrogen atoms have five valence electrons, therefore, three more electrons are required to complete the shell. This is accomplished by sharing three pairs of electrons as shown below:
Covalent compounds are usually gases, liquids, or relatively low melting solids. One of the rare exceptions is diamond, which melts above 3,500 ° C. This is due to the structure of diamond, which is a continuous lattice of covalently bonded carbon atoms, rather than a collection of individual molecules. In fact, any diamond crystal, regardless of its size, is one huge molecule.
A covalent bond occurs when the electrons of two nonmetal atoms combine. The resulting structure is called a molecule.
Polar covalent bond
In most cases, two covalently bonded atoms have different electronegativity and shared electrons do not belong to two atoms equally. Most of the time they are closer to one atom than to another. In a molecule of hydrogen chloride, for example, the electrons forming a covalent bond are located closer to the chlorine atom, since its electronegativity is higher than that of hydrogen. However, the difference in the ability to attract electrons is not so great that a complete transfer of an electron from a hydrogen atom to a chlorine atom occurs. Therefore, the bond between hydrogen and chlorine atoms can be regarded as a cross between an ionic bond (complete electron transfer) and a non-polar covalent bond (symmetric arrangement of a pair of electrons between two atoms). The partial charge on atoms is denoted by the Greek letter δ. This connection is called polar covalent bond, and the hydrogen chloride molecule is said to be polar, that is, it has a positively charged end (hydrogen atom) and a negatively charged end (chlorine atom).
The table below lists the main types of bonds and examples of substances:
Exchange and donor-acceptor mechanism of covalent bond formation
1) Exchange mechanism. Each atom gives one unpaired electron to a common electron pair.
2) Donor-acceptor mechanism. One atom (donor) provides an electron pair, and the other atom (acceptor) provides a free orbital for this pair.