covalent bond - a chemical bond formed by the socialization of a pair of valence electron clouds. The electrons that provide the bond are called common electron pair.

Holy covalent bond : directivity, saturation, polarity, polarizability - determine the chemical and physical properties connections.

The direction of communication determines molecular structure substances and geometric shape their molecules. The angles between two bonds are called bond angles.

Saturation - 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 the electron density due to differences in the electronegativity of the atoms. On this basis, covalent bonds are divided into non-polar and polar.

The polarizability of a bond is expressed in the displacement of the bond electrons under the influence of an external electric field, including another reacting particle. Polarizability is determined by the electron mobility. The polarity and polarizability of covalent bonds determine the reactivity of molecules with respect to polar reagents.

Ionic bond.

The ionic type of bond is possible only between atoms that differ sharply in properties. A sharp difference in the properties of the elements leads to the fact that the metal atom completely loses its valence electrons, and the non-metal atom adds them. formed positively and negatively charged ions in the molecules and the crystal lattice by the forces of electrostatic attraction. Such a bond is called ionic.

An example is the formation of a NaCL molecule in the gas phase.

Non-specific types of communication.

metal connection - chemical bond due to the presence of relatively free electrons. It is characteristic of both pure metals and their alloys and intermetallic compounds.

Mechanism of metal bonding: Positive metal ions are located in all nodes of the crystal lattice. Between them randomly, like gas molecules, valence electrons move, unhooked from atoms during the formation of ions. These electrons play the role of cement, holding the positive ions together; otherwise, the lattice would disintegrate under the action of repulsive forces between the ions. At the same time, electrons are also held by ions within the crystal lattice and cannot leave it. Communication forces are not localized and not directed. Therefore, in most cases, high coordination numbers appear (for example, 12 or 8).

Other properties: Freely moving electrons cause high electrical and thermal conductivity. Substances with a metallic bond often combine strength with ductility, since when atoms are displaced relative to each other, bonds do not break.

Van der Waals forces - forces of intermolecular interaction with an energy of 0.8 - 8.16 kJ / mol. This term originally denoted all such forces, in modern science it is usually applied to forces arising from the polarization of molecules and the formation of dipoles. Discovered by J. D. van der Waals in 1869.

Van der Waals forces include interactions between dipoles (permanent and induced). The name comes from the fact that these forces are the cause of the correction for internal pressure in the van der Waals equation of state for a real gas. These interactions mainly determine the forces responsible for the formation of the spatial structure of biological macromolecules.

Themes USE codifier: Covalent chemical bond, its varieties and mechanisms of formation. Characteristics of a covalent bond (polarity and bond energy). Ionic bond. Metal connection. hydrogen bond

Intramolecular chemical bonds

Let us first consider the bonds that arise between particles within molecules. Such connections are called intramolecular.

chemical bond between atoms chemical elements has an electrostatic nature and is formed due to interactions of external (valence) electrons, in more or less degree held by positively charged nuclei bonded atoms.

The key concept here is ELECTRONEGNATIVITY. It is she who determines the type chemical bond between atoms and the properties of this bond.

is the ability of an atom to attract (hold) external(valence) electrons. Electronegativity is determined by the degree of attraction of external electrons to the nucleus and depends mainly on the radius of the atom and the charge of the nucleus.

Electronegativity is difficult to determine unambiguously. L. Pauling compiled a table of relative electronegativity (based on the bond energies of diatomic molecules). The most electronegative element is fluorine with meaning 4 .

It is important to note that in different sources you can find different scales and tables of electronegativity values. This should not be frightened, since the formation of a chemical bond plays a role atoms, and it is approximately the same in any system.

If one of the atoms in the chemical bond A:B attracts electrons more strongly, then the electron pair is shifted towards it. The more electronegativity difference atoms, the more the electron pair is displaced.

If the electronegativity values ​​of the interacting atoms are equal or approximately equal: EO(A)≈EO(V), then the shared electron pair is not displaced to any of the atoms: A: B. Such a connection is called covalent non-polar.

If the electronegativity of the interacting atoms differ, but not much (the difference in electronegativity is approximately from 0.4 to 2: 0,4<ΔЭО<2 ), then the electron pair is shifted to one of the atoms. Such a connection is called covalent polar .

If the electronegativity of the interacting atoms differ significantly (the difference in electronegativity is greater than 2: ΔEO>2), then one of the electrons almost completely passes to another atom, with the formation ions. Such a connection is called ionic.

The main types of chemical bonds are − covalent, ionic And metallic connections. Let's consider them in more detail.

covalent chemical bond

covalent bond – it's a chemical bond formed by formation of a common electron pair A:B . In this case, two atoms overlap atomic orbitals. A covalent bond is formed by the interaction of atoms with a small difference in electronegativity (as a rule, between two non-metals) or atoms of one element.

Basic properties of covalent bonds

• orientation,
• saturability,
• polarity,
• polarizability.

These bond properties affect the chemical and physical properties of substances.

Direction of communication characterizes the chemical structure and form of substances. The angles between two bonds are called bond angles. For example, in a water molecule, the H-O-H bond angle is 104.45 o, so the water molecule is polar, and in the methane molecule, the H-C-H bond angle is 108 o 28 ′.

Saturability is the ability of atoms to form a limited number of covalent chemical bonds. The number of bonds that an atom can form is called.

Polarity bonds arise due to the uneven distribution of electron density between two atoms with different electronegativity. Covalent bonds are divided into polar and non-polar.

Polarizability connections are the ability of bond electrons to be displaced by an external electric field(in particular, the electric field of another particle). The polarizability depends on the electron mobility. The farther the electron is from the nucleus, the more mobile it is, and, accordingly, the molecule is more polarizable.

Covalent non-polar chemical bond

There are 2 types of covalent bonding - POLAR And NON-POLAR .

Example . Consider the structure of the hydrogen molecule H 2 . Each hydrogen atom carries 1 unpaired electron in its outer energy level. To display an atom, we use the Lewis structure - this is a diagram of the structure of the external energy level of an atom, when electrons are denoted by dots. Lewis point structure models are a good help when working with elements of the second period.

H. + . H=H:H

Thus, the hydrogen molecule has one common electron pair and one H–H chemical bond. This electron pair is not displaced to any of the hydrogen atoms, because the electronegativity of hydrogen atoms is the same. Such a connection is called covalent non-polar .

Covalent non-polar (symmetrical) bond - this is a covalent bond formed by atoms with equal electronegativity (as a rule, the same non-metals) and, therefore, with a uniform distribution of electron density between the nuclei of atoms.

The dipole moment of nonpolar bonds is 0.

Examples: H 2 (H-H), O 2 (O=O), S 8 .

Covalent polar chemical bond

covalent polar bond is a covalent bond that occurs between atoms with different electronegativity (usually, different non-metals) and is characterized displacement common electron pair to a more electronegative atom (polarization).

The electron density is shifted to a more electronegative atom - therefore, a partial negative charge (δ-) arises on it, and a partial positive charge arises on a less electronegative atom (δ+, delta +).

The greater the difference in the electronegativity of atoms, the higher polarity connections and even more dipole moment . Between neighboring molecules and charges opposite in sign, additional attractive forces act, which increases strength connections.

Bond polarity affects the physical and chemical properties of compounds. The reaction mechanisms and even the reactivity of neighboring bonds depend on the polarity of the bond. The polarity of a bond often determines polarity of the molecule and thus directly affects such physical properties as boiling point and melting point, solubility in polar solvents.

Examples: HCl, CO 2 , NH 3 .

Mechanisms for the formation of a covalent bond

A covalent chemical bond can occur by 2 mechanisms:

1. exchange mechanism the formation of a covalent chemical bond is when each particle provides one unpaired electron for the formation of a common electron pair:

BUT . + . B= A:B

2. The formation of a covalent bond is such a mechanism in which one of the particles provides an unshared electron pair, and the other particle provides a vacant orbital for this electron pair:

BUT: + B= A:B

In this case, one of the atoms provides an unshared electron pair ( donor), and the other atom provides a vacant orbital for this pair ( acceptor). As a result of the formation of a bond, both electron energy decreases, i.e. this is beneficial for the atoms.

A covalent bond formed by the donor-acceptor mechanism, is not different by properties from other covalent bonds formed by the exchange mechanism. The formation of a covalent bond by the donor-acceptor mechanism is typical for atoms either with a large number of electrons in the external energy level (electron donors), or vice versa, with a very small number of electrons (electron acceptors). The valence possibilities of atoms are considered in more detail in the corresponding.

A covalent bond is formed by the donor-acceptor mechanism:

- in a molecule carbon monoxide CO(the bond in the molecule is triple, 2 bonds are formed by the exchange mechanism, one by the donor-acceptor mechanism): C≡O;

- in ammonium ion NH 4 +, in ions organic amines, for example, in the methylammonium ion CH 3 -NH 2 + ;

- in complex compounds, a chemical bond between the central atom and groups of ligands, for example, in sodium tetrahydroxoaluminate Na the bond between aluminum and hydroxide ions;

- in nitric acid and its salts- nitrates: HNO 3 , NaNO 3 , in some other nitrogen compounds;

- in a molecule ozone O 3 .

Main characteristics of a covalent bond

A covalent bond, as a rule, is formed between the atoms of non-metals. The main characteristics of a covalent bond are length, energy, multiplicity and directivity.

Chemical bond multiplicity

Chemical bond multiplicity - this the number of shared electron pairs between two atoms in a compound. The multiplicity of the bond can be quite easily determined from the value of the atoms that form the molecule.

For example , in the hydrogen molecule H 2 the bond multiplicity is 1, because each hydrogen has only 1 unpaired electron in the outer energy level, therefore, one common electron pair is formed.

In the oxygen molecule O 2, the bond multiplicity is 2, because each atom has 2 unpaired electrons in its outer energy level: O=O.

In the nitrogen molecule N 2, the bond multiplicity is 3, because between each atom there are 3 unpaired electrons in the outer energy level, and the atoms form 3 common electron pairs N≡N.

Covalent bond length

Chemical bond length is the distance between the centers of the nuclei of atoms that form a bond. It is determined by experimental physical methods. The bond length can be estimated approximately, according to the additivity rule, according to which the bond length in the AB molecule is approximately equal to half the sum of the bond lengths in the A 2 and B 2 molecules:

The length of a chemical bond can be roughly estimated along the radii of atoms, forming a bond, or by the multiplicity of communication if the radii of the atoms are not very different.

With an increase in the radii of the atoms forming a bond, the bond length will increase.

For example

With an increase in the multiplicity of bonds between atoms (whose atomic radii do not differ, or differ slightly), the bond length will decrease.

For example . In the series: C–C, C=C, C≡C, the bond length decreases.

Bond energy

A measure of the strength of a chemical bond is the bond energy. Bond energy is determined by the energy required to break the bond and remove the atoms that form this bond to an infinite distance from each other.

The covalent bond is very durable. Its energy ranges from several tens to several hundreds of kJ/mol. The greater the bond energy, the greater the bond strength, and vice versa.

The strength of a chemical bond depends on the bond length, bond polarity, and bond multiplicity. The longer the chemical bond, the easier it is to break, and the lower the bond energy, the lower its strength. The shorter the chemical bond, the stronger it is, and the greater the bond energy.

For example, in the series of compounds HF, HCl, HBr from left to right the strength of the chemical bond decreases, because the length of the bond increases.

Ionic chemical bond

Ionic bond is a chemical bond based on electrostatic attraction of ions.

ions are formed in the process of accepting or giving away electrons by atoms. For example, the atoms of all metals weakly hold the electrons of the outer energy level. Therefore, metal atoms are characterized restorative properties the ability to donate electrons.

Example. The sodium atom contains 1 electron at the 3rd energy level. Easily giving it away, the sodium atom forms a much more stable Na + ion, with the electron configuration of the noble neon gas Ne. The sodium ion contains 11 protons and only 10 electrons, so the total charge of the ion is -10+11 = +1:

+11Na) 2 ) 8 ) 1 - 1e = +11 Na +) 2 ) 8

Example. The chlorine atom has 7 electrons in its outer energy level. To acquire the configuration of a stable inert argon atom Ar, chlorine needs to attach 1 electron. After the attachment of an electron, a stable chlorine ion is formed, consisting of electrons. The total charge of the ion is -1:

+17Cl) 2 ) 8 ) 7 + 1e = +17 Cl — ) 2 ) 8 ) 8

Note:

• The properties of ions are different from the properties of atoms!
• Stable ions can form not only atoms, but also groups of atoms. For example: ammonium ion NH 4 +, sulfate ion SO 4 2-, etc. Chemical bonds formed by such ions are also considered ionic;
• Ionic bonds are usually formed between metals And nonmetals(groups of non-metals);

The resulting ions are attracted due to electrical attraction: Na + Cl -, Na 2 + SO 4 2-.

Let us visually generalize difference between covalent and ionic bond types:

metal connection is the relationship that is formed relatively free electrons between metal ions forming a crystal lattice.

The atoms of metals on the outer energy level usually have one to three electrons. The radii of metal atoms, as a rule, are large - therefore, metal atoms, unlike non-metals, quite easily donate outer electrons, i.e. are strong reducing agents.

By donating electrons, metal atoms become positively charged ions . The detached electrons are relatively free are moving between positively charged metal ions. Between these particles there is a connection, because shared electrons hold metal cations in layers together , thus creating a sufficiently strong metal crystal lattice . In this case, the electrons continuously move randomly, i.e. new neutral atoms and new cations are constantly emerging.

Intermolecular interactions

Separately, it is worth considering the interactions that occur between individual molecules in a substance - intermolecular interactions . Intermolecular interactions are a type of interaction between neutral atoms in which new covalent bonds do not appear. The forces of interaction between molecules were discovered by van der Waals in 1869 and named after him. Van dar Waals forces. Van der Waals forces are divided into orientation, induction And dispersion . The energy of intermolecular interactions is much less than the energy of a chemical bond.

Orientation forces of attraction arise between polar molecules (dipole-dipole interaction). These forces arise between polar molecules. Inductive interactions is the interaction between a polar molecule and a non-polar one. A non-polar molecule is polarized due to the action of a polar one, which generates an additional electrostatic attraction.

A special type of intermolecular interaction is hydrogen bonds. - these are intermolecular (or intramolecular) chemical bonds that arise between molecules in which there are strongly polar covalent bonds - H-F, H-O or H-N. If there are such bonds in the molecule, then between the molecules there will be additional forces of attraction .

Mechanism of Education The hydrogen bond is partly electrostatic and partly donor-acceptor. In this case, an atom of a strongly electronegative element (F, O, N) acts as an electron pair donor, and hydrogen atoms connected to these atoms act as an acceptor. Hydrogen bonds are characterized orientation in space and saturation .

The hydrogen bond can be denoted by dots: H ··· O. The greater the electronegativity of an atom connected to hydrogen, and the smaller its size, the stronger the hydrogen bond. It is primarily characteristic of compounds fluorine with hydrogen , as well as to oxygen with hydrogen , less nitrogen with hydrogen .

Hydrogen bonds occur between the following substances:

— hydrogen fluoride HF(gas, solution of hydrogen fluoride in water - hydrofluoric acid), water H 2 O (steam, ice, liquid water):

— solution of ammonia and organic amines- between ammonia and water molecules;

— organic compounds in which O-H or N-H bonds: alcohols, carboxylic acids, amines, amino acids, phenols, aniline and its derivatives, proteins, solutions of carbohydrates - monosaccharides and disaccharides.

The hydrogen bond affects the physical and chemical properties of substances. Thus, the additional attraction between molecules makes it difficult for substances to boil. Substances with hydrogen bonds exhibit an abnormal increase in the boiling point.

For example As a rule, with an increase in molecular weight, an increase in the boiling point of substances is observed. However, in a number of substances H 2 O-H 2 S-H 2 Se-H 2 Te we do not observe a linear change in boiling points.

Namely, at boiling point of water is abnormally high - not less than -61 o C, as the straight line shows us, but much more, +100 o C. This anomaly is explained by the presence of hydrogen bonds between water molecules. Therefore, under normal conditions (0-20 o C), water is liquid by phase state.

covalent bond

Characteristics of a chemical bond. Hybridization.

LECTURE №3. Chemical bond and structure of molecules. Valence.

Only a few chemical elements in natural conditions are in a monatomic state (for example, inert gases). Free atoms of other elements form more complex systems - molecules with more stable electronic configurations. This phenomenon is called the formation of a chemical bond.

chemical bond - this is the interaction of two or more atoms, as a result of which a chemically stable two- or polyatomic system is formed. The formation of a chemical bond is accompanied by a decrease in the total energy of the system.

The theory of chemical bonding is based on ideas about electronic interactions. The most stable (strong) groupings of electrons are the completed outer electron layers of atoms of inert gases (two-electron for helium and eight-electron for other noble gases). The incomplete outer electron layers of all other elements are unstable, and when such atoms are combined with other atoms, their electronic shells are rearranged. A chemical bond is formed by valence electrons, but is carried out in different ways.

Valence are called electrons that participate in the formation of chemical bonds, mainly these are electrons of the last or penultimate energy level.

There are several types of chemical bonds: ionic, metallic, covalent, and hydrogen.

The simplest example of a covalent bond is the formation of a hydrogen molecule. Hydrogen atoms have an electron shell of one unpaired s-electron, i.e. one electron is missing to complete the level. When hydrogen atoms approach each other up to a certain distance, electrons with antiparallel spins interact with the formation general electron pair. A common electron pair is formed as a result of partial overlapping of s-orbitals, and in this case, the greatest density is created in the region of overlapping orbitals.

The bonding of atoms using shared electron pairs is called covalent.

A molecule with a covalent bond can be written in the form of two formulas: electronic (an electron is indicated by a dot) and structural (a shared electron pair is indicated by a bar).

1. Link length is the distance between the nuclei of atoms. Expressed in nm. A chemical bond is stronger the shorter its length. However, the measure of bond strength is its energy.

2. Bond energy - this is the amount of energy that is released during the formation of a chemical bond and, therefore, this is the work that must be spent on breaking the bond. Expressed in kJ/mol. The bond energy increases with decreasing bond length.

3. Under satiety understand the ability of atoms to form a limited number of covalent bonds. For example, a hydrogen atom, having one unpaired electron, can form one bond, and an excited carbon atom can form no more than four bonds. Due to the saturation of the bonds, the molecules have a certain composition. However, even with saturated covalent bonds, more complex molecules can be formed according to the donor-acceptor mechanism.

4. multiplicity determined by the number of common electron pairs between atoms, i.e. the number of chemical bonds. In the considered hydrogen molecule, as well as in the molecules of fluorine and chlorine, the bond between atoms is carried out due to one electron pair, such a bond is called single. In an oxygen molecule double, and in the nitrogen molecule - triple.

Moreover, a covalent bond can be of two types:

1) If electron clouds overlap in the direction of a straight line that connects the nuclei of atoms (i.e. along communication axes ), such a covalent bond is called sigma bond . Covalent sigma bonds are formed by overlapping orbitals: s-s (hydrogen molecule), s-p (hydrogen chloride) and p-p (chlorine molecule).

2) If p-orbitals directed perpendicular to the bond axis overlap, two areas of overlap are formed on both sides of the bond axis and such a bond is called pi bond .

Despite the fact that the energy of a pi bond is less than sigma, the total energy of a double, and even more so a triple bond, is higher than a single one.

5. Polarity bonds are determined by the location of a common electron pair, if it is distributed in space symmetrically with respect to the nuclei of both atoms, then such a covalent bond is called non-polar . An example is diatomic molecules consisting of atoms of the same element, i.e. simple substances.

In the case polar covalent bond , the molecule is formed by atoms of different elements and the electron cloud of the bond, in this case, is shifted to the atom with a higher relative electronegativity. For example, during the formation of the HCl molecule, the common electron pair is shifted to the chlorine atom, since it has a greater EO.

EO- this is the ability of the atoms of elements to attract common electron pairs to themselves. An atom more than an EO element takes on an effective negative charge d-, and the second atom takes on an effective positive charge d+. As a result, there is dipole. The measure of bond polarity is electric dipole moment .

6. Orientation covalent bond determines the spatial structure of molecules, i.e. their geometric shape. Directionality is quantified valence angle is the angle between chemical bonds. Covalent bonds formed by multivalent atoms always have a spatial orientation.

In most cases, when a bond is formed, the electrons of the bonded atoms are shared. This type of chemical bond is called a covalent bond (the prefix "co-" in Latin means compatibility, "valence" - having force). The binding electrons are predominantly located in the space between the bonded atoms. Due to the attraction of the nuclei of atoms to these electrons, a chemical bond is formed. Thus, a covalent bond is a chemical bond that occurs due to an increase in electron density in the region between chemically bonded atoms.

The first theory of the covalent bond belongs to the American physical chemist G.-N. Lewis. In 1916, he suggested that the bonds between two atoms are carried out by a pair of electrons, with an eight-electron shell usually forming around each atom (the octet rule).

One of the essential properties of a covalent bond is its saturation. With a limited number of outer electrons in the regions between the nuclei, a limited number of electron pairs are formed near each atom (and, consequently, the number of chemical bonds). It is this number that is closely related to the concept of the valency of an atom in a molecule (valency is the total number of covalent bonds formed by an atom). Another important property of a covalent bond is its orientation in space. This is manifested in approximately the same geometric structure of chemical particles with similar composition. A feature of the covalent bond is also its polarizability.

To describe a covalent bond, two methods are mainly used, based on different approximations in solving the Schrödinger equation: the method of molecular orbitals and the method of valence bonds. At present, the method of molecular orbitals is used almost exclusively in theoretical chemistry. However, the method of valence bonds, despite the great complexity of calculations, gives a more visual representation of the formation and structure of chemical particles.

Covalent bond parameters

The set of atoms that form a chemical particle differs significantly from the set of free atoms. The formation of a chemical bond leads, in particular, to a change in the atomic radii and their energy. There is also a redistribution of the electron density: the probability of finding electrons in the space between the bound atoms increases.

Chemical bond length

When a chemical bond is formed, atoms always approach each other - the distance between them is less than the sum of the radii of isolated atoms:

r(A−B) r(A) + r(B)

The radius of a hydrogen atom is 53 pm, that of a fluorine atom is 71 pm, and the distance between the nuclei of atoms in an HF molecule is 92 pm:

The internuclear distance between chemically bonded atoms is called the chemical bond length.

In many cases, the bond length between atoms in a molecule of a substance can be predicted by knowing the distances between these atoms in other chemicals. The bond length between carbon atoms in diamond is 154 pm, between halogen atoms in a chlorine molecule - 199 pm. The half-sum of distances between carbon and chlorine atoms calculated from these data is 177 pm, which coincides with the experimentally measured bond length in the CCl 4 molecule. At the same time, this is not always the case. For example, the distance between hydrogen and bromine atoms in diatomic molecules is 74 and 228 pm, respectively. The arithmetic mean of these numbers is 151 pm, but the actual distance between atoms in a hydrogen bromide molecule is 141 pm, that is, noticeably less.

The distance between atoms decreases significantly with the formation of multiple bonds. The higher the bond multiplicity, the shorter the interatomic distance.

Lengths of some simple and multiple bonds

Valence angles

The direction of covalent bonds is characterized by valence angles - the angles between the lines connecting the bonded atoms. The graphic formula of a chemical particle does not carry information about bond angles. For example, in the SO 4 2− sulfate ion, the bond angles between sulfur–oxygen bonds are 109.5 o , and in the tetrachloropalladate ion 2− 90 o . The combination of bond lengths and bond angles in a chemical particle determines its spatial structure. To determine bond angles, experimental methods are used to study the structure of chemical compounds. Valence angles can be estimated theoretically based on the electronic structure of a chemical particle.

Covalent bond energy

A chemical compound is formed from individual atoms only if it is energetically favorable. If the attractive forces prevail over the repulsive forces, the potential energy of the interacting atoms decreases, otherwise it increases. At some distance (equal to the bond length r 0) this energy is minimal.

Thus, when a chemical bond is formed, energy is released, and when it is broken, energy is absorbed. Energy E 0 , necessary to separate the atoms and remove them from each other at a distance at which they do not interact, is called binding energy. For diatomic molecules, the binding energy is defined as the energy of dissociation of a molecule into atoms. It can be measured experimentally.

In a hydrogen molecule, the binding energy is numerically equal to the energy that is released during the formation of an H 2 molecule from H atoms:

H + H \u003d H 2 + 432 kJ

The same energy must be expended to break the H-H bond:

H 2 \u003d H + H - 432 kJ

For polyatomic molecules, this value is conditional and corresponds to the energy of such a process in which a given chemical bond disappears, while all the others remain unchanged. If there are several identical bonds (for example, for a water molecule containing two oxygen-hydrogen bonds), their energy can be calculated using Hess' law. The values ​​of the energy of the decomposition of water into simple substances, as well as the energies of the dissociation of hydrogen and oxygen into atoms are known:

2H 2 O \u003d 2H 2 + O 2; 484 kJ/mol

H 2 \u003d 2H; 432 kJ/mol

O 2 \u003d 2O; 494 kJ/mol

Given that two water molecules contain 4 bonds, the oxygen-hydrogen bond energy is:

E(О−Н) \u003d (2. 432 + 494 + 484) / 4 \u003d 460.5 kJ / mol

In molecules of composition AB n the successive detachment of B atoms is accompanied by certain (not always identical) energy expenditures. For example, the energy values ​​(kJ/mol) of successive elimination of hydrogen atoms from a methane molecule differ significantly:

	427		368		519		335
CH 4	→	CH 3	→	CH 2	→	CH	→	FROM

In this case, the A–B bond energy is defined as the average value of the energy expended at all stages:

CH 4 \u003d C + 4H; 1649 kJ/mol

E(С−Н) = 1649 / 4 = 412 kJ/mol

The higher the energy of a chemical bond, the stronger the bond.. The bond is considered strong or strong if its energy exceeds 500 kJ/mol (for example, 942 kJ/mol for N 2), weak - if its energy is less than 100 kJ/mol (for example, 69 kJ/mol for NO 2). If during the interaction of atoms an energy of less than 15 kJ/mol is released, then it is considered that a chemical bond is not formed, but an intermolecular interaction is observed (for example, 2 kJ/mol for Xe 2). Bond strength usually decreases with increasing bond length.

A single bond is always weaker than multiple bonds - double and triple - between the same atoms.

Energies of some simple and multiple bonds

Polarity of a covalent bond

The polarity of a chemical bond depends on the difference in the electronegativity of the bonding atoms.

Electronegativity is a conditional value that characterizes the ability of an atom in a molecule to attract electrons. If, in a diatomic molecule A–B, the bonding electrons are attracted to the B atom more strongly than to the A atom, then the B atom is considered to be more electronegative.

The electronegativity scale was used by L. Pauling for quantitative characteristics of the ability of atoms to polarize covalent bonds. For a quantitative description of electronegativity, in addition to thermochemical data, data on the geometry of molecules (Sanderson method) or spectral characteristics (Gordy method) are also used. The Allred and Rochov scale is also widely used, in which the effective nuclear charge and the atomic covalent radius are used in the calculation. The method proposed by the American physical chemist R. Mulliken (1896-1986) has the clearest physical meaning. He defined the electronegativity of an atom as half the sum of its electron affinity and ionization potential. Electronegativity values ​​based on the Mulliken method and extended to a wide range of various objects are called absolute.

Fluorine has the highest electronegativity value. The least electronegative element is cesium. The higher the difference between the electronegativity of two atoms, the more polar is the chemical bond between them.

Depending on how the redistribution of electron density occurs during the formation of a chemical bond, several types of it are distinguished. The limiting case of chemical bond polarization is the complete transition of an electron from one atom to another. In this case, two ions are formed, between which an ionic bond occurs. In order for two atoms to form an ionic bond, their electronegativity must be very different. If the electronegativities of the atoms are equal (when molecules are formed from identical atoms), the bond is called non-polar covalent. Most often found polar covalent bond - it is formed between any atoms that have different values ​​of electronegativity.

Quantifying polarity("ionic") bonds can serve as the effective charges of atoms. The effective charge of an atom characterizes the difference between the number of electrons belonging to a given atom in a chemical compound and the number of electrons in a free atom. An atom of a more electronegative element attracts electrons more strongly. Therefore, the electrons are closer to him, and he receives some negative charge, which is called effective, and his partner has the same positive charge. If the electrons that form a bond between atoms belong equally to them, the effective charges are zero. In ionic compounds, the effective charges must coincide with the charges of the ions. And for all other particles they have intermediate values.

The best method for estimating the charges of atoms in a molecule is to solve the wave equation. However, this is possible only in the presence of a small number of atoms. Qualitatively, the charge distribution can be estimated using the electronegativity scale. Various experimental methods are also used. For diatomic molecules, the polarity of the bond can be characterized and the effective charges of the atoms can be determined based on the measurement of the dipole moment:

μ = q r,

where q is the charge of the dipole pole, which is equal to the effective charge for a diatomic molecule, r− internuclear distance.

The bond dipole moment is a vector quantity. It is directed from the positively charged part of the molecule to its negative part. Based on the measurement of the dipole moment, it was found that in the HCl molecule, the hydrogen atom has a positive charge of +0.2 fractions of the electron charge, and the chlorine atom has a negative charge of −0.2. Hence, the H–Cl bond is ionic by 20%. And the Na–Cl bond is 90% ionic.

Why can atoms combine with each other to form molecules? What is the reason for the possible existence of substances, which include atoms of completely different chemical elements? These are global issues affecting the fundamental concepts of modern physical and chemical science. You can answer them, having an idea about the electronic structure of atoms and knowing the characteristics of the covalent bond, which is the basic basis for most classes of compounds. The purpose of our article is to get acquainted with the mechanisms of formation of various types of chemical bonds and compounds containing them in their molecules.

The electronic structure of the atom

The electrically neutral particles of matter, which are its structural elements, have a structure that mirrors the structure of the solar system. As the planets revolve around the central star - the Sun, so the electrons in the atom move around the positively charged nucleus. To characterize the covalent bond, the electrons located at the last energy level and the most distant from the nucleus will be significant. Since their connection with the center of their own atom is minimal, they are able to be easily attracted by the nuclei of other atoms. This is very important for the occurrence of interatomic interactions leading to the formation of molecules. Why is the molecular form the main type of existence of matter on our planet? Let's figure it out.

Basic property of atoms

The ability of electrically neutral particles to interact, leading to a gain in energy, is their most important feature. Indeed, under normal conditions, the molecular state of matter is more stable than the atomic state. The main provisions of modern atomic and molecular theory explain both the principles of the formation of molecules and the characteristics of a covalent bond. Recall that there can be from 1 to 8 electrons per atom, in the latter case the layer will be complete, which means it will be very stable. Atoms of noble gases have such an external level structure: argon, krypton, xenon - inert elements that complete each period in the system of D. I. Mendeleev. The exception here is helium, which has not 8, but only 2 electrons in the last level. The reason is simple: in the first period there are only two elements whose atoms have a single electron layer. All other chemical elements have from 1 to 7 electrons on the last, incomplete layer. In the process of interaction between themselves, the atoms will tend to fill up with electrons up to an octet and restore the configuration of an atom of an inert element. Such a state can be achieved in two ways: by the loss of one's own or by the acceptance of foreign negatively charged particles. These forms of interaction explain how to determine what kind of bond - ionic or covalent - will arise between the reacting atoms.

Mechanisms for the Formation of a Stable Electronic Configuration

Imagine that two simple substances enter into the reaction of the compound: metallic sodium and gaseous chlorine. A substance of the class of salts is formed - sodium chloride. It has an ionic type of chemical bond. Why and how did it come about? Let us turn again to the structure of the atoms of the initial substances. Sodium has only one electron on the last layer, weakly bound to the nucleus due to the large radius of the atom. The ionization energy of all alkali metals, including sodium, is low. Therefore, the electron of the outer level leaves the energy level, is attracted by the nucleus of the chlorine atom and remains in its space. This creates a precedent for the transition of the Cl atom into the form of a negatively charged ion. Now we are no longer dealing with electrically neutral particles, but with charged sodium cations and chlorine anions. In accordance with the laws of physics, electrostatic attraction forces arise between them, and the compound forms an ionic crystal lattice. The mechanism of formation of the ionic type of chemical bond considered by us will help to clarify the specifics and main characteristics of the covalent bond more clearly.

Shared electron pairs

If an ionic bond arises between atoms of elements that differ greatly in electronegativity, i.e., metals and non-metals, then the covalent type appears when atoms of the same or different non-metallic elements interact. In the first case, it is customary to talk about non-polar, and in the other - about the polar form of a covalent bond. The mechanism of their formation is common: each of the atoms partially gives electrons for common use, which are combined in pairs. But the spatial arrangement of electron pairs relative to the nuclei of atoms will be different. On this basis, the types of covalent bonds are distinguished - non-polar and polar. Most often, in chemical compounds consisting of atoms of non-metallic elements, there are pairs consisting of electrons with opposite spins, i.e., rotating around their nuclei in opposite directions. Since the movement of negatively charged particles in space leads to the formation of electron clouds, which ultimately ends in their mutual overlap. What are the consequences of this process for atoms and what does it lead to?

Physical properties of a covalent bond

It turns out that a two-electron cloud appears between the centers of two interacting atoms, which has a high density. The electrostatic forces of attraction between the negatively charged cloud itself and the nuclei of atoms increase. A portion of energy is released and the distances between atomic centers decrease. For example, at the beginning of the formation of an H 2 molecule, the distance between the nuclei of hydrogen atoms is 1.06 A, after the clouds overlap and the formation of a common electron pair, it is 0.74 A. Examples of a covalent bond formed according to the above mechanism can be found both among simple and among complex inorganic substances. Its main distinguishing feature is the presence of common electron pairs. As a result, after the emergence of a covalent bond between atoms, for example, hydrogen, each of them acquires the electronic configuration of inert helium, and the resulting molecule has a stable structure.

Spatial shape of a molecule

Another very important physical property of a covalent bond is directionality. It depends on the spatial configuration of the substance molecule. For example, when two electrons overlap with a spherical cloud, the appearance of the molecule is linear (hydrogen chloride or hydrogen bromide). The shape of water molecules, in which s- and p-clouds hybridize, is angular, and very strong particles of gaseous nitrogen look like a pyramid.

The structure of simple substances - non-metals

Having found out what kind of bond is called covalent, what signs it has, now is the time to deal with its varieties. If atoms of the same non-metal - chlorine, nitrogen, oxygen, bromine, etc., interact with each other, then the corresponding simple substances are formed. Their common electron pairs are located at the same distance from the centers of atoms, without shifting. For compounds with a non-polar type of covalent bond, the following features are inherent: low boiling and melting points, insolubility in water, and dielectric properties. Next, we will find out which substances are characterized by a covalent bond, in which a shift of common electron pairs occurs.

Electronegativity and its influence on the type of chemical bond

The property of a certain element to attract electrons from the atom of another element in chemistry is called electronegativity. The scale of values ​​for this parameter, proposed by L. Pauling, can be found in all textbooks on inorganic and general chemistry. Its highest value - 4.1 eV - has fluorine, the smaller one - other active non-metals, and the lowest indicator is typical for alkali metals. If elements differing in their electronegativity react with each other, then inevitably one, more active, will attract negatively charged particles of an atom of a more passive element to its nucleus. Thus, the physical properties of a covalent bond directly depend on the ability of the elements to donate electrons for common use. The common pairs formed in this case are no longer located symmetrically with respect to the nuclei, but are shifted towards the more active element.

Features of compounds with a polar bond

Substances in whose molecules joint electron pairs are asymmetric with respect to the nuclei of atoms include hydrogen halides, acids, compounds of chalcogens with hydrogen, and acid oxides. These are sulfate and nitrate acids, oxides of sulfur and phosphorus, hydrogen sulfide, etc. For example, a hydrogen chloride molecule contains one common electron pair formed by unpaired electrons of hydrogen and chlorine. It is shifted closer to the center of the Cl atom, which is a more electronegative element. All substances with a polar bond in aqueous solutions dissociate into ions and conduct an electric current. The compounds that we have cited also have higher melting and boiling points compared to simple non-metal substances.

Methods for breaking chemical bonds

In organic chemistry, saturated hydrocarbons with halogens follow a radical mechanism. A mixture of methane and chlorine in the light and at ordinary temperature reacts in such a way that the chlorine molecules begin to split into particles carrying unpaired electrons. In other words, the destruction of a common electron pair and the formation of very active radicals -Cl are observed. They are able to influence methane molecules in such a way that they break the covalent bond between carbon and hydrogen atoms. An active particle -H is formed, and the free valency of the carbon atom takes on a chlorine radical, and chloromethane becomes the first product of the reaction. Such a mechanism for the splitting of molecules is called homolytic. If the common pair of electrons completely passes into the possession of one of the atoms, then they speak of a heterolytic mechanism characteristic of reactions taking place in aqueous solutions. In this case, polar water molecules will increase the rate of destruction of the chemical bonds of the dissolved compound.

Double and triple bonds

The vast majority of organic substances and some inorganic compounds contain in their molecules not one, but several common electron pairs. The multiplicity of the covalent bond reduces the distance between atoms and increases the stability of compounds. They are usually referred to as chemically resistant. For example, in a nitrogen molecule there are three pairs of electrons, they are indicated in the structural formula by three dashes and determine its strength. The simple substance nitrogen is chemically inert and can react with other compounds, such as hydrogen, oxygen or metals, only when heated or at elevated pressure, and also in the presence of catalysts.

Double and triple bonds are inherent in such classes of organic compounds as unsaturated diene hydrocarbons, as well as substances of the ethylene or acetylene series. Multiple bonds determine the main chemical properties: addition and polymerization reactions occurring at the points of their rupture.

In our article, we gave a general description of the covalent bond and examined its main types.