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: 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.
Covalent bond (atomic bond, homeopolar bond) - a chemical bond formed by the overlap (socialization) of paravalent electron clouds. The electronic clouds (electrons) that provide communication are called shared electron pair.
The characteristic properties of a covalent bond - directionality, saturation, polarity, polarizability - determine the chemical and physical properties connections.
The direction of the connection is determined by the molecular structure of the substance and the geometric shape of its molecule. The angles between two bonds are called bond angles.
Saturability is the ability of atoms to form a limited number of covalent bonds. The number of bonds formed by an atom is limited by the number of its outer atomic orbitals.
The polarity of the bond is due to the uneven distribution of electron density due to differences in the electronegativity of the atoms. On this basis, covalent bonds are divided into non-polar and polar (non-polar - a diatomic molecule consists of identical atoms (H 2, Cl 2, N 2) and the electron clouds of each atom are distributed symmetrically relative to these atoms; polar - a diatomic molecule consists of atoms of different chemical elements , and the general electron cloud shifts towards one of the atoms, thereby forming an asymmetry in the distribution of electric charge in the molecule, generating a dipole moment of the molecule).
The polarizability of a bond is expressed in the displacement of the bond electrons under the influence of an external electric field, including that of another reacting particle. Polarizability is determined by electron mobility. The polarity and polarizability of covalent bonds determines the reactivity of molecules towards polar reagents.
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A covalent bond is formed by a pair of electrons shared between two atoms, and these electrons must occupy two stable orbitals, one from each atom.
A + + B → A: B
As a result of socialization, electrons form a filled energy level. A bond is formed if their total energy at this level is less than in the initial state (and the difference in energy will be nothing more than the bond energy).
Filling of atomic (along the edges) and molecular (in the center) orbitals in the H 2 molecule with electrons. The vertical axis corresponds to the energy level, electrons are indicated by arrows reflecting their spins.
According to the theory of molecular orbitals, the overlap of two atomic orbitals leads, in the simplest case, to the formation of two molecular orbitals (MO): linking MO And anti-binding (loosening) MO. The shared electrons are located on the lower energy bonding MO.
Types of covalent bond
There are three types of covalent chemical bonds, differing in the mechanism of formation:
1. Simple covalent bond. For its formation, each atom 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 shared electron pair. This connection is called non-polar covalent bond. Simple substances have such a connection, for example: O 2, N 2, Cl 2. But not only nonmetals of the same type can form a covalent nonpolar bond. Non-metal elements whose electronegativity is of equal importance can also form a covalent nonpolar bond, for example, in the PH 3 molecule the bond is covalent nonpolar, since the EO of hydrogen is equal to the EO of phosphorus.
· If the atoms are different, then the degree of possession of a shared pair of electrons is determined by the difference in the electronegativity of the atoms. An atom with greater electronegativity attracts a pair of bonding electrons more strongly toward itself, and its true charge becomes negative. An atom with lower electronegativity acquires, accordingly, a positive charge of the same magnitude. If a compound is formed between two different non-metals, then such a compound is called covalent polar bond.
2. Donor-acceptor bond. To form this type of covalent bond, both electrons are provided by one of the atoms - donor. The second of the atoms involved in the formation of a bond is called acceptor. In the resulting molecule, the formal charge of the donor increases by one, and the formal charge of the acceptor decreases by one.
3. Semipolar connection. It can be considered as a polar donor-acceptor bond. This type of covalent bond is formed between an atom with a lone pair of electrons (nitrogen, phosphorus, sulfur, halogens, etc.) and an atom with two unpaired electrons (oxygen, sulfur). The formation of a semipolar bond occurs in two stages:
1. Transfer of one electron from an atom with a lone pair of electrons to an atom with two unpaired electrons. As a result, an atom with a lone pair of electrons turns into a radical cation (a positively charged particle with an unpaired electron), and an atom with two unpaired electrons turns into a radical anion (a negatively charged particle with an unpaired electron).
2. Sharing of unpaired electrons (as in the case of a simple covalent bond).
When a semipolar bond is formed, an atom with a lone pair of electrons increases its formal charge by one, and an atom with two unpaired electrons decreases its formal charge by one.
σ bond and π bond
Sigma (σ)-, pi (π)-bonds are an approximate description of the types of covalent bonds in molecules of various compounds; the 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 -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 σ-bond plane. For example, let's take ethylene, acetylene and benzene.
In the ethylene molecule C 2 H 4 there is a double bond CH 2 = CH 2, its electronic formula: H:C::C:H. The nuclei of all ethylene atoms are located in the same plane. The three electron clouds of each carbon atom form three covalent bonds with other atoms in the same plane (with angles between them of approximately 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 a σ bond; the second, less strong covalent bond is called an -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 σ bonds between the same carbon atoms. Two -bonds are located above the sphere of action of the σ-bond in two mutually perpendicular planes.
All six carbon atoms of the cyclic benzene molecule C 6 H 6 lie in the same plane. There are σ bonds between carbon atoms in the plane of the ring; Each carbon atom has the same bonds with hydrogen atoms. Carbon atoms spend three electrons to make these bonds. Clouds of fourth valence electrons of carbon atoms, shaped like figures of eight, are located perpendicular to the plane of the benzene molecule. Each such cloud overlaps equally with the electron clouds of neighboring carbon atoms. In a benzene molecule, not three separate -bonds are formed, but a single -electronic system of six electrons, common to all carbon atoms. The bonds between the carbon atoms in the benzene molecule are exactly the same.
Examples of substances with covalent bonds
A simple covalent bond connects atoms in the molecules of simple gases (H 2, Cl 2, etc.) and compounds (H 2 O, NH 3, CH 4, CO 2, HCl, etc.). Compounds with a donor-acceptor bond - ammonium NH 4 +, tetrafluoroborate anion BF 4 - etc. Compounds with a semipolar bond - nitrous oxide N 2 O, O - -PCl 3 +.
Crystals with covalent bonds are dielectrics or semiconductors. Typical examples of atomic crystals (atoms in which are interconnected by covalent (atomic) bonds are diamond, germanium and silicon.
The only one known person a substance with an example of a covalent bond between a metal and a carbon is cyanocobalamin, known as vitamin B12.
Ionic bond- a very strong chemical bond formed between atoms with a large difference (> 1.5 on the Pauling scale) of electronegativity, in which the common electron pair is completely transferred to an atom with greater electronegativity. This is the attraction of ions as oppositely charged bodies. An example is the compound CsF, in which the “degree of ionicity” is 97%. Let's consider the method of formation using sodium chloride NaCl as an example. The electronic configuration of sodium and chlorine atoms can be represented as: 11 Na 1s2 2s2 2p 6 3s1; 17 Cl 1s2 2s2 2p6 3s2 3р5. These are atoms with incomplete energy levels. Obviously, to complete them, it is easier for a sodium atom to give up one electron than to gain seven, and for a chlorine atom it is easier to gain one electron than to give up seven. During a chemical interaction, the sodium atom completely gives up one electron, and the chlorine atom accepts it. Schematically, this can be written as follows: Na. - l e -> Na+ sodium ion, stable eight-electron 1s2 2s2 2p6 shell due to the second energy level. :Cl + 1е --> .Cl - chlorine ion, stable eight electron shell. Electrostatic attraction forces arise between the Na+ and Cl- ions, resulting in the formation of a compound. Ionic bonding is an extreme case of polarization of a polar covalent bond. Formed between a typical metal and non-metal. In this case, the electrons from the metal are completely transferred to the non-metal. Ions are formed.
If a chemical bond is formed between atoms that have a very large difference in electronegativity (EO > 1.7 according to Pauling), then the common electron pair is completely transferred to the atom with a higher EO. The result of this is the formation of a compound of oppositely charged ions:
An electrostatic attraction occurs between the resulting ions, which is called ionic bonding. Or rather, this look is convenient. In fact, the ionic bond between atoms in its pure form is not realized anywhere or almost nowhere; usually, in fact, the bond is partly ionic and partly covalent in nature. At the same time, the bond of complex molecular ions can often be considered purely ionic. The most important differences between ionic bonds and other types of chemical bonds are non-directionality and non-saturation. That is why crystals formed due to ionic bonds gravitate towards various dense packings of the corresponding ions.
Characteristics Such compounds have good solubility in polar solvents (water, acids, etc.). This occurs due to the charged parts of the molecule. In this case, the dipoles of the solvent are attracted to the charged ends of the molecule, and, as a result of Brownian motion, they “tear” the molecule of the substance into pieces and surround them, preventing them from connecting again. The result is ions surrounded by solvent dipoles.
When such compounds are dissolved, energy is usually released, since the total energy of the formed solvent-ion bonds is greater than the energy of the anion-cation bond. Exceptions are many salts of nitric acid (nitrates), which absorb heat when dissolved (solutions cool). Last fact explained on the basis of laws that are considered in physical chemistry.
Definition
A covalent bond is a chemical bond formed by atoms sharing their valence electrons. Required condition 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 AOs leads to the formation of two molecular orbitals (MO): a bonding MO and an antibonding (antibonding) MO. The shared electrons are located on the lower energy bonding MO:
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Covalent bond (atomic bond, homeopolar bond) - a bond between two atoms due to electron sharing of two electrons - one from each atom:
A. + B. -> A: B
For this reason, the homeopolar relationship is directional. The pair of electrons that perform the bond belongs simultaneously to both bonded atoms, for example:
| .. | .. | .. | |||||||||
| : | Cl | : | Cl | : | H | : | O | : | H | ||
| .. | .. | .. |
Types of covalent bond
There are three types of covalent chemical bonds, differing in the mechanism of their formation:
1. Simple covalent bond. For its formation, each atom 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 shared electron pair, such a bond is called a non-polar covalent bond. If the atoms are different, then the degree of possession of a shared pair of electrons is determined by the difference in the electronegativity of the atoms, an atom with a higher electronegativity has a pair of bonding electrons to a greater extent, and therefore its true charge has a negative sign, an atom with a lower electronegativity acquires the same charge, but with a positive sign.
Sigma (σ)-, pi (π)-bonds are an approximate description of the types of covalent bonds in molecules of organic compounds; the 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 σ bond plane. For example, take ethylene, acetylene and benzene.
In the ethylene molecule C 2 H 4 there is a double bond CH 2 = CH 2, its electronic formula: H:C::C:H. The nuclei of all ethylene atoms are located in the same plane. The three electron clouds of each carbon atom form three covalent bonds with other atoms in the same plane (with angles between them of approximately 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 a σ bond; the second, weaker covalent bond is called a 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 benzene molecule C 6 H 6 lie in the same plane. There are σ bonds between carbon atoms in the plane of the ring; Each carbon atom has the same bonds with hydrogen atoms. Carbon atoms spend three electrons to make these bonds. Clouds of fourth valence electrons of carbon atoms, shaped like figures of eight, are located perpendicular to the plane of the benzene molecule. Each such cloud overlaps equally with the electron clouds of neighboring carbon atoms. In a 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 sharing of electrons (to form common electron pairs), which occurs during the overlap of electron clouds. The formation of a covalent bond involves the electron clouds of two atoms. There are two main types of covalent bonds:
- A covalent nonpolar bond is formed between atoms of a nonmetal of the same chemical element. Simple substances, for example O 2, have such a connection; N 2; C 12.
- A polar covalent bond is formed between atoms of different nonmetals.
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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CHEMICAL BONDING, the mechanism by which atoms join together to form molecules. There are several types of such bonds, based either on the attraction of opposite charges, or on the formation of stable configurations through the exchange of electrons.... ... Scientific and technical encyclopedic dictionary
Chemical bond- CHEMICAL BOND, the interaction of atoms, causing their combination into molecules and crystals. The forces acting during the formation of a chemical bond are mainly electrical in nature. The formation of a chemical bond is accompanied by a restructuring... ... Illustrated Encyclopedic Dictionary
The mutual attraction of atoms, leading to the formation of molecules and crystals. It is customary to say that in a molecule or in a crystal there are chemical structures between neighboring atoms. The valence of an atom (which is discussed in more detail below) shows the number of bonds... Great Soviet Encyclopedia
chemical bond- mutual attraction of atoms, leading to the formation of molecules and crystals. The valence of an atom shows the number of bonds formed by a given atom with neighboring ones. The term “chemical structure” was introduced by Academician A. M. Butlerov in... ... encyclopedic Dictionary in metallurgy
An ionic bond is a strong chemical bond formed between atoms with a large difference in electronegativity, in which the shared electron pair is completely transferred to the atom with a higher electronegativity. An example is the compound CsF... Wikipedia
Chemical bonding is a phenomenon of interaction of atoms caused by the overlap of electron clouds of bonding particles, which is accompanied by a decrease in the total energy of the system. The term “chemical structure” was first introduced by A. M. Butlerov in 1861... ... Wikipedia
A covalent bond is the bonding of atoms using common (shared between them) electron pairs. In the word “covalent,” the prefix “co-” means “joint participation.” And “valens” translated into Russian means strength, ability. In this case, we mean the ability of atoms to bond with other atoms.
When a covalent bond is formed, atoms combine their electrons as if into a common “piggy bank” - a molecular orbital, which is formed from the atomic shells of individual atoms. This new shell contains as complete a number of 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 bonding developed on this basis was called valence bond method (VS method). The BC method is based on the following provisions:
1) A covalent bond is formed by two electrons with opposite spins, and this electron pair belongs to two atoms.
2) The more the electron clouds overlap, the stronger the covalent bond.
Combinations of two-electron two-center bonds, reflecting the electronic structure of the molecule, are called valence schemes. Examples of constructing valence circuits:
Valence schemes most clearly embody the representations Lewis on the formation of a chemical bond by sharing electrons with the formation of an electron shell of a noble gas: for hydrogen– of two electrons (shell He), For nitrogen– of eight electrons (shell Ne).
29. Non-polar and polar covalent bonds.
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 various elements, then the total 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, 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.
30. Methods of expressing covalent bonds.
There are two main ways of education 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 covalent bonds created is accompanied by the release of more energy than is expended on excitation of the atom. Since the valence of an atom depends on the number of unpaired electrons, excitation leads to an increase in valence. For nitrogen, oxygen, and fluorine atoms, the number of unpaired electrons does not increase, because there are no vacancies within the second level orbitals*, and the movement of electrons to the third quantum level requires significantly more energy than that which would be released during the formation of additional bonds. Thus, when an atom is excited, transitions of electrons to freeorbitals possible only within one energy level.
2) Covalent bonds can be formed due to paired electrons present in 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 by the first method, therefore the term “donor-acceptor” does not mean any special type of bond, but only the method of its formation.
A chemical bond is the interaction of particles (ions or atoms), which occurs in the process of exchanging electrons located at the last electronic level. There are several types of such bonds: 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 ones. And to be more precise, in its polar form.
A polar covalent bond is a chemical bond between the valence electron clouds of neighboring atoms. The prefix “co-” means “together” in this case, and the stem “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 Lenngrum. This happened in 1919. In his work, the scientist explained that a bond in which electrons common to two atoms are observed is different from a metallic or ionic one. This means 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 took on the matter 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 sign of which is the large release of heat when two electrons combine. It looks something like this: two helium atoms approach each other, each having one electron in their orbit. Then these two clouds come closer and form a new one, similar to a shell of helium, 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. When a molecule is formed, excess heat is dissipated into the environment.
Classification
In chemistry, there are two types of covalent bonds:
- A covalent nonpolar bond formed between two atoms of the same nonmetallic element, such as oxygen, hydrogen, nitrogen, carbon.
- A polar covalent bond occurs between atoms of different nonmetals. A good example could be a hydrogen chloride molecule. When atoms of two elements combine with each other, the unpaired electron from hydrogen partially transfers to the last electron 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 lies in the fact that one atom of the pair provides both electrons, becoming a donor, and the atom receiving them, accordingly, 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 connection - e e can be considered a subtype of donor-acceptor. Only in this case do atoms unite, one of which has a complete electron orbital (halogens, phosphorus, nitrogen), and the second - two unpaired electrons (oxygen). The formation of a connection takes place in two stages:
- first, one electron is removed from the lone pair and added 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 physical and chemical properties, such as directionality, 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 the geometric shape that the two atoms form upon joining.
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 a molecule occurs because the electron cloud formed from two different electrons is uneven around its entire circumference. This occurs due to the difference in negative charge in each of them. It is this property that determines whether a bond is polar or nonpolar. When two atoms of the same element combine, the electron cloud is symmetrical, which means the covalent bond is nonpolar. And if atoms unite different elements, 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 influence of external physical or chemical agents, for example, an electric or magnetic field, or 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 electron density distribution in the electron cloud during the formation of the molecule.
A 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 atomic nucleus.
Visualization of the relationship in the formula record
For example, we can take the chlorine atom. Its outermost electronic level contains seven electrons. In the formula, they are arranged in three pairs and one unpaired electron around the symbol of the element in the form of dots.
If you write a chlorine molecule in the same way, you will see that two unpaired electrons have formed a pair common to two atoms; it is called shared. In this case, 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 shared to form an electronic configuration that is similar to the atoms of noble elements.
That is, during the formation of molecules, in order to stabilize them, it is necessary that all atoms have a complete external electronic level. For example, hydrogen atoms, combining into a molecule, repeat the electronic shell of helium, chlorine atoms become similar at the electronic level to 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 at such a distance from each other that 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 atoms and the length of the bond. The larger the atom, the longer the bond between the nuclei.
An option is possible when an atom forms not one, but several covalent polar bonds. Then so-called bond angles are formed between the nuclei. They can be from ninety to one hundred and eighty degrees. They determine the geometric formula of the molecule.