Unified theory there is no chemical bond; chemical bonds are conventionally divided into covalent (a universal type of bond), ionic (a special case of a covalent bond), metallic and hydrogen.
Covalent bond
The formation of a covalent bond is possible by three mechanisms: exchange, donor-acceptor and dative (Lewis).
According to metabolic mechanism The formation of a covalent bond occurs due to the sharing of common electron pairs. In this case, each atom tends to acquire a shell of an inert gas, i.e. obtain a completed external energy level. The formation of a chemical bond by exchange type is depicted using Lewis formulas, in which each valence electron of an atom is represented by dots (Fig. 1).
Rice. 1 Formation of a covalent bond in the HCl molecule by the exchange mechanism
With the development of the theory of atomic structure and quantum mechanics, the formation of a covalent bond is represented as the overlap of electronic orbitals (Fig. 2).
Rice. 2. Formation of a covalent bond due to the overlap of electron clouds
The greater the overlap of atomic orbitals, the stronger the bond, the shorter the bond length, and the greater the bond energy. Covalent bond can be formed due to the overlap of different orbitals. As a result of the overlap of s-s, s-p orbitals, as well as d-d, p-p, d-p orbitals with lateral lobes, the formation of bonds occurs. A bond is formed perpendicular to the line connecting the nuclei of 2 atoms. One and one bond are capable of forming a multiple (double) covalent bond, characteristic of organic substances of the class of alkenes, alkadienes, etc. One and two bonds form a multiple (triple) covalent bond, characteristic of organic substances of the class of alkynes (acetylenes).
Formation of a covalent bond by donor-acceptor mechanism Let's look at the example of the ammonium cation:
NH 3 + H + = NH 4 +
7 N 1s 2 2s 2 2p 3
The nitrogen atom has a free lone pair of electrons (electrons not involved in the formation of chemical bonds within the molecule), and the hydrogen cation has a free orbital, so they are an electron donor and acceptor, respectively.
Let us consider the dative mechanism of covalent bond formation using the example of a chlorine molecule.
17 Cl 1s 2 2s 2 2p 6 3s 2 3p 5
The chlorine atom has both a free lone pair of electrons and vacant orbitals, therefore, it can exhibit the properties of both a donor and an acceptor. Therefore, when a chlorine molecule is formed, one chlorine atom acts as a donor and the other as an acceptor.
Main characteristics of a covalent bond are: saturation (saturated bonds are formed when an atom attaches as many electrons to itself as its valence capabilities allow; unsaturated bonds are formed when the number of attached electrons is less than the valence capabilities of the atom); directionality (this value is related to the geometry of the molecule and the concept of “bond angle” - the angle between bonds).
Ionic bond
There are no compounds with a pure ionic bond, although this is understood as a chemically bonded state of atoms in which a stable electronic environment of the atom is created when complete transition total electron density to an atom of a more electronegative element. Ionic bonding is possible only between atoms of electronegative and electropositive elements that are in the state of oppositely charged ions - cations and anions.
DEFINITION
Ion are electrically charged particles formed by the removal or addition of an electron to an atom.
When transferring an electron, metal and nonmetal atoms tend to form a stable electron shell configuration around their nucleus. A nonmetal atom creates a shell of the subsequent inert gas around its core, and a metal atom creates a shell of the previous inert gas (Fig. 3).
Rice. 3. Formation of an ionic bond using the example of a sodium chloride molecule
Molecules in which pure form exists ionic bond found in the vapor state of the substance. The ionic bond is very strong, and therefore substances with this bond have a high melting point. Unlike covalent bonds, ionic bonds are not characterized by directionality and saturation, since the electric field created by ions acts equally on all ions due to spherical symmetry.
Metal connection
The metallic bond is realized only in metals - this is the interaction that holds metal atoms in a single lattice. Only the valence electrons of the metal atoms belonging to its entire volume participate in the formation of a bond. In metals, electrons are constantly stripped from atoms and move throughout the entire mass of the metal. Metal atoms, deprived of electrons, turn into positively charged ions, which tend to accept moving electrons. This continuous process forms the so-called “electron gas” inside the metal, which firmly binds all the metal atoms together (Fig. 4).
The metallic bond is strong, therefore metals are characterized by a high melting point, and the presence of “electron gas” gives metals malleability and ductility.
Hydrogen bond
A hydrogen bond is a specific intermolecular interaction, because its occurrence and strength depend on chemical nature substances. It is formed between molecules in which a hydrogen atom is bonded to an atom with high electronegativity (O, N, S). The occurrence of a hydrogen bond depends on two reasons: firstly, the hydrogen atom associated with an electronegative atom does not have electrons and can easily be incorporated into the electron clouds of other atoms, and, secondly, having a valence s-orbital, the hydrogen atom is able to accept a lone pair electrons of an electronegative atom and form a bond with it through the donor-acceptor mechanism.
Covalent bond(from the Latin “co” together and “vales” having force) is carried out due to the electron pair belonging to both atoms. Formed between non-metal atoms.
The electronegativity of nonmetals is quite high, so that during the chemical interaction of two nonmetal atoms, complete transfer of electrons from one to another (as in the case) is impossible. In this case, electron pooling is required to complete.
As an example, we will 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 p5 - seven electrons in the outer level
Each of the two atoms is missing one electron in order to have a complete outer shell of electrons. And each of the atoms allocates one electron “for common use.” Thus, the octet rule is satisfied. This is best represented using the Lewis formulas:
Formation of covalent bond
The shared electrons now belong to both atoms. The hydrogen atom has two electrons (its own and the 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 the 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 identical atoms. For example:
This diagram explains why hydrogen and chlorine exist as diatomic molecules. Thanks to the pairing and sharing of two electrons, it is possible to fulfill the octet rule for both atoms.
In addition to single bonds, a double or triple covalent bond can be formed, as, for example, in molecules of oxygen O 2 or nitrogen N 2. Nitrogen atoms have five valence electrons, so three more electrons are required to complete the shell. This is achieved by sharing three pairs of electrons, as shown below:
Covalent compounds are usually gases, liquids, or relatively low melting point solids. One of the rare exceptions is diamond, which melts above 3,500 °C. This is explained by the structure of diamond, which is a continuous lattice of covalently bonded carbon atoms, and not 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 hydrogen chloride molecule, for example, the electrons that form a covalent bond are located closer to the chlorine atom because its electronegativity is higher than that of hydrogen. However, the difference in the ability to attract electrons is not large enough for complete electron transfer from the hydrogen atom to the chlorine atom to occur. Therefore, the bond between hydrogen and chlorine atoms can be considered as a cross between an ionic bond (complete electron transfer) and a non-polar covalent bond (a symmetrical 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 contributes 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 an empty orbital for that pair.
Thanks to which molecules of inorganic and organic substances are formed. A chemical bond appears through the interaction of electric fields that are created by the nuclei and electrons of atoms. Therefore, the formation of a covalent chemical bond is associated with electrical nature.
What is a connection
This term refers to the result of the action of two or more atoms, which lead to the formation of a strong polyatomic system. The main types of chemical bonds are formed when the energy of reacting atoms decreases. In the process of bond formation, atoms try to complete their electron shell.
Types of communication
In chemistry, there are several types of bonds: ionic, covalent, metallic. Covalent chemical bonds have two types: polar and non-polar.
What is the mechanism for its creation? A covalent nonpolar chemical bond is formed between atoms of identical nonmetals that have the same electronegativity. In this case, common electron pairs are formed.
Non-polar bond
Examples of molecules that have a nonpolar covalent chemical bond include halogens, hydrogen, nitrogen, and oxygen.
This connection was first discovered in 1916 by the American chemist Lewis. At first he put forward a hypothesis, and it was confirmed only after experimental confirmation.
Covalent chemical bonding is related to electronegativity. For non-metals it has a high value. During the chemical interaction of atoms, the transfer of electrons from one atom to another is not always possible; as a result, they combine. A genuine covalent chemical bond appears between the atoms. 8th grade regular school curriculum involves a detailed consideration of several types of communication.
Substances that have this type of bond under normal conditions are liquids, gases, as well as solids that have a low melting point.
Types of covalent bond
Let's look in more detail at this issue. What are the types of chemical bonds? Covalent bonds exist in exchange and donor-acceptor versions.
The first type is characterized by the donation of one unpaired electron by each atom to the formation of a common electronic bond.
Electrons combined into a common bond must have opposite spins. As an example of this type of covalent bond, consider hydrogen. When its atoms come closer, their electron clouds penetrate into each other, which in science is called overlapping of electron clouds. As a result, the electron density between the nuclei increases, and the energy of the system decreases.
At a minimum distance, the hydrogen nuclei repel each other, resulting in a certain optimal distance.
In the case of the donor-acceptor type of covalent bond, one particle has electrons and is called a donor. The second particle has a free cell in which a pair of electrons will be located.
Polar molecules
How are covalent polar chemical bonds formed? They arise in situations where the nonmetal atoms being bonded have different electronegativity. IN similar cases shared electrons are located closer to the atom whose electronegativity value is higher. As an example of a covalent polar bond, we can consider the bonds that arise in the hydrogen bromide molecule. Here the public electrons, which are responsible for forming a covalent bond, are closer to bromine than to hydrogen. The reason for this phenomenon is that bromine has a higher electronegativity than hydrogen.
Methods for determining covalent bonds
How to define covalent polar chemical bonds? To do this, you need to know the composition of the molecules. If it contains atoms different elements, there is a polar covalent bond in the molecule. Nonpolar molecules contain atoms of one chemical element. Among the tasks offered as part of school course chemistry, there are also those that involve identifying the type of connection. Tasks of this type are included in the final certification tasks in chemistry in grade 9, as well as in the tests of the unified state exam in chemistry in grade 11.
Ionic bond
What is the difference between covalent and ionic chemical bonds? If a covalent bond is characteristic of nonmetals, then an ionic bond is formed between atoms that have significant differences by electronegativity. For example, this is typical for compounds of elements of the first and second groups of the main subgroups of the PS (alkali and alkaline earth metals) and elements of the 6th and 7th groups of the main subgroups of the periodic table (chalcogens and halogens).
It is formed as a result of the electrostatic attraction of ions with opposite charges.
Features of ionic bonding
Since the force fields of oppositely charged ions are distributed evenly in all directions, each of them is capable of attracting particles of opposite sign. This characterizes the non-directionality of the ionic bond.
The interaction of two ions with opposite signs does not imply complete mutual compensation of individual force fields. This helps to maintain the ability to attract ions in other directions, therefore, unsaturation of the ionic bond is observed.
In an ionic compound, each ion has the ability to attract to itself a number of others of opposite sign to form a crystal lattice of an ionic nature. There are no molecules in such a crystal. Each ion is surrounded in a substance by a certain specific number of ions of a different sign.
Metal connection
This type chemical bond has certain individual characteristics. Metals have an excess number of valence orbitals and a deficiency of electrons.
When individual atoms come together, their valence orbitals overlap, which facilitates the free movement of electrons from one orbital to another, creating a bond between all metal atoms. These free electrons are the main feature of a metallic bond. It does not have saturation and directionality, since the valence electrons are distributed evenly throughout the crystal. The presence of free electrons in metals explains some of their physical properties: metallic luster, ductility, malleability, thermal conductivity, opacity.
Type of covalent bond
It is formed between a hydrogen atom and an element that has high electronegativity. There are intra- and intermolecular hydrogen bonds. This type of covalent bond is the weakest; it appears due to the action of electrostatic forces. The hydrogen atom has a small radius, and when this one electron is displaced or given away, hydrogen becomes a positive ion, acting on the atom with high electronegativity.
Among the characteristic properties of a covalent bond are: saturation, directionality, polarizability, polarity. Each of these indicators has a specific meaning for the compound being formed. For example, directionality is determined by the geometric shape of the 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.
Characteristic properties covalent bonds - directionality, saturation, polarity, polarizability - determine the chemical and physical properties of compounds.
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 different atoms chemical elements, and the total electron cloud shifts towards one of the atoms, thereby forming an asymmetry of the distribution electric charge in a 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.
Education Communications
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 covalent non-polar 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 - an approximate description of the types of covalent bonds in molecules various connections, 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 atomic crystals (the atoms in which are interconnected by covalent (atomic) bonds can be 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 bonding 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 solvent dipoles are attracted to the charged ends of the molecule, and, as a result, Brownian motion, “tear apart” 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 include many salts nitric acid(nitrates), which absorb heat when dissolved (solutions cool). Last fact explained on the basis of laws that are considered in physical chemistry.
Rice. 2.1. The formation of molecules from atoms is accompanied by redistribution of electrons of valence orbitals and leads to gain in energy, since the energy of molecules turns out to be less than the energy of non-interacting atoms. The figure shows a diagram of the formation of a nonpolar covalent chemical bond between hydrogen atoms.
§2 Chemical bond
Under normal conditions, the molecular state is more stable than the atomic state (Fig. 2.1). The formation of molecules from atoms is accompanied by a redistribution of electrons in valence orbitals and leads to a gain in energy, since the energy of molecules is less than the energy of non-interacting atoms(Appendix 3). The forces that hold atoms in molecules are collectively called chemical bond.
The chemical bond between atoms is carried out by valence electrons and is electrical in nature . There are four main types of chemical bonds: covalent,ionic,metal And hydrogen.
1 Covalent bond
A chemical bond carried out by electron pairs is called atomic or covalent . Compounds with covalent bonds are called atomic or covalent .
When a covalent bond occurs, an overlap of electron clouds of interacting atoms occurs, accompanied by the release of energy (Fig. 2.1). In this case, a cloud with an increased density of negative charge appears between the positively charged atomic nuclei. Due to the action of Coulomb forces of attraction between unlike charges, an increase in the density of the negative charge favors the bringing together of nuclei.
A covalent bond is formed by unpaired electrons in the outer shells of atoms . In this case, electrons with opposite spins form electron pair(Fig. 2.2), common to interacting atoms. If one covalent bond (one common electron pair) has arisen between atoms, then it is called single, double, double, etc.
Energy is a measure of the strength of a chemical bond. E sv spent on breaking the bond (gain in energy when forming a compound from individual atoms). This energy is usually measured per 1 mole. substances and are expressed in kilojoules per mole (kJ∙mol –1). The energy of a single covalent bond lies in the range of 200–2000 kJmol –1.
Rice. 2.2. Covalent bond is the most general view chemical bond arising due to the sharing of an electron pair through an exchange mechanism (A), when each of the interacting atoms supplies one electron, or through a donor-acceptor mechanism (b) when an electron pair is shared by one atom (donor) to another atom (acceptor).
A covalent bond has the properties saturation and focus . The saturation of a covalent bond is understood as the ability of atoms to form a limited number of bonds with their neighbors, determined by the number of their unpaired valence electrons. The directionality of a covalent bond reflects the fact that the forces holding atoms near each other are directed along the straight line connecting the atomic nuclei. Besides, covalent bond can be polar or non-polar .
In case non-polar In a covalent bond, the electron cloud formed by a common pair of electrons is distributed in space symmetrically relative to the nuclei of both atoms. A nonpolar covalent bond is formed between atoms of simple substances, for example, between identical atoms of gases that form diatomic molecules (O 2, H 2, N 2, Cl 2, etc.).
In case polar In a covalent bond, the electron cloud of the bond is shifted toward one of the atoms. The formation of polar covalent bonds between atoms is characteristic of complex substances. An example is the molecules of volatile inorganic compounds: HCl, H 2 O, NH 3, etc.
The degree of displacement of the total electron cloud towards one of the atoms during the formation of a covalent bond (degree of bond polarity ) determined mainly by the charge of atomic nuclei and the radius of interacting atoms .
The greater the charge of an atomic nucleus, the more strongly it attracts a cloud of electrons. At the same time, the larger the radius of the atom, the weaker the outer electrons are held near the atomic nucleus. The combined effect of these two factors is expressed in the different ability of different atoms to “pull” the cloud of covalent bonds towards themselves.
The ability of an atom in a molecule to attract electrons to itself is called electronegativity. . Thus, electronegativity characterizes the ability of an atom to polarize a covalent bond: the greater the electronegativity of an atom, the more strongly the electron cloud of the covalent bond is shifted towards it .
A number of methods have been proposed to quantify electronegativity. In this case, the clearest physical meaning has the method proposed by the American chemist Robert S. Mulliken, who determined electronegativity of an atom as half the sum of its energy E e electron affinity and energy E i ionization of atom:
.
(2.1)
Ionization energy An atom is the energy that must be expended to “tear off” an electron from it and remove it to an infinite distance. Ionization energy is determined by photoionization of atoms or by bombarding atoms with electrons accelerated in an electric field. The smallest value of photon or electron energy that becomes sufficient to ionize atoms is called their ionization energy E i. This energy is usually expressed in electron volts (eV): 1 eV = 1.610 –19 J.
Atoms are most willing to give up outer electrons metals, which contain a small number of unpaired electrons (1, 2 or 3) on the outer shell. These atoms have the lowest ionization energy. Thus, the magnitude of the ionization energy can serve as a measure of the greater or lesser “metallicity” of an element: the lower the ionization energy, the more pronounced the metalproperties element.
In the same subgroup of the periodic system of elements of D.I. Mendeleev, with an increase in the atomic number of an element, its ionization energy decreases (Table 2.1), which is associated with an increase in the atomic radius (Table 1.2), and, consequently, with a weakening of the bond of external electrons with a core. For elements of the same period, ionization energy increases with increasing atomic number. This is due to a decrease in atomic radius and an increase in nuclear charge.
Energy E e, which is released when an electron is added to a free atom, is called electron affinity(also expressed in eV). The release (rather than absorption) of energy when a charged electron attaches to some neutral atoms is explained by the fact that the most stable atoms in nature are those with filled outer shells. Therefore, for those atoms in which these shells are “a little unfilled” (i.e., 1, 2 or 3 electrons are missing before filling), it is energetically favorable to attach electrons to themselves, turning into negatively charged ions 1. Such atoms include, for example, halogen atoms (Table 2.1) - elements of the seventh group (main subgroup) of D.I. Mendeleev’s periodic system. The electron affinity of metal atoms is usually zero or negative, i.e. It is energetically unfavorable for them to attach additional electrons; additional energy is required to keep them inside the atoms. The electron affinity of nonmetal atoms is always positive and the greater, the closer the nonmetal is located to a noble (inert) gas in the periodic table. This indicates an increase non-metallic properties as we approach the end of the period.
From all that has been said, it is clear that the electronegativity (2.1) of atoms increases in the direction from left to right for elements of each period and decreases in the direction from top to bottom for elements of the same group of the Mendeleev periodic system. It is not difficult, however, to understand that to characterize the degree of polarity of a covalent bond between atoms, it is not the absolute value of electronegativity that is important, but the ratio of the electronegativities of the atoms forming the bond. That's why in practice they use relative electronegativity values(Table 2.1), taking the electronegativity of lithium as unity.
To characterize the polarity of a covalent chemical bond, the difference in the relative electronegativity of atoms is used. Typically, the bond between atoms A and B is considered purely covalent if | A – B|0.5.