What types of communication are characteristic of simple substances. Chemical communications

Chemical communications

All interactions leading to unification chemical particles (atoms, molecules, ions, etc.) in substances are divided into chemical bonds and intermolecular bonds (intermolecular interactions).

Chemical ties - Communications directly between atoms. There are ionic, covalent and metallic communications.

Intermolecular ties - Communication between molecules. This is a hydrogen bond, ion-dipole communication (due to the formation of this connection occurs, for example, the formation of hydrate ion shell), dipole-dipole (by the formation of the formation of this connection, the molecules of polar substances are combined, for example, in liquid acetone), etc.

Ion communication - Chemical bond formed by electrostatic attraction of differently charged ions. In binary compounds (compounds of two elements), it is formed in the case when the dimensions of the binding atoms are very different from each other: some atoms are large, other small - that is, some atoms easily give electrons, while others tend to take them (usually these are the atoms of elements forming them Typical metals and atoms of elements forming typical non-metals); Electricity of such atoms is also very different.
The ion connection is non-directional and not satiable.

Covalent communication - Chemical bond arising from the formation of a common pair of electrons. Covalent bond is formed between small atoms with the same or close radii. Prerequisite - the presence of unpaired electrons in both binding atoms (exchange mechanism) or a vapor pair in one atom and free orbital (donor-acceptor mechanism):

According to the number of general electronic couples, covalent bonds are divided into
• simple (single) - one pair of electrons,
• double - two pairs of electrons,
• triple - Three pairs of electrons.

Double and triple ties are called multiple connections.

On the distribution of the electron density between the covalent bonds binding atoms is divided into notolar and polar. The non-polar connection is formed between the same atoms, the polar - between different.

Electricity - measure of the ability of an atom in the substance to attract general electronic pairs to itself.
Electronic pairs of polar ties are shifted towards more electronegative elements. The displacement of electronic pairs is called polarization of communication. Partial (excessive) charges formed during the polarization are + and -, for example:.

According to the nature of the overlapping of electronic clouds ("orbitals"), a covalent bond is divided into-β-β-cell.
- Communication is formed due to the direct overlap of electronic clouds (along a direct connecting nucleus), -cuzv - due to lateral overlap (on both sides of the plane in which the nuclei of atoms).

Covalent bond has or saturation, as well as polarizability.
To explain and predict the mutual direction of covalent bonds, a hybridization model is used.

Hybridization of atomic orbitals and electronic clouds - the estimated leveling of atomic orbitals by energy, and electron clouds in form when the covalent bond atom is produced.
Most often there are three types of hybridization: sp.-, sp. 2 I. sp. 3-hybridization. For example:
sp.-Hypebridization - in C 2 H 2 molecules, BEH 2, CO 2 (linear structure);
sp. 2-hybridization - in C 2 H 4 molecules, C 6 H 6, BF 3 (flat triangular shape);
sp. 3-hybridization - in CCl 4, Sih 4, CH 4 molecules (tetrahedral form); NH 3 (pyramidal form); H 2 O (corner shape).

Metal communication - chemical bond formed by the generalization of valence electrons of all associated metallic crystal atoms. As a result, a single electronic cloud of a crystal is formed, which is easily shifted under the action of electrical voltage - hence the high electrical conductivity of metals.
Metal communication is formed in the case when the binding atoms are large and therefore tend to give electrons. Simple substances with metallic bond - metals (Na, Ba, Al, Cu, Au, etc.), complex substances - intermetallic compounds (AlCr 2, Ca 2 Cu, Cu 5 Zn 8, etc.).
Metal connection does not have a focus of saturation. It is preserved in metal melts.

Hydrogen communications - Intermolecular communication, formed by partial accepting a pair of electrons of a highly elected-negative atom of a hydrogen atom with a large positive partial charge. It is formed in those cases where in one molecule there is an atom with a mean pair of electrons and high electronegitability (F, O, N), and a hydrogen atom associated with a strong polar bond with one of such atoms. Examples of intermolecular hydrogen ties:

H-O - H ··· OH 2, H-O - H ··· NH 3, H-O - H ··· F - H, H-F ··· H-F.

Intramolecular hydrogen bonds exist in polypeptide molecules, nucleic acids, proteins, etc.

The strength measure of any communication is the energy of communication.
Communication energy - the energy required for breaking this chemical bond In 1 mole of substance. Unit of measurements - 1 kJ / mol.

The energy of the ionic and covalent bond is one order, the energy of hydrogen bonds is an order of magnitude less.

The covalent bond energy depends on the size of the binding atoms (communication length) and the multiplicity of communication. The less atoms and more multiplicity of communication, the greater its energy.

The energy of ionic communication depends on the size of the ions and from their charges. The fewer ions and more their charge, the greater the binding energy.

Structure of matter

By type of structure, all substances are divided into molecular and nemolecular. Among organic substances Molecular substances predominate, among inorganic - non-elecular.

According to the type of chemical bond, the substances are divided into substances with covalent bonds, substances with ionic bonds (ionic substances) and substances with metallic ties (metals).

Substances with covalent bonds can be molecular and neneticular. This significantly affects their physical properties.

Molecular substances consist of molecules interconnected with weak intermolecular bonds, these include: H 2, O 2, N 2, CL 2, BR 2, S 8, P 4 and other simple substances; CO 2, SO 2, N 2 O 5, H 2 O, HCl, HF, NH 3, CH 4, C 2 H 5 OH, organic polymers and many other substances. These substances do not have high strength, have low melting and boiling points, do not conduct electricitySome of them are soluble in water or other solvents.

Nemocecular substances with covalent bonds or atomic substances (diamond, graphite, Si, SiO 2, SiC and others) form very strong crystals (exception - layered graphite), they are insoluble in water and other solvents, have high melting and boiling temperatures, most of They do not conduct electric current (except graphite with electrical conductivity, and semiconductors - silicon, Germany, etc.)

All ionic substances naturally are non-elastic. These are solid refractory substances, solutions and melts of which are carried out electrical out. Many of them are soluble in water. It should be noted that in ionic substances whose crystals consist of complex ions, there are covalent bonds, for example: (Na +) 2 (SO 4 2-), (K +) 3 (PO 4 3-), (NH 4 + ) (NO 3-), etc. Covalent bonds are associated atoms from which complex ions consist.

Metals (substances with metallic ties) Very diverse in their physical properties. Among them there is a liquid (HG), very soft (Na, k) and very solid metals (W, NB).

The characteristic physical properties of metals are their high electrical conductivity (in contrast to semiconductors, decreases with increasing temperature), high heat capacity and plasticity (in pure metals).

In the hard state, almost all substances consist of crystals. By type of structure and type of chemical bond crystals ("crystalline lattices") divide on atomic (non-eracular substances crystals with covalent bond), ionic (crystals of ion substances) molecular (crystals of molecular substances with covalent bond) and metal (Crystals of substances with metallic communications).

Tasks and tests on the topic "Topic 10." Chemical Communication. The structure of the substance "."

• Types of chemical bond - structure of substance 8-9 class

  Lessons: 2 tasks: 9 tests: 1

• Tasks: 9 tests: 1

Having worked out this topic, you must learn the following concepts: Chemical Communications, Intermolecular Communication, Ion Communications, Covalent Communication, Metal Communications, Hydrogen Communications, Easy Communication, Double Communication, Triple Communications, Multiple Communication, Non-Polar Communication, Polar Communication, Electricity, Communication Polarization , - and-α, hybridization of atomic orbitals, bond energy.

You should know the classification of substances by the type of structure, by type of chemical bond, the dependence of the properties of simple and complex substances on the type of chemical bond and the type of "crystal lattice".

You should be able to: determine the type of chemical bond in the substance, the type of hybridization, to draw up the formation of relations, to use the concept of electronegativity, a number of electrical negotiations; Know how electronenence is changing chemical elements One period, and one group to determine the polarity of a covalent bond.

Making sure that everything you need is assimilated, go to the tasks. We wish you success.

• O. S. Gabrielyan, G. G. Lysova. Chemistry 11 CL. M., Drop, 2002.
• E. Rudzitis, F. G. Feldman. Chemistry 11 CL. M., Enlightenment, 2001.

The unified theory of chemical bond does not exist, conditionally chemical bonds are divided into a covalent (universal type of communication), ionic (private case of covalent bond), metallic and hydrogen.

Covalent communication

The formation of covalent communication is possible in three mechanisms: exchange, donor-acceptor and dative (Lewis).

According to exchange mechanism The formation of covalent communications occurs due to the generalization of common electronic pairs. At the same time, each atom seeks to purchase an inert gas shell, i.e. Get a completed external energy level. The formation of chemical bonds on exchange type is depicted using Lewis formulas, in which each valence electro atom is depicted by points (Fig. 1).

Fig. 1 Education of a covalent bond in the HCl molecule on the exchange mechanism

With the development of the theory of the structure of the atom and quantum mechanics The formation of a covalent bond is represented as overlapping electronic orbitals (Fig. 2).

Fig. 2. Education of covalent communication due to overlapping electronic clouds

The greater the overlapping of atomic orbitals, the stronger the connection, less the length of the communication and its more energy. Covalent bond can be formed by overlapping different orbital. As a result of overlapping S-S, S-P orbitals, as well as D-D, P-P, d-P orbital Side blades occurs education - communication. Perpendicular to the line connecting the core of 2 atoms is formed - the connection. One - and one - the relationship is capable of forming a multiple (double) covalent bond, characteristic of the organic substances of the class of alkenes, alkadiennes, etc. One - and two - connections form a multiple (triple) covalent bond, characteristic of the organic substances of the alkine class (acetylenes).

Education covalent donor-acceptor mechanism Consider on the example of ammonium cation:

NH 3 + H + \u003d NH 4 +

7 N 1S 2 2S 2 2P 3

The nitrogen atom has a free marginal pair of electrons (electrons not participating in the formation of chemical bonds inside the molecule), and hydrogen cation is a free orbital, so they are a donor and an electron acceptor, respectively.

The duty mechanism for the formation of a covalent connection will consider on the example of the chlorine molecule.

17 Cl 1S 2 2S 2 2P 6 3S 2 3P 5

The chlorine atom has a free marginal pair of electrons and vacant orbital, therefore, can show properties and donor and acceptor. Therefore, in the formation of a chlorine molecule, one chlorine atom acts as a donor, and the other is an acceptor.

Main characteristics of covalent bond are: saturability (rich connections are formed when the atom attaches to itself so much electrons as its valence capabilities allow; unsaturated bonds are formed when the number of electrons connected is less than the flu valence capabilities); Direction (this value is associated with the geometry of the molecule and the concept of "valence angle" - angle between connections).

Ion communication

There are no compounds with a pure ion bond, although this is a chemically related state of atoms, in which the steady electron environment of the atom is created with a full transition of a general electron density to an atom of a more electronegative element. Ionic communication is possible only between the atoms of electronegative and electropositive elements that are in a state of variemetically charged ions - cations and anions.

Definition

Ion Called electrically charged particles formed by separating or attaching an electron to the atom.

When the electron is transmitted, the atoms of metals and non-metals tend to form a stable configuration of the electronic shell around their kernel. The nenetal atom creates a shell of the subsequent inert gas around its kernel, and the metal atom is the previous inert gas (Fig. 3).

Fig. 3. Education of ion communication on the example of sodium chloride molecule

Molecules in which in pure form there is an ion connection are found in a vapor state of the substance. The ionic relationship is very durable, in connection with this substance with this bond have a high melting point. In contrast to covalent for ionic communication, direction and saturation is not characteristic, since the electric field created by ions acts the same to all ions due to spherical symmetry.

Metal connection

Metal bond is realized only in metals - this is the interaction that holds the metals atoms in a single lattice. Only valence electrons of metal atoms belonging to all its volume are involved in the formation of communication. In metals from atoms, electrons are constantly separated, which move throughout the entire mass of the metal. Metal atoms, devoid of electrons, are converted into positively charged ions that seek to accept moving electrons. This continuous process forms inside the metal so-called "electronic gas", which firmly connects all atoms of metal (Fig. 4).

The metal bond is strong, therefore, the metals are characterized by a high melting point, and the presence of "electronic gas" gives the metals with pupidity and plasticity.

Hydrogen communications

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 associated with an atom with high electronegitability (O, N, S). The occurrence of the hydrogen bond depends on two reasons, firstly, the hydrogen atom associated with an electronegative atom does not have electrons and can easily be embedded in electronic clouds of other atoms, and, secondly, possessing a valence S-orbital, a hydrogen atom can take a watery pair electrons of the electronegative atom and to form a bond with a donorly acceptor mechanism with it.

Any interaction between atoms is possible only in the presence of a chemical connection. Such a connection is the cause of the formation of a stable multiatomic system - molecular ion, molecules, crystal lattice. Durable chemical bond requires a lot of energy for the break, so it is the basic value for measuring the strength of the communication.

Chemical Communication Conditions

The formation of chemical bond is always accompanied by excretion of energy. This process is due to the reduction. potential energy Systems of interacting particles - molecules, ions, atoms. The potential energy of the resulting system of interacting elements is always less than the energy of unrelated outgoing particles. Thus, the basis for the occurrence of chemical bonds in the system is the decline in the potential energy of its elements.

Nature of chemical interaction

Chemical bond is a consequence of the interaction of electromagnetic fields arising around electrons and nuclei of atoms of those substances that participate in the formation of a new molecule or crystal. After opening the theory of the structure of the atom, the nature of this interaction has become more accessible to study.

For the first time the idea of \u200b\u200bOb. electrical Nature. Chemical Communications emerged from the English physics of Davy, which suggested that the molecules are formed due to the electrical attraction of multi-dimensionally charged particles. This idea was interested in the Swedish chemist and natural resource I.Ya. Blycelus, who developed an electrochemical theory of chemical bond.

The first theory explaining the processes of chemical interaction of substances was imperfect, and over time it had to refuse.

Theory of Butlerova

A more successful attempt to explain the nature of the chemical bond substances was taken by Russian scientist A.M. Butlerov. The basis of its theory, this scientist laid such assumptions:

• Atoms in the connected state are associated with each other in a certain order. The change in this order is the cause of the formation of a new substance.
• Atoms are binding to each other according to the laws of valence.
• The properties of the substance depends on the order of the compound of atoms in the molecule of the substance. Another order of arrangement becomes the reason for changing the chemical properties of the substance.
• Atoms related to each other are most strongly affected by each other.

Butlerova's theory explained properties chemical substances Not only by their composition, but also by the order of atoms. Such an internal order A.M. Butlers called the "chemical structure".

The theory of Russian scientist made it possible to bring order in the classification of substances and provided the opportunity to determine the structure of molecules by their chemical properties. The theory gave an answer to the question: why molecules containing the same number of atoms have different chemical properties.

Prerequisites for creating chemical communication theories

In his theory chemical structure Butlers did not touch the question of what a chemical connection is. For this, then there was too little data on the inner structure of the substance. Only after the opening of the planetary model of the atom, the American scientist Lewis began to develop a hypothesis that chemical bond arises through the formation of an electronic pair, which simultaneously belongs to two atoms. Subsequently, this idea has become a foundation for developing a covalent communication theory.

Covalent Chemical Communication

Sustainable chemical compound can be formed when overlapping electronic clouds of two adjacent atoms. The result of such a mutual intersection becomes increasing electron density in the inter-seat space. The nuclei of atoms, as you know, are charged positively, and therefore trying to attract a negatively charged electronic cloud as close as possible. This attraction is much stronger than repulsion strength between two positively charged nuclei, so this connection is stable.

For the first time, the calculations of the chemical bond were performed by the chemists of Gateler and London. They addressed the relationship between two hydrogen atoms. The simplest visual view of it may look like this:

As can be seen, the electron pair occupies a quantum place in both hydrogen atoms. Such a two-center placing electron was called "Covalent Chemical Communication". Covalent bond is typical for molecules of simple substances and their unmetall connections. The substances created as a result of a covalent bond usually do not conduct an electric current or are semiconductors.

Ion communication

The chemical connection of the ion type occurs with the mutual electrical attraction of the two oppositely charged ions. Ions may be simple, consisting of one atom of the substance. In compounds of this type, simple ions are most often positively charged atoms of metals 1.2 groups that have lost their electron. The formation of negative ions is inherent in the atoms of typical non-metals and the bases of their acids. Therefore, among typical ionic compounds there are many alkali metal halides, such as CSF, NaCl, and others.

Unlike covalent communication, the ion does not have a saturation: a different number of oppositely charged ions can join the Ion or group of ions. The number of attached particles is limited only by linear dimensions of interacting ions, as well as a condition in which the forces of attraction of oppositely charged ions should be greater than the pushing force of the same charged particles participating in the connection ion type.

Hydrogen communications

Even before the creation of the theory of the chemical structure, the experimental means it was observed that hydrogen compounds with various non-metals have several unusual properties. For example, the boiling point of fluoride hydrogen and water is significantly higher than it could be expected.

These and other features of hydrogen compounds can be explained by the ability of an H + atom to form another chemical bond. This type of connection was called "Hydrogen Communication". The causes of the occurrence of hydrogen bonds lie in the properties of electrostatic forces. For example, in the fluoroor molecule, the general electronic cloud is so shifted towards fluorine as the space around the atom of this substance saturated negative electric field. Around the hydrogen atom, devoid of its only electron, the field is much weaker, and has a positive charge. As a result, there is an additional relationship between the positive fields of the H + electronic clouds and the negative F -.

Chemical connection of metals

Atoms of all metals are located in space in a certain way. The order of the atoms of metals is called a crystal lattice. At the same time, electrons of various atoms weakly interact with each other, forming a general electronic cloud. This type of interaction between atoms and electrons was called "Metal Communication".

It is free movement of electrons in metals that can be explained physical properties Metal substances: electrical conductivity, thermal conductivity, strength, federation and others.

Each atom has some number of electrons.

Entering B. chemical reactionsThe atoms give, acquire, or generalize electrons achieving the most stable electronic configuration. The most stable is the configuration with the lowest energy (as in the atoms of noble gases). This pattern is called "octet rules" (Fig. 1).

Fig. one.

This rule applies to all types of connections. Electronic connections Between atoms allow them to form sustainable structures, from the simplest crystals to complex biomolecules forming, ultimately live systems. They differ from crystals with continuous metabolism. In this case, many chemical reactions proceed by mechanisms electronic transferwho play a crucial role in the energy processes in the body.

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

The nature of the chemical bond is universal: this is an electrostatic force of attraction between negatively charged electrons and positively charged nuclei, determined by the configuration of electrons of the outer shell of atoms. The ability of an atom to form chemical connections is called valence, or degree of oxidation. With valence associated with the concept of valence electrons - electrons forming chemical bonds, that is, located at the most high-energy orbital. Accordingly, the outer shell of the atom containing these orbital is called valentine's sheath. Currently, it is not enough to indicate the presence of a chemical bond, and it is necessary to clarify its type: ionic, covalent, dipole-dipole, metallic.

First type of communication -ionic communication

In accordance with the electronic theory of Lewis and Kossel's valence, atoms can achieve a stable electronic configuration in two ways: first, losing electrons, turning into cations, secondly, acquiring them, turning into anions. As a result of electronic transfer, thanks to the electrostatic strength of attraction between ions with charges of the opposite sign, a chemical bond called the cossel " electrovalent"(Now it is called ionic).

In this case, the anions and cations form a stable electronic configuration with a filled outdoor electronic shell. Typical ionic bonds are formed from cations T and II groups of periodic system and anions of non-metallic elements VI and VII groups (16 and 17 subgroups - respectively, chalcogenovand halogen). Communication in ionic compounds are unsaturated and non-directional, so the possibility of electrostatic interaction with other ions is preserved. In fig. 2 and 3 are examples of ionic connections that correspond to the electronic co-axle transfer models.

Fig. 2.

Fig. 3. Ion connection in the table salt molecule (NaCl)

Here it is appropriate to remind some properties that explain the behavior of substances in nature, in particular, consider the idea of acidsand basins.

The aqueous solutions of all these substances are electrolytes. They change in different ways indicators. The mechanism of action of indicators was opened by F.V. Ostelad. It showed that the indicators are weak acids or bases, the painting of which is dissolved in the unfair and dissociated states.

The bases are able to neutralize the acids. Not all bases are soluble in water (for example, some are insoluble organic compoundsnot containing - on-groups, in particular, triethylamine N (C 2N 5) 3); Soluble bases are called alkalis.

Aqueous solutions acids enter the 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 salt formation, Co. 2 I. N. 2 O..

The properties of acids and bases describe several theories. In accordance with the theory of S.A. Arrhenius, acid is a substance that dissociates with the formation of ions N. +, whereas the base forms ions IS HE -. This theory does not take into account the existence of organic bases that do not have hydroxyl groups.

In accordance with S. protonnathe theory of Brensted and Lowry, acid is a substance containing molecules or ions that give protons ( donorsprotons), and the base is a substance consisting of molecules or ions taking protons ( acceptorsprotons). Note that in aqueous solutions hydrogen ions exist in hydrated form, that is, in the form of hydroxonium ions H 3 O. +. This theory describes the reaction not only with water and hydroxide ions, but also carried out in the absence of a solvent or with a non-aqueous solvent.

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

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

1) NH. 4 + I. NH. 3

2) HCLand Cl ‑

Here in each conjugate pair of acid and the base differ on one proton. Each acid has a conjugate base. A weak conjugate base corresponds to severe acid, and a severe conjugate base.

The theory of Brensteda Lowei allows you to explain the uniqueness of the role of water for the livelihood of the biosphere. Water, depending on the substance interacting with it, can exhibit properties or acids, or base. For example, in reactions with aqueous solutions of acetic acid, water is the base, and with an aqueous solutions of ammonia - acid.

1) CH 3 coxy + H 2 O. ↔ H 3 O. + + CH 3 SOO -. Here, the molecule of acetic acid is by the proton of the water molecule;

2) NH 3. + H 2 O. ↔ NH 4. + + IS HE -. Here, the ammonia molecule accepts the proton from the water molecule.

Thus, water can form two conjugate pairs:

1) H 2 O. (acid) and IS HE - (conjugate base)

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

In the first case, the water is diagnosed with proton, and in the second - accepts it.

This property is called amphiprotonality. Substances that can enter into reactions in quality and acids and grounds are called amphoteric. In the wilderness, such substances are common. For example, amino acids are capable of forming salts and with acids, and with bases. Therefore, peptides easily form coordination compounds with those present metal ions.

Thus, the characteristic property of the ion connection is the complete movement of the naps of the binding electrons to one of the cores. This means that there is an area between ions, where the electronic density is almost zero.

Second Communication Type -covalent communication

Atoms can form stable electronic configuration by combining electrons.

Such a connection is formed when the pair of electrons is generalized by one from everyone Atom. In this case, the Common Communication Electrons are distributed between atoms equally. Examples of covalent communication can be called gomoidernydihomatomy molecules N. 2 , N. 2 , F. 2. The same type of communication is available at allotropics O. 2 and ozone O. 3 and in the polyatomic molecule S. 8, as well as heteroantore molecules chloroodor Nsl, carbon dioxide Co. 2, metha SH 4, ethanol FROM 2 N. 5 IS HE, sulfur hexafluoride Sf. 6, acetylene FROM 2 N. 2. In all these molecules, electrons are equally common, and their connections are saturated and directed equally (Fig. 4).

For biologists, it is important that in double and triple bonds, covalent radii atoms are reduced compared to single bond.

Fig. four. Covalent bond in the CL 2 molecule.

The ionic and covalent types of connections are two limiting cases of many existing types of chemical bonds, and in practice most intermediate bonds.

The compounds of two elements located in the opposite ends of one or different periods of the Mendeleev system are preferably forming ionic ties. As it rates the elements within the period, the ionic nature of their compounds is reduced, and covalent - increases. For example, halides and oxides of the elements of the left periodic table form primarily ion connections ( NaCl, Agbr, Baso 4, Caco 3, Kno 3, Cao, Naoh), and the same connections of the elements of the right part of the table - covalent ( H 2 O, CO 2, NH 3, NO 2, CH 4, phenol C 6 H 5 OH, glucose C 6 H 12 O 6, ethanol From 2N 5 he).

A covalent bond, in turn, has another modification.

In polyhytomic ions and in complex biological molecules, both electrons can occur only from oneatom. It is called donorelectronic couple. Atom, a compatible with a donor of this pair of electrons, is called acceptorelectronic couple. Such a kind of covalent communication is named coordination (donor-acceptor, ordative) commonwealth(Fig. 5). This type of communication is most important for biology and medicine, since the chemistry of the most important D-elements for metabolism is largely described by coordination bonds.

PC. five.

As a rule, in complex connection Metal atom acts as an electronic pair; On the contrary, with ionic and covalent bonds, the metal atom is an electron donor.

The essence of the covalent bond and its varieties - coordination communications - can be clarified with the help of another theory of acids and the grounds proposed by GG. Lewis. He somewhat expanded the semantic concept of the terms "Acid" and "Base" on the theory of Brenstead-Lowry. Lewis's theory explains the nature of the formation of complex ions and the participation of substances in the reactions of nucleophilic substitution, that is, in the formation of the COP.

According to Lewis, acid is a substance capable of forming a covalent connection by accepting an electronic pair from the base. The Lewis base is called a substance with a mean-free electron pair, which, by turning the electrons, forms a covalent bond with Lewisic acid.

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

Consequently, according to this theory, cations are leewasic acids, and anions are Lewis bases. An example is the following reactions:

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

Polarizabilitydetermined by the electronic structure, charge and sizes of the ion; Anions are higher than that of the cations. The greatest polarizability among cations - the cations of a larger charge and smaller, for example, HG 2+, CD 2+, PB 2+, Al 3+, TL 3+. A strong polarizing action possesses N. +. Since the influence of the polarization of the ions is bilateral, it significantly changes the properties of the compounds formed by them.

Third Communication Type -dipole-dipole communication

In addition to the listed types of communication, the dipole-dipole distinguish intermolecularinteractions called also vantherval masses .

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

Mix the interactions of three types: Permanent dipole - Permanent dipole ( dipole-dipole attraction); Permanent dipole - induced dipole ( induction attraction); Instant dipole - induced dipole ( dispersion attraction, or London forces; Fig. 6).

Fig. 6.

Dipole-dipole moment possess only molecules with polar covalent bonds ( HCl, NH 3, SO 2, H 2 O, C 6 H 5 Cl), and the communication force is 1-2 debay(1D \u003d 3.338 × 10 -30 pendant meter - CL × M).

In biochemistry, one more type of communication is distinguished - hydrogen communication, which is an extreme case dipole-dipole attraction. This relationship is formed by attraction between the hydrogen atom and the electronegative atom of a small size, most often - oxygen, fluorine and nitrogen. With large atoms with similar electronegility (for example, with chlorine and gray), hydrogen bond is significantly weaker. The hydrogen atom is characterized by one essential feature: when it is distinguished by the binding electrons, its kernel - proton - is taken off and ceases to be applied by electrons.

Therefore, the atom turns into a major dipole.

Hydrogen bond, unlike Vanderwals, is formed not only for intermolecular interactions, but also inside one molecule - intramolecularhydrogen bond. Hydrogen bonds play an important role in a biochemistry, for example, to stabilize the structure of proteins in the form of a-helix, or for the formation of a double DNA helix (Fig. 7).

Fig.7.

Hydrogen and Vanderwalts bonds are much weaker than ionic, covalent and coordination. The energy of intermolecular connections is indicated in Table. one.

Table 1. Energy of intermolecular power

Note: The degree of intermolecular interactions reflect the indicators of the enthalpy of melting and evaporation (boiling). Ion compounds are required to separate ions much more energy than for the separation of molecules. Enthalpy melting ionic compounds is significantly higher than molecular compounds.

Fourth Communication Type -metal communication

Finally, there is another type of intermolecular ties - metal: Communication of positive metal lattice ions with free electrons. In biological objects, this type of communication is not found.

Of summary Types of links It turns out one detail: an important parameter of an atom or a metal ion - electrons donor, as well as an atom - the electron acceptance is its the size.

Without going into details, we note that the covalent radii of atoms, the ionic radii of metals and Vanderwali radii of interacting molecules increase as they increase their sequence number in the periodic groups. At the same time, the values \u200b\u200bof the radii ions are the smallest, and the vantherwalvas radius - the largest. As a rule, when moving down the group, the radii of all elements increase, both both covalent and Vanderwals.

The greatest value for biologists and doctors have coordination(donor-acceptor) Communications considered by coordination chemistry.

Medical bioornery. GK Barashkov

3.3.1 Covalent Communication - This is a two-center two-electron bond, formed by overlapping electronic clouds carrying unpaired electrons with anti-parallel spins. As a rule, it is formed between atoms of one chemical element.

It is quantitatively characterized by valence. Valuation of element - It is its ability to form a certain number of chemical bonds due to the free electrons located atomic valence zone.

Covalent bond forms only the pair of electrons in between atoms. It is called a divided pair. The remaining pairs of electrons are called watered pairs. They fill the shells and do not take part in the binding. The relationship between atoms can be carried out not only one, but also two and even three divided pairs. Such connections are called double and T. local - multiple connections.

3.3.1.1 Covalent non-polar connection. Communication carried out by the formation of electronic pairs, to the same extent belonging to both atoms, is called covalent notolary. It arises between atoms with almost equal to electronegitability (0.4\u003e ΔEo\u003e 0) and, consequently, the uniform distribution of electron density between the nuclei of atoms in homo-tenor molecules. For example, H 2, O 2, N 2, CL 2, etc., the dipole moment of such connections is zero. The link in the limit hydrocarbons (for example, in CH 4) is considered almost non-polar, because Δ EO \u003d 2.5 (C) - 2.1 (H) \u003d 0.4.

3.3.1.2 Covalent polar communication. If the molecule is formed by two different atoms, the zone of overlapping electron clouds (orbital) is shifted towards one of the atoms, and such a connection is called polar . With such a connection, the probability of finding electrons near the kernel of one of the atoms is higher. For example, NCl, H 2 S, pH 3.

Polar (asymmetric) Covalent Communication - Communication between atoms with different electronegility (2\u003e ΔEo\u003e 0.4) and the asymmetrical distribution of a common electron pair. As a rule, it is formed between two non-metals.

The electronic density of such a bond is shifted towards a more electronegative atom, which leads to the appearance of a partial negative charge on it (Delt minus), and on a less electronegative atom - a partial positive charge  (Delta Plus)

C      C      C  N     H   C  MG .

The direction of displacement of the electrons is also indicated by the arrow:

CCl, Co, Cn, On, Cmg.

The greater the difference in the electronegativity of the associated atoms, the higher the polarity of the communication and its more dipole moment. Between the opposite sign partial charges there are additional attraction forces. Therefore, than a polar connection, it is stronger.

Besides polarizum covalent communication has a property saturacy - the ability of an atom to form so many covalent ties as it has energetically accessible atomic orbitals. The third property of a covalent connection is its focus.

3.3.2 ion connection. The driving force of its formation is all the same aspiration of atoms to an octetic shell. But in some cases, such an octetic shell may occur only when electron transmission from one atom to another. Therefore, as a rule, an ionic connection is formed between the metal and non-metallol.

Consider as an example the reaction between sodium atoms (3S 1) and fluorine (2S 2 3S 5). Electricity difference in connection NAF

EO \u003d 4.0 - 0.93 \u003d 3.07

Sodium, giving the Fectour 3S 1 -Electron, becomes Na + ion and remains with 2s 2 2p 6 with a 6 o 2p 6 shell, which corresponds to the electronic configuration of the neon atom. The exact same electronic configuration acquires Fluoros, accepting one electron, given by sodium. As a result, the forces of electro-static attraction between oppositely charged ions occur.

Ion communication - Extreme case of polar covalent bond based on electrostatic attraction of ions. Such a link occurs with a large difference in the electronegatenes associated atoms (EO\u003e 2), when less electronegative atom almost completely gives its valence electrons and turns into a cation, and the other, more electronegative atom, these electrons attach and becomes an anion. The interaction of ions of the opposite sign does not depend on the direction, and the Coulomb forces do not have the property of saturation. By virtue of this alive Communication No spatial directional and saturacy Since each ion is associated with a certain number of counterions (coordination number of ion). Therefore, ion-related compounds do not have a molecular structure and are solids forming ion crystalline lattices, with high temperatures of melting and boiling, they are highly solar, often saline, in aqueous solutions of electrically conductive. For example, MGS, NaCl, and 2 O 3. There are practically no compounds with purely ionic connections, since some share of covalency always remains due to the fact that the total transition of one electron to another atom is not observed; In the most "ion" substances, the share of ionic communications does not exceed 90%. For example, in NAF, the polarization of communication is about 80%.

In organic compounds, ionic connections are quite rare, because The carbon atom is not inclined to lose or acquire electrons with the formation of ions.

Valence Elements in compounds with ionic connections are very often characterized degree of oxidation which, in turn, corresponds to the magnitude of the element ion charge in this connection.

Degree of oxidation - This is a conditional charge that acquires an atom as a result of redistribution of electronic density. It is quantitatively characterized by the number of shifted electrons from the less electric-king element to more electronegative. A positively charged ion is formed from that element that gave its electrons, and the negative ion from the element that these electrons accepted.

Element located in higher oxidation (Maximum positive), already gave all his valence electrons that are in AVZ. And since their number is determined by the number of the group in which it is an element, the highest degree of oxidation for most elements and will be equal group number . Concerning lower oxidation (maximum negative), then it appears when forming an eight-electron shell, that is, in the case when AVZ is filled in completely. For nemmetalov It is calculated by the formula Group number - 8 . For metals equal zero because they cannot receive electrons.

For example, AVZ sulfur has the form: 3S 2 3P 4. If the atom gives all electrons (six), it will acquire the highest degree of oxidation +6 equal VI if there are two necessary to complete the steady shell, it will acquire a low degree of oxidation –2 equal Group number - 8 \u003d 6 - 8 \u003d -2.

3.3.3 Metal communication. Most metals have a number of properties that are common and different from the properties of other substances. Such properties are relatively high melting temperatures, the ability to reflect light, high heat and electrical conductivity. These features are explained by the existence of a special type of interaction in metals. – metal communication.

In accordance with the position in the periodic system, the metals atoms have a small number of valence electrons that are sufficiently poorly related to their cores and can easily break away from them. As a result of this, positively charged ions localized in certain positions of the crystal lattice appear in the crystal lattice of the metal, and a large number of delocalized (free) electrons that are relatively freely moving in the field of positive centers and communicate between all metal atoms due to electrostatic attraction.

This is an important difference between metal ties from covalent, which have a strict orientation in space. Communication forces in metals are not localized and not directed, and free electrons forming "electronic gas" cause high heat and electrical conductivity. Therefore, in this case, it is impossible to talk about the direction of bonds, since the valence electrons are almost evenly distributed over the crystal. This is exactly what is explained, for example, the plasticity of metals, i.e. the possibility of displacement of ions and atoms in any direction

3.3.4 donor-acceptor communication. In addition to the mechanism for the formation of a covalent bond, according to which the overall electron pair occurs when two electrons interacts, there is also a special donor-acceptor mechanism . It lies in the fact that a covalent bond is formed as a result of the transition of an already existing (meaningful) electronic pair donora (electrons supplier) in the overall use of the donor and acceptor (Supplier of free atomic orbital).

After formation, it does not differ from covalent. The donor-acceptor mechanism is well illustrated by the ammonium ion formation (Figure 9) (sprockets indicate the electrons of the outer level of the nitrogen atom):

Figure 9- Ammonium Ion Education Scheme

The electronic formula of AVZ nitrogen atom 2S 2 2P 3, that is, it has three unpaired electrons, which come into a covalent bond with three hydrogen atoms (1S 1), each of which has one valence electron. At the same time, an ammonia molecule NH 3 is formed, in which a mean-free electronic pair of nitrogen is preserved. If the hydrogen proton (1S 0) is suitable for this molecule, which does not have electrons, then nitrogen will transmit its pair of electrons (donor) to this atomic orbital hydrogen (acceptor), resulting in ammonium ion. It has every hydrogen atom associated with a nitrogen atom with a common electron pair, one of which is implemented according to the donor-acceptor mechanism. It is important to note that n-N communicationFormed by various mechanisms, no differences in properties do not have. The indicated phenomenon is due to the fact that at the time of the formation of the coupling of the orbital of 2S and 2R electrons of the nitrogen atom change its form. As a result, four are completely identical in the form of orbital.

Atoms with a large number of electrons usually perform as donors, but having a small number of unpaired electrons. For elements II of the period, such an opportunity, except for the nitrogen atom, is available at oxygen (two vapor pairs) and at fluorine (three different pairs). For example, ion of hydrogen H + in aqueous solutions is never in a free state, since ion hydroxonium H 3 O + hydroxony ion is always formed from the water molecules H 2 O and Ion H +, although hydroxony is present in all aqueous solutions, although it is stored for ease of writing H + symbol.

3.3.5 Hydrogen bond. A hydrogen atom associated with a strongly electronegative element (nitrogen, oxygen, fluorine, etc.), which "tightens" the overall electronic pair is lack of electrons and acquires an effective positive charge. Therefore, it is capable of interacting with a different pair of electrons of another electronegative atom (which acquires an effective negative charge) of the same (intramolecular communication) or another molecule (intermolecular communication). As a result, it occurs hydrogen communications which is graphically designated by points:

This connection is much weaker than other chemical ties (energy of its formation 10 – 40 kJ / mol) and mainly has partially electrostatic, partially donor-acceptor character.

An extremely important role to hydrogen bonds in biological macromolecules, such inorganic compounds as H 2 O, H 2 F 2, NH 3. For example, the bonds of ONV in H 2 O have a noticeable polar character with an excess of negative charge - on an oxygen atom. A hydrogen atom, on the contrary, acquires a small positive charge  + and can interact with watertic vapors of electrons of an oxygen atom of a neighboring water molecule.

The interaction between water molecules is sufficiently strong, such that, even in steams of water, dimers and three-dimensional trimers (H 2 O) 2, (H 2 O) 3, etc., may arise in solutions. Long chains of the associates of this type can occur:

since the oxygen atom has two meaningless pairs of electrons.

The presence of hydrogen bonds explains high water boiling temperatures, alcohols, carboxylic acids. Due to hydrogen bonds, water is characterized as high compared to H 2 E (E \u003d S, SE, TE) with melting and boiling temperatures. If the hydrogen bonds were absent, then the water would melt at -100 ° C, and boiled at -80 ° C. Typical cases of association are observed for alcohols and organic acids.

Hydrogen bonds may occur both between different molecules and inside the molecule if there are groups with donor and acceptor abilities in this molecule. For example, it is the intramolecular hydrogen bonds that play a major role in the formation of peptide chains, which determine the structure of proteins. N-bonds affect the physical and chemical properties of the substance.

Type of hydrogen bonds do not form atoms of other elements Since the forces of the electrostatic attraction of the variepete ends of the dipoles of polar bonds (O-H, N-H, etc.) are quite weak and act only at low distances. Hydrogen, having the smallest atomic radius, allows you to get close to such dipoles so much that the attraction force becomes noticeable. No other element with a large atomic radius is capable of forming such connections.

3.3.6 Intermolecular interaction forces (van der Waals strength). In 1873, the Dutch scientist I. Van der Waals suggested that there were forces that determine the attraction between molecules. These forces later received the name of Van der Waals forces – The most universal view of the intermolecular communication. The energy of the van der Waals communication is less hydrogen and is 2-20 kJ / ∙ mole.

Depending on the method of force are divided into:

1) Orientational (dipole-dipole or ion-dipole) - arise between polar molecules or between ions and polar molecules. When the polar molecules are rapprocheted in such a way that the positive side of one dipole is focused on the negative side of another dipole (Figure 10).

2) induction (dipole - induced dipole or an ion-induced dipole) - arise between polar molecules or ions and non-polar molecules, but capable of polarization. Diples can affect non-polar molecules, turning them into an indicated (induced) dipoles. (Figure 11).

3) Dispersion (induced dipole - induced dipole) - arise between non-polar molecules capable of polarization. In any molecule or atom of noble gas, fluctuations of electrical density occur, as a result of which instantal dipoles appear, which in turn induce instant dipoles in neighboring molecules. The motion of instant dipoles becomes consistent, their appearance and decay occurs synchronously. As a result of the interaction of instant dipoles, the energy of the system decreases (Figure 12).