Types of covalent bonds polar nonpolar hcl, h2o h2, cl2, n2. Determine the type of chemical bond: HCL, Na2S, NH3, I2, MnO2 Nature of the chemical bond

Task No. 1

From the list provided, select two compounds that contain an ionic chemical bond.

• 1. Ca(ClO 2) 2
• 2. HClO 3
• 3.NH4Cl
• 4. HClO 4
• 5.Cl2O7

Answer: 13

In the vast majority of cases, the presence of an ionic type of bond in a compound can be determined by the fact that its structural units simultaneously include atoms of a typical metal and atoms of a non-metal.

Based on this feature, we establish that there is an ionic bond in compound number 1 - Ca(ClO 2) 2, because in its formula you can see atoms of the typical metal calcium and atoms of non-metals - oxygen and chlorine.

However, there are no more compounds containing both metal and non-metal atoms in this list.

Among the compounds indicated in the task is ammonium chloride, in which the ionic bond is realized between the ammonium cation NH 4 + and the chloride ion Cl − .

Task No. 2

From the list provided, select two compounds in which the type of chemical bond is the same as in the fluorine molecule.

1) oxygen

2) nitric oxide (II)

3) hydrogen bromide

4) sodium iodide

Write down the numbers of the selected connections in the answer field.

Answer: 15

The fluorine molecule (F2) consists of two atoms of one nonmetal chemical element, therefore the chemical bond in this molecule is covalent, nonpolar.

A covalent nonpolar bond can only be realized between atoms of the same nonmetal chemical element.

Of the proposed options, only oxygen and diamond have a covalent nonpolar type of bond. The oxygen molecule is diatomic, consisting of atoms of one nonmetal chemical element. Diamond has an atomic structure and in its structure, each carbon atom, which is a non-metal, is bonded to 4 other carbon atoms.

Nitric oxide (II) is a substance consisting of molecules formed by atoms of two different non-metals. Since the electronegativity of different atoms is always different, the shared electron pair in a molecule is biased towards the more electronegative element, in this case oxygen. Thus, the bond in the NO molecule is polar covalent.

Hydrogen bromide also consists of diatomic molecules consisting of hydrogen and bromine atoms. The shared electron pair forming the H-Br bond is shifted toward the more electronegative bromine atom. The chemical bond in the HBr molecule is also polar covalent.

Sodium iodide is a substance of ionic structure formed by a metal cation and an iodide anion. The bond in the NaI molecule is formed due to the transfer of an electron from 3 s-orbitals of the sodium atom (the sodium atom is converted into a cation) to underfilled 5 p-orbital of the iodine atom (the iodine atom turns into an anion). This chemical bond is called ionic.

Task No. 3

From the list provided, select two substances whose molecules form hydrogen bonds.

• 1. C 2 H 6
• 2. C 2 H 5 OH
• 3.H2O
• 4. CH 3 OCH 3
• 5. CH 3 COCH 3

Write down the numbers of the selected connections in the answer field.

Answer: 23

Explanation:

Hydrogen bonds occur in substances with a molecular structure that contain covalent bonds H-O, H-N, H-F. Those. covalent bonds of a hydrogen atom with atoms of three chemical elements with the highest electronegativity.

Thus, obviously, there are hydrogen bonds between molecules:

2) alcohols

3) phenols

4) carboxylic acids

5) ammonia

6) primary and secondary amines

7) hydrofluoric acid

Task No. 4

From the list provided, select two compounds with ionic chemical bonds.

• 1.PCl 3
• 2.CO2
• 3. NaCl
• 4.H2S
• 5. MgO

Write down the numbers of the selected connections in the answer field.

Answer: 35

Explanation:

In the vast majority of cases, a conclusion about the presence of an ionic type of bond in a compound can be drawn from the fact that the structural units of the substance simultaneously include atoms of a typical metal and atoms of a non-metal.

Based on this feature, we establish that there is an ionic bond in compounds numbered 3 (NaCl) and 5 (MgO).

Note*

In addition to the above characteristic, the presence of an ionic bond in a compound can be said if its structural unit contains an ammonium cation (NH 4 +) or its organic analogues - alkylammonium cations RNH 3 +, dialkylammonium R 2 NH 2 +, trialkylammonium cations R 3 NH + or tetraalkylammonium R 4 N +, where R is some hydrocarbon radical. For example, the ionic type of bond occurs in the compound (CH 3) 4 NCl between the cation (CH 3) 4 + and the chloride ion Cl −.

Task No. 5

From the list provided, select two substances with the same type of structure.

4) table salt

Write down the numbers of the selected connections in the answer field.

Answer: 23

Task No. 8

From the proposed list, select two substances of non-molecular structure.

2) oxygen

3) white phosphorus

5) silicon

Write down the numbers of the selected connections in the answer field.

Answer: 45

Task No. 11

From the proposed list, select two substances whose molecules contain a double bond between carbon and oxygen atoms.

3) formaldehyde

4) acetic acid

5) glycerin

Write down the numbers of the selected connections in the answer field.

Answer: 34

Task No. 14

From the list provided, select two substances with ionic bonds.

1) oxygen

3) carbon monoxide (IV)

4) sodium chloride

5) calcium oxide

Write down the numbers of the selected connections in the answer field.

Answer: 45

Task No. 15

From the proposed list, select two substances with the same type of crystal lattice as a diamond.

1) silica SiO 2

2) sodium oxide Na 2 O

3) carbon monoxide CO

4) white phosphorus P 4

5) silicon Si

Write down the numbers of the selected connections in the answer field.

Answer: 15

Task No. 20

From the list provided, select two substances whose molecules have one triple bond.

• 1. HCOOH
• 2.HCOH
• 3. C 2 H 4
• 4. N 2
• 5. C 2 H 2

Write down the numbers of the selected connections in the answer field.

Answer: 45

Explanation:

In order to find the correct answer, let's draw the structural formulas of the compounds from the list presented:

Thus, we see that there is a triple bond in nitrogen and acetylene molecules. Those. correct answers 45

Task No. 21

From the proposed list, select two substances whose molecules contain a covalent nonpolar bond.

Characteristics of chemical bonds

The doctrine of chemical bonding forms the basis of all theoretical chemistry. A chemical bond is understood as the interaction of atoms that binds them into molecules, ions, radicals, and crystals. There are four types of chemical bonds: ionic, covalent, metallic and hydrogen. Different types of bonds can be found in the same substances.

1. In bases: between the oxygen and hydrogen atoms in hydroxo groups the bond is polar covalent, and between the metal and the hydroxo group it is ionic.

2. In salts of oxygen-containing acids: between the non-metal atom and the oxygen of the acidic residue - covalent polar, and between the metal and the acidic residue - ionic.

3. In ammonium, methylammonium, etc. salts, between the nitrogen and hydrogen atoms there is a polar covalent, and between ammonium or methylammonium ions and the acid residue - ionic.

4. In metal peroxides (for example, Na 2 O 2), the bond between the oxygen atoms is covalent, nonpolar, and between the metal and oxygen is ionic, etc.

The reason for the unity of all types and types of chemical bonds is their identical chemical nature - electron-nuclear interaction. The formation of a chemical bond in any case is the result of electron-nuclear interaction of atoms, accompanied by the release of energy.

Methods for forming a covalent bond

Covalent chemical bond is a bond that arises between atoms due to the formation of shared electron pairs.

Covalent compounds are usually gases, liquids, or relatively low-melting solids. One of the rare exceptions is diamond, which melts above 3,500 °C. This is 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.

The mechanism of formation of such a bond can be exchange or donor-acceptor.

In most cases, two covalently bonded atoms have different electronegativity and the shared electrons do not belong to the 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. Such a bond is called a 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).

1. The exchange mechanism operates when atoms form shared electron pairs by combining unpaired electrons.

1) H 2 - hydrogen.

The bond occurs due to the formation of a common electron pair by the s-electrons of hydrogen atoms (overlapping s-orbitals).

2) HCl - hydrogen chloride.

The bond occurs due to the formation of a common electron pair of s- and p-electrons (overlapping s-p orbitals).

3) Cl 2: In a chlorine molecule, a covalent bond is formed due to unpaired p-electrons (overlapping p-p orbitals).

4) N ​​2: In the nitrogen molecule, three common electron pairs are formed between the atoms.

Donor-acceptor mechanism of covalent bond formation

Donor has an electron pair acceptor- free orbital that this pair can occupy. In the ammonium ion, all four bonds with hydrogen atoms are covalent: three were formed due to the creation of common electron pairs by the nitrogen atom and hydrogen atoms according to the exchange mechanism, one - through the donor-acceptor mechanism. Covalent bonds are classified by the way the electron orbitals overlap, as well as by their displacement towards one of the bonded atoms. Chemical bonds formed as a result of overlapping electron orbitals along a bond line are called σ - connections(sigma bonds). The sigma bond is very strong.

The p orbitals can overlap in two regions, forming a covalent bond through lateral overlap.

Chemical bonds formed as a result of the “lateral” overlap of electron orbitals outside the bond line, i.e., in two regions, are called pi bonds.

According to the degree of displacement of common electron pairs to one of the atoms they connect, a covalent bond can be polar or non-polar. A covalent chemical bond formed between atoms with the same electronegativity is called non-polar. Electron pairs are not displaced towards any of the atoms, since atoms have the same electronegativity - the property of attracting valence electrons from other atoms. For example,

that is, molecules of simple non-metal substances are formed through a covalent non-polar bond. A covalent chemical bond between atoms of elements whose electronegativity differs is called polar.

For example, NH 3 is ammonia. Nitrogen is a more electronegative element than hydrogen, so the shared electron pairs are shifted towards its atom.

Characteristics of a covalent bond: bond length and energy

The characteristic properties of a covalent bond are its length and energy. Bond length is the distance between atomic nuclei. The shorter the length of a chemical bond, the stronger it is. However, a measure of bond strength is bond energy, which is determined by the amount of energy required to break the bond. It is usually measured in kJ/mol. Thus, according to experimental data, the bond lengths of the H 2, Cl 2 and N 2 molecules, respectively, are 0.074, 0.198 and 0.109 nm, and the bond energies, respectively, are 436, 242 and 946 kJ/mol.

Ions. Ionic bond

There are two main possibilities for an atom to obey the octet rule. The first of these is the formation of ionic bonds. (The second is the formation of a covalent bond, which will be discussed below). When an ionic bond is formed, a metal atom loses electrons, and a non-metal atom gains electrons.

Let's imagine that two atoms “meet”: an atom of a group I metal and a non-metal atom of group VII. A metal atom has a single electron at its outer energy level, while a non-metal atom just lacks one electron for its outer level to be complete. The first atom will easily give the second its electron, which is far from the nucleus and weakly bound to it, and the second will provide it with a free place on its outer electronic level. Then the atom, deprived of one of its negative charges, will become a positively charged particle, and the second will turn into a negatively charged particle due to the resulting electron. Such particles are called ions.

This is a chemical bond that occurs between ions. Numbers showing the number of atoms or molecules are called coefficients, and numbers showing the number of atoms or ions in a molecule are called indices.

Metal connection

Metals have specific properties that differ from the properties of other substances. Such properties are relatively high melting temperatures, the ability to reflect light, and high thermal and electrical conductivity. These features are due to the existence of a special type of bond in metals - a metallic bond.

Metallic bonding is a bond between positive ions in metal crystals due to the attraction of electrons moving freely throughout the crystal. The atoms of most metals at the outer level contain a small number of electrons - 1, 2, 3. These electrons come off easily, and the atoms turn into positive ions. The detached electrons move from one ion to another, binding them into a single whole. Connecting with ions, these electrons temporarily form atoms, then break off again and combine with another ion, etc. A process occurs endlessly, which can be schematically depicted as follows:

Consequently, in the volume of the metal, atoms are continuously converted into ions and vice versa. The bond in metals between ions through shared electrons is called metallic. The metallic bond has some similarities with the covalent bond, since it is based on the sharing of external electrons. However, with a covalent bond, the outer unpaired electrons of only two neighboring atoms are shared, while with a metallic bond, all atoms take part in the sharing of these electrons. That is why crystals with a covalent bond are brittle, but with a metal bond, as a rule, they are ductile, electrically conductive and have a metallic luster.

Metallic bonding is characteristic of both pure metals and mixtures of various metals - alloys in solid and liquid states. However, in the vapor state, metal atoms are connected to each other by a covalent bond (for example, sodium vapor fills yellow light lamps to illuminate the streets of large cities). Metal pairs consist of individual molecules (monatomic and diatomic).

A metal bond also differs from a covalent bond in strength: its energy is 3-4 times less than the energy of a covalent bond.

Bond energy is the energy required to break a chemical bond in all molecules that make up one mole of a substance. The energies of covalent and ionic bonds are usually high and amount to values ​​of the order of 100-800 kJ/mol.

Hydrogen bond

Chemical bond between positively polarized hydrogen atoms of one molecule(or parts thereof) and negatively polarized atoms of highly electronegative elements having shared electron pairs (F, O, N and less often S and Cl), another molecule (or parts thereof) is called hydrogen. The mechanism of hydrogen bond formation is partly electrostatic, partly d honoror-acceptor character.

Examples of intermolecular hydrogen bonding:

In the presence of such a connection, even low-molecular substances can, under normal conditions, be liquids (alcohol, water) or easily liquefied gases (ammonia, hydrogen fluoride). In biopolymers - proteins (secondary structure) - there is an intramolecular hydrogen bond between carbonyl oxygen and the hydrogen of the amino group:

Polynucleotide molecules - DNA (deoxyribonucleic acid) - are double helices in which two chains of nucleotides are linked to each other by hydrogen bonds. In this case, the principle of complementarity operates, i.e., these bonds are formed between certain pairs consisting of purine and pyrimidine bases: the thymine (T) is located opposite the adenine nucleotide (A), and the cytosine (C) is located opposite the guanine (G).

Substances with hydrogen bonds have molecular crystal lattices.

169338 0

Each atom has a certain number of electrons.

When entering into chemical reactions, atoms donate, gain, or share electrons, achieving the most stable electronic configuration. The configuration with the lowest energy (as in noble gas atoms) turns out to be the most stable. This pattern is called the “octet rule” (Fig. 1).

Rice. 1.

This rule applies to everyone types of connections. Electronic connections between atoms allow them to form stable structures, from the simplest crystals to complex biomolecules that ultimately form living systems. They differ from crystals in their continuous metabolism. At the same time, many chemical reactions proceed according to the mechanisms electronic transfer, which play a critical role in energy processes in the body.

A chemical bond is the force that holds together two or more atoms, ions, molecules, or any combination of these.

The nature of a chemical bond is universal: it is an electrostatic force of attraction between negatively charged electrons and positively charged nuclei, determined by the configuration of the electrons of the outer shell of atoms. The ability of an atom to form chemical bonds is called valence, or oxidation state. The concept of valence electrons- electrons that form chemical bonds, that is, located in the highest energy orbitals. Accordingly, the outer shell of the atom containing these orbitals is called valence shell. Currently, it is not enough to indicate the presence of a chemical bond, but it is necessary to clarify its type: ionic, covalent, dipole-dipole, metallic.

The first type of connection isionic connection

According to Lewis and Kossel's electronic valence theory, atoms can achieve a stable electronic configuration in two ways: first, by losing electrons, becoming cations, secondly, acquiring them, turning into anions. As a result of electron transfer, due to the electrostatic force of attraction between ions with charges of opposite signs, a chemical bond is formed, called by Kossel “ electrovalent"(now called ionic).

In this case, anions and cations form a stable electronic configuration with a filled outer electron shell. Typical ionic bonds are formed from cations T and II groups of the periodic system and anions of non-metallic elements of groups VI and VII (16 and 17 subgroups, respectively, chalcogens And halogens). The bonds of ionic compounds are unsaturated and non-directional, so they retain the possibility of electrostatic interaction with other ions. In Fig. Figures 2 and 3 show examples of ionic bonds corresponding to the Kossel model of electron transfer.

Rice. 2.

Rice. 3. Ionic bond in a molecule of table salt (NaCl)

Here it is appropriate to recall some properties that explain the behavior of substances in nature, in particular, consider the idea of acids And reasons.

Aqueous solutions of all these substances are electrolytes. They change color differently indicators. The mechanism of action of indicators was discovered by F.V. Ostwald. He showed that indicators are weak acids or bases, the color of which differs in the undissociated and dissociated states.

Bases can neutralize acids. Not all bases are soluble in water (for example, some organic compounds that do not contain OH groups are insoluble, in particular, triethylamine N(C 2 H 5) 3); soluble bases are called alkalis.

Aqueous solutions of acids undergo characteristic reactions:

a) with metal oxides - with the formation of salt and water;

b) with metals - with the formation of salt and hydrogen;

c) with carbonates - with the formation of salt, CO 2 and N 2 O.

The properties of acids and bases are described by several theories. In accordance with the theory of S.A. Arrhenius, an acid is a substance that dissociates to form ions N+ , while the base forms ions HE- . This theory does not take into account the existence of organic bases that do not have hydroxyl groups.

In accordance with proton According to the theory of Brønsted and Lowry, an acid is a substance containing molecules or ions that donate protons ( donors protons), and a base is a substance consisting of molecules or ions that accept protons ( acceptors protons). Note that in aqueous solutions, hydrogen ions exist in hydrated form, that is, in the form of hydronium ions H3O+ . This theory describes reactions not only with water and hydroxide ions, but also those carried out in the absence of a solvent or with a non-aqueous solvent.

For example, in the reaction between ammonia N.H. 3 (weak base) and hydrogen chloride in the gas phase, solid ammonium chloride is formed, and in an equilibrium mixture of two substances there are always 4 particles, two of which are acids, and the other two are bases:

This equilibrium mixture consists of two conjugate pairs of acids and bases:

1)N.H. 4+ and N.H. 3

2) HCl And Cl ‑

Here, in each conjugate pair, the acid and base differ by one proton. Every acid has a conjugate base. A strong acid has a weak conjugate base, and a weak acid has a strong conjugate base.

The Brønsted-Lowry theory helps explain the unique role of water for the life of the biosphere. Water, depending on the substance interacting with it, can exhibit the properties of either an acid or a base. For example, in reactions with aqueous solutions of acetic acid, water is a base, and in reactions with aqueous solutions of ammonia, it is an acid.

1) CH 3 COOH + H2O ↔ H3O + + CH 3 COO- . Here, an acetic acid molecule donates a proton to a water molecule;

2) NH 3 + H2O ↔ NH 4 + + HE- . Here, an ammonia molecule accepts a proton from a water molecule.

Thus, water can form two conjugate pairs:

1) H2O(acid) and HE- (conjugate base)

2) H 3 O+ (acid) and H2O(conjugate base).

In the first case, water donates a proton, and in the second, it accepts it.

This property is called amphiprotonism. Substances that can react as both acids and bases are called amphoteric. Such substances are often found in living nature. For example, amino acids can form salts with both acids and bases. Therefore, peptides easily form coordination compounds with the metal ions present.

Thus, a characteristic property of an ionic bond is the complete movement of the bonding electrons to one of the nuclei. This means that between the ions there is a region where the electron density is almost zero.

The second type of connection iscovalent connection

Atoms can form stable electronic configurations by sharing electrons.

Such a bond is formed when a pair of electrons is shared one at a time from everyone atom. In this case, the shared bond electrons are distributed equally between the atoms. Examples of covalent bonds include homonuclear diatomic molecules H 2 , N 2 , F 2. The same type of connection is found in allotropes O 2 and ozone O 3 and for a polyatomic molecule S 8 and also heteronuclear molecules hydrogen chloride HCl, carbon dioxide CO 2, methane CH 4, ethanol WITH 2 N 5 HE, sulfur hexafluoride SF 6, acetylene WITH 2 N 2. All these molecules share the same electrons, and their bonds are saturated and directed in the same way (Fig. 4).

It is important for biologists that double and triple bonds have reduced covalent atomic radii compared to a single bond.

Rice. 4. Covalent bond in a Cl 2 molecule.

Ionic and covalent types of bonds are two extreme cases of the many existing types of chemical bonds, and in practice most bonds are intermediate.

Compounds of two elements located at opposite ends of the same or different periods of the periodic system predominantly form ionic bonds. As elements move closer together within a period, the ionic nature of their compounds decreases, and the covalent character increases. For example, the halides and oxides of elements on the left side of the periodic table form predominantly ionic bonds ( NaCl, AgBr, BaSO 4, CaCO 3, KNO 3, CaO, NaOH), and the same compounds of elements on the right side of the table are covalent ( H 2 O, CO 2, NH 3, NO 2, CH 4, phenol C6H5OH, glucose C 6 H 12 O 6, ethanol C 2 H 5 OH).

The covalent bond, in turn, has one more modification.

In polyatomic ions and in complex biological molecules, both electrons can only come from one atom. It is called donor electron pair. An atom that shares this pair of electrons with a donor is called acceptor electron pair. This type of covalent bond is called coordination (donor-acceptor, ordative) communication(Fig. 5). This type of bond is most important for biology and medicine, since the chemistry of the d-elements most important for metabolism is largely described by coordination bonds.

Fig. 5.

As a rule, in a complex compound the metal atom acts as an acceptor of an electron pair; on the contrary, in ionic and covalent bonds the metal atom is an electron donor.

The essence of the covalent bond and its variety - the coordination bond - can be clarified with the help of another theory of acids and bases proposed by GN. Lewis. He somewhat expanded the semantic concept of the terms “acid” and “base” according to the Brønsted-Lowry theory. Lewis's theory explains the nature of the formation of complex ions and the participation of substances in nucleophilic substitution reactions, that is, in the formation of CS.

According to Lewis, an acid is a substance capable of forming a covalent bond by accepting an electron pair from a base. A Lewis base is a substance that has a lone electron pair, which, by donating electrons, forms a covalent bond with Lewis acid.

That is, Lewis's theory expands the range of acid-base reactions also to reactions in which protons do not participate at all. Moreover, the proton itself, according to this theory, is also an acid, since it is capable of accepting an electron pair.

Therefore, according to this theory, the cations are Lewis acids and the anions are Lewis bases. An example would be the following reactions:

It was noted above that the division of substances into ionic and covalent is relative, since complete electron transfer from metal atoms to acceptor atoms does not occur in covalent molecules. In compounds with ionic bonds, each ion is in the electric field of ions of the opposite sign, so they are mutually polarized, and their shells are deformed.

Polarizability determined by the electronic structure, charge and size of the ion; for anions it is higher than for cations. The highest polarizability among cations is for cations of greater charge and smaller size, for example, Hg 2+, Cd 2+, Pb 2+, Al 3+, Tl 3+. Has a strong polarizing effect N+ . Since the influence of ion polarization is two-way, it significantly changes the properties of the compounds they form.

The third type of connection isdipole-dipole connection

In addition to the listed types of communication, there are also dipole-dipole intermolecular interactions, also called van der Waals .

The strength of these interactions depends on the nature of the molecules.

There are three types of interactions: permanent dipole - permanent dipole ( dipole-dipole attraction); permanent dipole - induced dipole ( induction attraction); instantaneous dipole - induced dipole ( dispersive attraction, or London forces; rice. 6).

Rice. 6.

Only molecules with polar covalent bonds have a dipole-dipole moment ( HCl, NH 3, SO 2, H 2 O, C 6 H 5 Cl), and the bond strength is 1-2 Debaya(1D = 3.338 × 10‑30 coulomb meters - C × m).

In biochemistry, there is another type of connection - hydrogen connection that is a limiting case dipole-dipole attraction. This bond is formed by the attraction between a hydrogen atom and a small electronegative atom, most often oxygen, fluorine and nitrogen. With large atoms that have similar electronegativity (such as chlorine and sulfur), the hydrogen bond is much weaker. The hydrogen atom is distinguished by one significant feature: when the bonding electrons are pulled away, its nucleus - the proton - is exposed and is no longer shielded by electrons.

Therefore, the atom turns into a large dipole.

A hydrogen bond, unlike a van der Waals bond, is formed not only during intermolecular interactions, but also within one molecule - intramolecular hydrogen bond. Hydrogen bonds play an important role in biochemistry, for example, to stabilize the structure of proteins in the form of an a-helix, or for the formation of a double helix of DNA (Fig. 7).

Fig.7.

Hydrogen and van der Waals bonds are much weaker than ionic, covalent and coordination bonds. The energy of intermolecular bonds is indicated in table. 1.

Table 1. Energy of intermolecular forces

Note: The degree of intermolecular interactions is reflected by the enthalpy of melting and evaporation (boiling). Ionic compounds require significantly more energy to separate ions than to separate molecules. The enthalpy of melting of ionic compounds is much higher than that of molecular compounds.

The fourth type of connection ismetal connection

Finally, there is another type of intermolecular bonds - metal: connection of positive ions of a metal lattice with free electrons. This type of connection does not occur in biological objects.

From a brief review of bond types, one detail becomes clear: an important parameter of a metal atom or ion - an electron donor, as well as an atom - an electron acceptor, is its size.

Without going into details, we note that the covalent radii of atoms, the ionic radii of metals and the van der Waals radii of interacting molecules increase as their atomic number increases in groups of the periodic table. In this case, the values ​​of the ion radii are the smallest, and the van der Waals radii are the largest. As a rule, when moving down the group, the radii of all elements increase, both covalent and van der Waals.

Of greatest importance for biologists and physicians are coordination(donor-acceptor) bonds considered by coordination chemistry.

Medical bioinorganics. G.K. Barashkov

There is no unified theory of chemical bonds; 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 exchange 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. A covalent bond can be formed by overlapping 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 the total electron density is completely transferred to the 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 non-metal 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 ionic bonds exist in their pure form are 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 the chemical nature of the substance. 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.