Definition
It is called a covalent bond 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 (MOs): a bonding MO and an antibonding (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:
- covalent non-polar bond 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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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 view 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. atomic orbital 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.
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:
- 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.
- 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".
- 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.
It's no secret that chemistry is a rather complex and diverse science. Many different reactions, reagents, chemicals and other complex and incomprehensible terms - they all interact with each other. But the main thing is that we deal with chemistry every day, no matter if we listen to the teacher in the lesson and learn new material or we brew tea, which in general is also chemical process.
It can be concluded that chemistry is a must, to understand it and to know how our world or some of its separate parts works is interesting, and, moreover, useful.
Now we have to deal with such a term as a covalent bond, which, by the way, can be both polar and non-polar. By the way, the very word "covalent" is formed from the Latin "co" - together and "vales" - having power.
Term occurrences
Let's start with the fact that The term "covalent" was first introduced in 1919 by Irving Langmuir - laureate Nobel Prize. The concept of "covalent" implies a chemical bond in which both atoms share electrons, which is called co-ownership. Thus, it differs, for example, from a metallic one, in which electrons are free, or from an ionic one, where one gives electrons to another. It should be noted that it is formed between non-metals.
Based on the foregoing, we can draw a small conclusion about what this process is. It arises between atoms due to the formation of common electron pairs, and these pairs arise on the outer and pre-outer sublevels of electrons.
Examples, substances with a polar:
Types of covalent bond
Two types are also distinguished - these are polar, and, accordingly, non-polar bonds. We will analyze the features of each of them separately.
Covalent polar - education
What is the term "polar"?
It usually happens that two atoms have different electronegativity, therefore, common electrons do not belong to them equally, but they are always closer to one than to the other. For example, a molecule of hydrogen chloride, in which the electrons of the covalent bond are located closer to the chlorine atom, since its electronegativity is higher than that of hydrogen. However, in reality, the difference in electron attraction is small enough for complete transfer of an electron from hydrogen to chlorine.
As a result, at polarity, the electron density shifts to a more electronegative one, and a partial negative charge arises on it. In turn, the nucleus, whose electronegativity is lower, has, accordingly, a partial positive charge.
We conclude: polar arises between various non-metals, which differ in the value of electronegativity, and electrons are located closer to the nucleus with greater electronegativity.
Electronegativity - the ability of some atoms to attract the electrons of others, thereby forming chemical reaction.
Examples of covalent polar, substances with a covalent polar bond:
The formula of a substance with a covalent polar bond
Covalent non-polar, difference between polar and non-polar
And finally, non-polar, we will soon find out what it is.
The main difference between non-polar and polar is symmetry. If, in the case of a polar bond, the electrons were located closer to one atom, then with a non-polar bond, the electrons are arranged symmetrically, that is, equally with respect to both.
It is noteworthy that non-polar arises between non-metal atoms of one chemical element.
Eg, substances with a non-polar covalent bond:
Also, a set of electrons is often called simply an electron cloud, based on this we conclude that the electron cloud of communication, which forms a common pair of electrons, is distributed in space symmetrically, or evenly with respect to the nuclei of both.
Examples of a covalent non-polar bond and a scheme for the formation of a covalent non-polar bond
But it is also useful to know how to distinguish between covalent polar and non-polar.
covalent non-polar are always atoms of the same substance. H2. CL2.
This article has come to an end, now we know what this chemical process is, we know how to determine it and its varieties, we know the formulas for the formation of substances, and in general a little more about our complex world, success in chemistry and the formation of new formulas.
A covalent bond is the binding of atoms with the help of common (shared between them) electron pairs. In the word "covalent" the prefix "co-" means "joint participation." And "valenta" in translation into Russian - strength, ability. In this case, we mean the ability of atoms to bond with other atoms.
When a covalent bond is formed, atoms unite their electrons, as it were, into a common "piggy bank" - a molecular orbital, which is formed from the atomic shells of individual atoms. This new shell contains as many complete electrons as possible and replaces the atoms with their own incomplete atomic shells.
Ideas about the mechanism of formation of the hydrogen molecule were extended to more complex molecules. The theory of chemical bond developed on this basis was called valence bond method (VS method). The VS method is based on the following provisions:
1) A covalent bond is formed by two electrons with oppositely directed spins, and this electron pair belongs to two atoms.
2) The stronger the covalent bond, the more the electron clouds overlap.
Combinations of two-electron two-center bonds, reflecting the electronic structure of the molecule, are called valence schemes. Examples of building valence schemes:
In valence schemes, representations are most clearly embodied Lewis on the formation of a chemical bond through the socialization of electrons with the formation of an electron shell of a noble gas: for hydrogen- from two electrons (shell He), For nitrogen- of eight electrons (shell Ne).
29. Non-polar and polar covalent bond.
If a diatomic molecule consists of atoms of one element, then the electron cloud is distributed in space symmetrically with respect to the nuclei of atoms. Such a covalent bond is called non-polar. 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.
As a result of the formation of a polar covalent bond, a more electronegative atom acquires a partial negative charge, and an atom with a lower electronegativity acquires a partial positive charge. These charges are commonly referred to as the effective charges of the atoms in the molecule. They may be fractional.
30. Methods for expressing a covalent bond.
There are two main ways to create covalent bond * .
1) An electron pair forming a bond can be formed due to unpaired electrons, available in unexcited atoms. An increase in the number of created covalent bonds is accompanied by the release of more energy than is spent on excitation of the atom. Since the valence of an atom depends on the number unpaired electrons, excitation leads to an increase in valency. At atoms of nitrogen, oxygen, fluorine, the number of unpaired electrons does not increase, because within the second level there are no free orbitals*, and the movement of electrons to the third quantum level requires much more energy than that which would be released during the formation of additional bonds. Thus, when an atom is excited, the transitions of electrons to freeorbitals possible only within the same energy level.
2) Covalent bonds can be formed due to the paired electrons present on the outer electron layer of the atom. In this case, the second atom must have a free orbital on the outer layer. An atom that provides its electron pair to form a covalent bond * is called a donor, and an atom that provides an empty orbital is called an acceptor. A covalent bond formed in this way is called a donor-acceptor bond. In the ammonium cation, this bond is absolutely identical in its properties to the other three covalent bonds, formed in the first way, so the term “donor-acceptor” does not mean some special type of connection, but only a way of its formation.
