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Each atom has a certain number of electrons.

Entering 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.

Chemical bond is the force that holds together two or more atoms, ions, molecules, or any combination thereof.

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 of T and II groups periodic table 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 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 are insoluble organic compounds, not containing - OH groups, 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 With acetic acid, water is a base, and 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, characteristic property ionic bond - the complete movement of two 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 covalent bond can be called 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 limiting cases of the set existing types 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, halides and oxides of elements on the left side 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' theory explains the nature of education 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 brief overview types of bonds, 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, ionic radii of metals and van der Waals radii of interacting molecules increase as their serial number 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

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 - 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.

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 Greek letterδ. 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 in metals special type connection - metal connection.

Metallic bond is a bond between positive ions in metal crystals, carried out 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 lamps yellow light for lighting 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.

A chemical bond is the force that holds the particles that form a substance together.

Depending on the particles that hold these forces, bonds are divided into intramolecular and intermolecular.

Intramolecular bonds.

1. Covalent bond.

A covalent bond is a shared pair of electrons between two nonmetal atoms.

Let us consider the example of a hydrogen molecule (H2), in which a covalent bond is realized.

A hydrogen molecule consists of two hydrogen atoms (H), which have one electron at the outer energy level:

Atoms tend to completely fill their orbitals. This is why two atoms come together. They share their unpaired electrons, creating a shared electron pair. The electrons became paired:

This shared electron pair is a covalent chemical bond. A covalent bond is indicated either by a line connecting the atoms or by two dots that indicate a shared electron pair:

Imagine that there are two desk neighbors. These are two atoms. They need to draw a picture that has red and Blue colour. They have a common pair of pencils (one red, the other blue) - this is a common electronic pair. Both desk neighbors use these pencils. Thus, these two neighbors are connected by a common pair of pencils, i.e. covalent chemical bond.

There are two mechanisms for the formation of covalent chemical bonds.

1. Exchange mechanism of covalent bond formation.

In this case, each atom provides electrons to form a covalent bond. We looked at this mechanism when we got acquainted with the covalent bond:

1. Donor-acceptor mechanism of covalent bond formation.

In this case, the common electron pair, so to speak, is unequal.

One atom has an LEP - a lone electron pair (two electrons in one orbital). And he provides it entirely for the formation of a covalent bond. This atom is called donor– because it provides both electrons to form a chemical bond.

And the second atom has only a free orbital. It accepts an electron pair. This atom is called acceptor– it accepts both electrons.

A classic example is the formation of ammonium ion NH 4 +. It is formed by the interaction of H + ion and ammonia (NH 3). The hydrogen cation H+ is an empty s orbital.

This particle will be an acceptor.

The nitrogen volume in ammonia has an LEP (lone electron pair).

The nitrogen atom in ammonia will be a donor:

In this case, both the blue and red pencil were brought by one of the desk neighbors. He “treats” the second one. And they both use pencils.

The specific reactions that produce such an ion will be discussed later in the appropriate sections. For now, you just need to remember the principle by which a covalent bond is formed through the donor-acceptor mechanism.

There are two types of covalent bonds. There are covalent polar and nonpolar bonds.

Covalent polar bond occurs between atoms non-metals with different electronegativity values. That is, between different non-metal atoms.

An atom with a high electronegativity value will pull the common electron pair towards itself.

Covalent non-polar bond occurs between atoms nonmetals with the same electronegativity values. This condition is met if a bond occurs between atoms one chemical element-non-metal. Because different atoms may have electronegativity very close to each other, but they will still be different.

The shared electron pair will not move towards any atom, since each atom “pulls” it with the same force: the shared electron pair will be in the middle.

And of course, a covalent bond can be single, double or triple:

1. Ionic bond.

An ionic bond occurs between metal and non-metal atoms. Since a metal and a non-metal have a large difference in electronegativity, the electron pair fully is drawn towards a more electronegative atom - a non-metal atom.

The configuration of a completely filled energy level is not achieved through the formation of a common electron pair. The nonmetal takes an electron from the metal and fills its outer level. But it is easier for metal to give up its electrons (it has few of them) and it also has a completely filled level.

Thus, the metal, having given up electrons, acquires a negative charge and becomes a cation. And a non-metal, having received electrons, acquires a negative charge and becomes an anion.

An ionic chemical bond is electrostatic attraction of a cation to an anion.

Ionic bonding occurs in metal salts, oxides and hydroxides. And in other substances in which a metal atom is bonded to a non-metal atom (Li 3 N, CaH 2).

Here you should pay attention to one important feature: the ionic bond takes place between the cation and anions in of all salts. We most generally describe it as a metal-nonmetal bond. But it is necessary to understand that this is done only for simplification. The salt may not contain a metal atom. For example, in ammonium salts (NH 4 Cl, (NH 4) 2 SO 4. The ammonium ion NH 4 + is attracted to the anion of the salt - this is an ionic bond.

Frankly speaking, there is no ionic bonding. An ionic bond is just an extreme degree of polar covalent bonding. Any bond has its own percentage of “ionicity” - this depends on the difference in electronegativity. But in school curriculum, and even more so in the Unified State Exam requirements, ionic and covalent bonds are completely two different concepts that cannot be confused.

1. Metal connection.

All the splendor of the metal connection can only be understood together with the metal crystal lattice. Therefore, we will consider the metallic bond later, when we disassemble crystal lattices.

All you need to know for now is that the metallic bond is realized in simple substances– metals.

Intermolecular bonds.

Intermolecular bonds are much weaker than intramolecular bonds, since they do not involve a common electron pair.

1. Hydrogen bonds.

Hydrogen bonds occur in substances in which a hydrogen atom is bonded to an atom with a high electronegativity value (F, O, Cl, N).

In this case, the bond with hydrogen atoms becomes highly polar. An electron pair moves from the hydrogen atom to a more electronegative atom. Due to this displacement, a partial positive charge (δ+) appears on the hydrogen, and a partial negative charge (δ-) appears on the electronegative atom.

For example, in a hydrogen fluoride molecule:

The δ+ of one molecule is attracted by the δ- of another molecule. This is a hydrogen bond. Graphically in the diagram it is indicated by a dotted line:

A water molecule can form four hydrogen bonds:

Hydrogen bonds provide more low temperatures boiling and melting of substances between the molecules of which they arise. Compare hydrogen sulfide and water. Water contains hydrogen bonds - it is a liquid under normal conditions, while hydrogen sulfide is a gas.

1. Van der Waals forces.

These are very weak intermolecular interactions. The principle of occurrence is the same as that of hydrogen bonds. Very weak partial charges arise from vibrations of a common electron pair. And momentary forces of attraction arise between these charges.

Rarely chemical substances consist of individual, unrelated atoms of chemical elements. Under normal conditions, only a small number of gases called noble gases have this structure: helium, neon, argon, krypton, xenon and radon. Most often, chemical substances do not consist of isolated atoms, but of their combinations into various groups. Such associations of atoms can number a few, hundreds, thousands, or even more atoms. The force that holds these atoms in such groups is called chemical bond.

In other words, we can say that a chemical bond is an interaction that provides the connection of individual atoms into more complex structures (molecules, ions, radicals, crystals, etc.).

The reason for the formation of a chemical bond is that the energy of more complex structures is less than the total energy of the individual atoms that form it.

So, in particular, if the interaction of atoms X and Y produces a molecule XY, this means that the internal energy of the molecules of this substance is lower than the internal energy of the individual atoms from which it was formed:

E(XY)< E(X) + E(Y)

For this reason, when chemical bonds are formed between individual atoms, energy is released.

Electrons of the outer electron layer with the lowest binding energy with the nucleus, called valence. For example, in boron these are electrons of the 2nd energy level - 2 electrons per 2 s- orbitals and 1 by 2 p-orbitals:

When a chemical bond is formed, each atom tends to obtain the electronic configuration of noble gas atoms, i.e. so that there are 8 electrons in its outer electron layer (2 for elements of the first period). This phenomenon is called the octet rule.

It is possible for atoms to achieve the electron configuration of a noble gas if initially single atoms share some of their valence electrons with other atoms. In this case, common electron pairs are formed.

Depending on the degree of electron sharing, covalent, ionic and metallic bonds can be distinguished.

Covalent bond

Covalent bonds most often occur between atoms of nonmetal elements. If the nonmetal atoms forming a covalent bond belong to different chemical elements, such a bond is called a polar covalent bond. The reason for this name lies in the fact that atoms of different elements also have different abilities to attract a common electron pair. Obviously, this leads to a displacement of the common electron pair towards one of the atoms, as a result of which a partial negative charge is formed on it. In turn, a partial positive charge is formed on the other atom. For example, in a hydrogen chloride molecule the electron pair is shifted from the hydrogen atom to the chlorine atom:

Examples of substances with polar covalent bonds:

CCl 4, H 2 S, CO 2, NH 3, SiO 2, etc.

A covalent nonpolar bond is formed between nonmetal atoms of the same chemical element. Since the atoms are identical, their ability to attract shared electrons is also the same. In this regard, no displacement of the electron pair is observed:

The above mechanism for the formation of a covalent bond, when both atoms provide electrons to form common electron pairs, is called exchange.

There is also a donor-acceptor mechanism.

When a covalent bond is formed by the donor-acceptor mechanism, a shared electron pair is formed due to the filled orbital of one atom (with two electrons) and the empty orbital of another atom. An atom that provides a lone pair of electrons is called a donor, and an atom with a vacant orbital is called an acceptor. Atoms that have paired electrons, for example N, O, P, S, act as donors of electron pairs.

For example, according to the donor-acceptor mechanism, the formation of the fourth covalent N-H connections in the ammonium cation NH 4 +:

In addition to polarity, covalent bonds are also characterized by energy. Bond energy is the minimum energy required to break a bond between atoms.

The binding energy decreases with increasing radii of bonded atoms. Since we know that atomic radii increase down the subgroups, we can, for example, conclude that the strength of the halogen-hydrogen bond increases in the series:

HI< HBr < HCl < HF

Also, the bond energy depends on its multiplicity - the greater the bond multiplicity, the greater its energy. Bond multiplicity refers to the number of shared electron pairs between two atoms.

Ionic bond

An ionic bond can be considered as an extreme case of a polar covalent bond. If in a covalent-polar bond the common electron pair is partially shifted to one of the pair of atoms, then in an ionic bond it is almost completely “given” to one of the atoms. The atom that donates electron(s) acquires a positive charge and becomes cation, and the atom that has taken electrons from it acquires a negative charge and becomes anion.

Thus, an ionic bond is a bond formed by the electrostatic attraction of cations to anions.

The formation of this type of bond is typical during the interaction of atoms typical metals and typical non-metals.

For example, potassium fluoride. The potassium cation is formed by the removal of one electron from a neutral atom, and the fluorine ion is formed by the addition of one electron to the fluorine atom:

An electrostatic attraction force arises between the resulting ions, resulting in the formation of an ionic compound.

When a chemical bond was formed, electrons from the sodium atom passed to the chlorine atom and oppositely charged ions were formed, which have a complete external energy level.

It has been established that electrons from the metal atom are not completely detached, but are only shifted towards the chlorine atom, as in a covalent bond.

Most binary compounds that contain metal atoms are ionic. For example, oxides, halides, sulfides, nitrides.

Ionic bonding also occurs between simple cations and simple anions (F −, Cl −, S 2-), as well as between simple cations and complex anions (NO 3 −, SO 4 2-, PO 4 3-, OH −). Therefore, ionic compounds include salts and bases (Na 2 SO 4, Cu(NO 3) 2, (NH 4) 2 SO 4), Ca(OH) 2, NaOH).

Metal connection

This type of bond is formed in metals.

Atoms of all metals have electrons in their outer electron layer that have a low binding energy with the nucleus of the atom. For most metals, the process of losing outer electrons is energetically favorable.

Due to such a weak interaction with the nucleus, these electrons in metals are very mobile and the following process continuously occurs in each metal crystal:

M 0 - ne - = M n + , where M 0 is a neutral metal atom, and M n + is a cation of the same metal. The figure below provides an illustration of the processes taking place.

That is, electrons “rush” across a metal crystal, detaching from one metal atom, forming a cation from it, joining another cation, forming a neutral atom. This phenomenon was called “electron wind,” and the collection of free electrons in a crystal of a nonmetal atom was called “electron gas.” This type of interaction between metal atoms is called a metallic bond.

Hydrogen bond

If a hydrogen atom in a substance is bonded to an element with high electronegativity (nitrogen, oxygen, or fluorine), that substance is characterized by a phenomenon called hydrogen bonding.

Since a hydrogen atom is bonded to an electronegative atom, a partial positive charge is formed on the hydrogen atom, and a partial negative charge is formed on the atom of the electronegative element. In this regard, electrostatic attraction becomes possible between a partially positively charged hydrogen atom of one molecule and an electronegative atom of another. For example, hydrogen bonding is observed for water molecules:

It is the hydrogen bond that explains the abnormally high melting point of water. In addition to water, strong hydrogen bonds are also formed in substances such as hydrogen fluoride, ammonia, oxygen-containing acids, phenols, alcohols, and amines.