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Each atom has some number of electrons.

When entering into chemical reactions, the atoms are given, they acquire, or communicate electrons, reaching 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 stable 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 an external electronic shell filled. 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, non-soluble, some organic compounds that do not contain - on-groups, in particular, triethylamine N (C 2 H 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 of hydrogen ions exist in hydrated form, that is, in the form of hydroxony 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 elements of the left part of the periodic table form predominantly ionic 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 the complex compound, the metal atom acts as an acceptor of 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.

From a brief overview of types of bonds, one piece is found: an important parameter of an atom or metal ion - electrons donor, as well as an atom - electron acceptor 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

.

You know that atoms can be combined with each other with the formation of both simple and complex substances. At the same time, various types of chemical bonds are formed: ionic, covalent (non-polar and polar), metallic and hydrogen. One of the most significant properties of the atoms of elements that determine which link is formed between them - ion or covalent, - this is electronegativity, i.e. The ability of atoms in conjunction to attract electrons.

The conditional quantitative assessment of electronegability gives a scale of relative electrical negotiations.

In periods there is a general trend of the growth of electrotric and elements, and in groups - their falls. Elements for electrotypes are placed in a row, on the basis of which you can compare the electronegativity of elements in different periods.

The type of chemical communication depends on how large the difference between the values \u200b\u200bof the electronegateness of the connecting atoms of the elements. The more differed in electronegability atoms of elements forming the connection, the chemical bond is polar. It is impossible to carry out a sharp boundary between types of chemical bonds. In most compounds, the type of chemical bond is intermediate; For example, a strong-polar covalent chemical bond is close to ion connection. Depending on how the limiting cases are closer in its nature, the chemical bond is referred to either to ionic or to covalent polar communication.

Ion connection.

The ionic communication is formed when the interaction of atoms, which differ sharply from each other by electronegativity. For example, typical metals of lithium (Li), sodium (Na), potassium (k), calcium (CA), strontium (SR), Barium (BA) form ion bond with typical non-metals, mainly with halogens.

In addition to alkali metal halides, ionic communication is also formed in such compounds as alkali and salt. For example, in sodium hydroxide (NaOH) and sodium sulfate (Na 2 SO 4), ionic bonds exist only between sodium and oxygen atoms (other connections - covalent polar).

Covalent non-polar connection.

In the interaction of atoms with the same electrotricant, molecules with a covalent non-polar bond are formed. Such a link exists in the molecules of the following simple substances: H 2, F 2, CL 2, O 2, N 2. Chemical bonds in these gases are formed by general electronic pairs, i.e. When overlapping the corresponding electron clouds, due to the electron-nuclear interaction, which performs when atoms rapprochet.

By compiling electronic formulas, it should be remembered that each general electron pair is a conditional image of an increased electron density resulting from overlapping the corresponding electronic clouds.

Covalent polar communication.

When the interaction of atoms, the value of the electrotability of which is different, but does not sharply, there is a displacement of a common electron pair to a more electronegative atom. This is the most common type of chemical bond, which is found in both inorganic and organic compounds.

Tone relations that are formed by donor-acceptor mechanism, for example, in hydroxonium and amine ions are fully applied to covalent bonds.

Metal connection.

Communication, which is formed as a result of the interaction of relax free electrons with metal ions, is called a metal tie. This type of communication is characteristic of simple metals.

The essence of the process of formation of the metallic bond is as follows: Metal atoms easily give valence electrons and turn into positive charged ions. Relatively free electrons that broke away from the atom move between the projection ions of metals. Between them there is a metal connection, i.e. electrons, as it were, cementing positive ions of the crystal-leic lattice of metals.

Hydrogen bond.

Communication that is formed between hydrogen atoms of one molecule and an atom of a strong electronegative element (O, N, F) another molecule is called hydrogen bond.

A question may arise: why exactly the hydrogen forms such a specific chemical relationship?

This is explained by the fact that the atomic radius of hydrogen is very small. In addition, when displaced or full of its single electron, hydrogen acquires a relatively high positive charge, due to which hydrogen of one molecule interacts with the atoms of electronegative elements having a partial negative charge in the composition of other molecules (HF, H 2 O, NH 3) .

Consider some examples. Usually we depict the composition of the water with the chemical formula H 2 O. However, this is not exactly accurate. It would be more correct to design a water (H 2 O) N formula (H 2 O) N, where n \u003d 2,3,4, etc. This is due to the fact that individual water molecules are interconnected by hydrogen bonds.

Hydrogen bond is made to designate points. It is much weaker than an ionic or covalent bond, but stronger than the usual intermolecular interaction.

The presence of hydrogen bonds explains the increase in water with a decrease in temperature. This is due to the fact that when the temperature decreases, the molecules are strengthened and therefore the density of their "packaging" decreases.

When studying organic chemistry, such a question arose: why the boiling temperatures of alcohols are much higher than the corresponding hydrocarbons? This is explained by the fact that hydrogen bonds are formed between alcohol molecules.

The increase in the boiling point of alcohols is also occurring the vicinity of the enlargement of their molecules.

Hydrogen bond is also characteristic of many other organic compounds (phenols, carboxylic acids, etc.). From the courses of organic chemistry and general biology, you know that the presence of hydrogen communication is explained by the secondary structure of proteins, the structure of the DNA double helix, i.e. the phenomenon of complimentaryness.

In the section, help solve chemistry, please. Specify the type of communication in NH3, CaCl2 molecules, Al2O3, Bas ... asked by the author Evgeny_1991. The best answer is 1) NH3 Type of Communication Cove. Polar. Three unpaired nitrogen electrons and one hydrogen take part in the formation of communication. Pi-bonds are not. Hybridization SP3. The shape of the pyramidal molecule (one orbital does not participate in hybridization, the tetrahedron turns into a pyramid)
CaCl2 ion connection type. In the formation of communication, two electrons calcium on s orbitals are involved, which take two chlorine atoms, completing their third level. PI-bonds no, type hybridization sp. They are located in the space at an angle of 180 degrees
Al2O3 ion connection type. In the formation of communication, three electrons with S and p are aluminum orbital, which take oxygen, completing its second level. O \u003d Al-O-Al \u003d O. There are pi-ties between oxygen and aluminum. The type of hybridization SP is likely.
Bas connection type ionic. Two electrons barium takes sulfur. Ba \u003d S is one PI communication. Hybridization Sp. Flat molecule.
2) AGNO3
silver restored on the cathode
To AG + + E \u003d AG
water oxidizes on the anode
A 2H2O - 4E \u003d O2 + 4H +
according to the Faraday law (as already there ...) the mass (volume) of the substance elected on the cathode is proportional to the number of electricity passed through the solution
m (AG) \u003d ME / ZF * I * T \u003d 32.23 g
V (O2) \u003d VE / F * I * T \u003d 1.67 L

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 connection. 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 the bonds of H-n, formed by various mechanisms, do not have any differences in properties. 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).

Figure 10 - Orientational interaction

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).

Figure 11 - Induction interaction

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).

Figure 12 - Dispersion interaction