Rarely chemical substances consist of individual, unrelated atoms of chemical elements. Under normal conditions, only a small number of gases called noble gases have such a structure: helium, neon, argon, krypton, xenon and radon. Most often, chemical substances do not consist of disparate atoms, but of their combinations into various groups. Such combinations of atoms can include several units, hundreds, thousands, or even more atoms. The force that keeps these atoms in such groupings is called chemical bond .
In other words, we can say that a chemical bond is an interaction that ensures the bonding 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 an XY molecule is formed during the interaction of X and Y atoms, 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.
In the formation of chemical bonds, the 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 an electronic configuration of noble gas atoms, i.e. so that in its outer electron layer there are 8 electrons (2 for elements of the first period). This phenomenon is called the octet rule.
It is possible for atoms to achieve the electronic 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 socialization of electrons, covalent, ionic and metallic bonds can be distinguished.
covalent bond
A covalent bond occurs most often between atoms of non-metal elements. If the atoms of non-metals forming a covalent bond belong to different chemical elements, such a bond is called a covalent polar bond. The reason for this name lies in the fact that atoms of different elements also have a different ability to attract a common electron pair to themselves. Obviously, this leads to a shift 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 a covalent polar bond:
СCl 4 , H 2 S, CO 2 , NH 3 , SiO 2 etc.
A covalent non-polar bond is formed between atoms of non-metals of the same chemical element. Since the atoms are identical, their ability to pull shared electrons is 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 for the formation of 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 common 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 an unshared electron pair is called a donor, and an atom with a free orbital is called an acceptor. The donors of electron pairs are atoms that have paired electrons, for example, N, O, P, S.
For example, according to the donor-acceptor mechanism, the fourth N-H covalent bond is formed in the ammonium cation NH 4 +:
In addition to polarity, covalent bonds are also characterized by energy. The bond energy is the minimum energy required to break a bond between atoms.
The binding energy decreases with increasing radii of the bound 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. The bond multiplicity is the number of common electron pairs between two atoms.
Ionic bond
An ionic bond can be considered as the limiting case of a covalent polar bond. If in a covalent-polar bond the common electron pair is partially shifted to one of the pair of atoms, then in the ionic one it is almost completely “given away” to one of the atoms. The atom that has donated an electron(s) acquires a positive charge and becomes cation, and the atom that took electrons from it acquires a negative charge and becomes anion.
Thus, an ionic bond is a bond formed due to the electrostatic attraction of cations to anions.
The formation of this type of bond is characteristic of the interaction of atoms of typical metals and typical nonmetals.
For example, potassium fluoride. A potassium cation is obtained as a result of the detachment of one electron from a neutral atom, and a fluorine ion is formed by attaching one electron to a fluorine atom:
Between the resulting ions, a force of electrostatic attraction arises, as a result of which an ionic compound is formed.
During the formation of a chemical bond, electrons from the sodium atom passed to the chlorine atom and oppositely charged ions were formed, which have a completed external energy level.
It has been established that electrons do not completely detach from the metal atom, but only shift towards the chlorine atom, as in a covalent bond.
Most binary compounds that contain metal atoms are ionic. For example, oxides, halides, sulfides, nitrides.
An ionic bond 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.
The atoms of all metals have electrons on the outer electron layer that have a low binding energy with the atomic nucleus. For most metals, the loss of external electrons is energetically favorable.
In view of 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 - \u003d M n +, where M 0 is a neutral metal atom, and M n + cation of the same metal. The figure below shows an illustration of the ongoing processes.
That is, electrons “rush” along the metal crystal, detaching from one metal atom, forming a cation from it, joining another cation, forming a neutral atom. This phenomenon was called “electronic wind”, and the set of free electrons in the crystal of a non-metal atom was called “electron gas”. This type of interaction between metal atoms is called a metallic bond.
hydrogen bond
If a hydrogen atom in any substance is bonded to an element with a high electronegativity (nitrogen, oxygen, or fluorine), such a substance is characterized by such a phenomenon as a hydrogen bond.
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 electronegative atom. In this regard, electrostatic attraction becomes possible between the partially positively charged hydrogen atom of one molecule and the 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, amines.
Metal connection. Properties of a metallic bond.
A metallic bond is a chemical bond due to the presence of relatively free electrons. It is characteristic both for pure metals and their alloys and intermetallic compounds.
Metal bond mechanism
At all nodes crystal lattice positive metal ions are located. Between them randomly, like gas molecules, valence electrons move, unhooked from atoms during the formation of ions. These electrons play the role of cement, holding the positive ions together; otherwise, the lattice would disintegrate under the action of repulsive forces between the ions. At the same time, electrons are also held by ions within the crystal lattice and cannot leave it. Communication forces are not localized and not directed. For this reason, high coordination numbers (eg 12 or 8) appear in most cases. When two metal atoms approach each other, their outer shell orbitals overlap to form molecular orbitals. If a third atom comes up, its orbital overlaps with the orbitals of the first two atoms, giving one more molecular orbital. When there are many atoms, a huge number of three-dimensional molecular orbitals arise, extending in all directions. Due to the multiple overlapping of orbitals, the valence electrons of each atom are influenced by many atoms.
Characteristic crystal lattices
Most metals form one of the following highly symmetrical, close-packed lattices: body-centered cubic, face-centered cubic, and hexagonal.
In a cubic body-centered lattice (bcc), atoms are located at the vertices of the cube and one atom is located at the center of the volume of the cube. Metals have a cubic body-centered lattice: Pb, K, Na, Li, β-Ti, β-Zr, Ta, W, V, α-Fe, Cr, Nb, Ba, etc.
In a face-centered cubic lattice (fcc), atoms are located at the vertices of the cube and at the center of each face. Metals of this type have a lattice: α-Ca, Ce, α-Sr, Pb, Ni, Ag, Au, Pd, Pt, Rh, γ-Fe, Cu, α-Co, etc.
In a hexagonal lattice, atoms are located at the vertices and the center of the hexagonal bases of the prism, and three atoms are located in the middle plane of the prism. Metals have such a packing of atoms: Mg, α-Ti, Cd, Re, Os, Ru, Zn, β-Co, Be, β-Ca, etc.
Other properties
Freely moving electrons cause high electrical and thermal conductivity. Substances with a metallic bond often combine strength with ductility, since when atoms are displaced relative to each other, bonds do not break. Another important property is metallic aromaticity.
Metals conduct heat and electricity well, they are strong enough, they can be deformed without breaking. Some metals are malleable (they can be forged), some are malleable (they can be drawn into wire). These unique properties are explained by a special type of chemical bond that connects metal atoms to each other - a metallic bond.
Metals in the solid state exist in the form of crystals of positive ions, as if “floating” in a sea of electrons freely moving between them.
The metallic bond explains the properties of metals, in particular their strength. Under the action of a deforming force, the metal lattice can change its shape without cracking, unlike ionic crystals.
The high thermal conductivity of metals is explained by the fact that if you heat a piece of metal on one side, then the kinetic energy of electrons will increase. This increase in energy will propagate in the "electronic sea" throughout the sample at great speed.
The electrical conductivity of metals also becomes clear. If a potential difference is applied to the ends of a metal sample, then the cloud of delocalized electrons will shift in the direction of the positive potential: this flow of electrons moving in the same direction is the familiar electric current.
Metal connection. Properties of a metallic bond. - concept and types. Classification and features of the category "Metal bond. Metal bond properties." 2017, 2018.
All metals have the following characteristics:
A small number of electrons in the outer energy level (except for some exceptions, which may have 6.7 and 8);
Big atomic radius;
Low ionization energy.
All this contributes to the easy separation of the outer unpaired electrons from the nucleus. In this case, the atom has a lot of free orbitals. The scheme for the formation of a metallic bond will just show the overlap of numerous orbital cells of different atoms with each other, which, as a result, form a common intracrystalline space. Electrons are fed into it from each atom, which begin to wander freely in different parts of the lattice. Periodically, each of them attaches to an ion at a crystal site and turns it into an atom, then detaches again, forming an ion.
In this way, a metallic bond is a bond between atoms, ions, and free electrons in a common metal crystal. An electron cloud that moves freely within a structure is called an "electron gas". They explain most physical properties metals and their alloys.
How exactly does a metallic chemical bond realize itself? Various examples can be given. Let's try to consider on a piece of lithium. Even if you take it the size of a pea, there will be thousands of atoms. Let's imagine that each of these thousands of atoms donates its single valence electron to the common crystalline space. At the same time, knowing electronic building given element, you can see the number of empty orbitals. Lithium will have 3 of them (p-orbitals of the second energy level). Three for each atom out of tens of thousands - this is the common space inside the crystal, in which the "electron gas" moves freely.
A substance with a metallic bond is always strong. After all, the electron gas does not allow the crystal to collapse, but only shifts the layers and immediately restores. It shines, has a certain density (most often high), fusibility, malleability and plasticity.
Where else is a metallic bond realized? Substance examples:
Metals in the form of simple structures;
All metal alloys with each other;
All metals and their alloys in liquid and solid state.
There are just an incredible number of specific examples, because there are more than 80 metals in the periodic system!
The mechanism of education in general view is expressed by the following notation: Me 0 - e - ↔ Me n+. It is obvious from the diagram which particles are present in the metal crystal.
Any metal is capable of donating electrons, turning into a positively charged ion.
On the example of iron: Fe 0 -2e - = Fe 2+
Where are the separated negatively charged particles - electrons? The minus is always attracted to the plus. The electrons are attracted to another (positively charged) iron ion in the crystal lattice: Fe 2+ + 2e - \u003d Fe 0
The ion becomes a neutral atom. And this process is repeated many times.
It turns out that the free electrons of iron are in constant motion throughout the entire volume of the crystal, breaking away and joining the ions at the lattice sites. Another name for this phenomenon is delocalized electron cloud. The term "delocalized" means - free, not tied.
A metallic bond is a chemical bond due to the presence of relatively free electrons. It is typical for both pure metals and their alloys and intermetallic compounds.
Metal bond mechanism
In all nodes of the crystal lattice there are positive metal ions. Between them randomly, like gas molecules, valence electrons move, unhooked from atoms during the formation of ions. These electrons play the role of cement, holding the positive ions together; otherwise, the lattice would disintegrate under the action of repulsive forces between the ions. At the same time, electrons are also held by ions within the crystal lattice and cannot leave it. Communication forces are not localized and not directed.
Therefore, in most cases, high coordination numbers appear (for example, 12 or 8). When two metal atoms approach each other, their outer shell orbitals overlap to form molecular orbitals. If a third atom comes up, its orbital overlaps with those of the first two atoms, resulting in another molecular orbital. When there are many atoms, there is a huge number of three-dimensional molecular orbitals, extending in all directions. Due to the multiple overlapping of orbitals, the valence electrons of each atom are influenced by many atoms.
Characteristic crystal lattices
Most metals form one of the following highly symmetrical, close-packed lattices: body-centered cubic, face-centered cubic, and hexagonal.
In a body-centered cubic lattice (bcc), the atoms are located at the vertices of the cube and one atom is located at the center of the volume of the cube. Metals have a cubic body-centered lattice: Pb, K, Na, Li, β-Ti, β-Zr, Ta, W, V, α-Fe, Cr, Nb, Ba, etc.
In a face-centered cubic lattice (fcc), atoms are located at the vertices of the cube and at the center of each face. Metals of this type have a lattice: α-Ca, Ce, α-Sr, Pb, Ni, Ag, Au, Pd, Pt, Rh, γ-Fe, Cu, α-Co, etc.
In a hexagonal lattice, atoms are located at the vertices and the center of the hexagonal bases of the prism, and three atoms are located in the middle plane of the prism. Metals have such a packing of atoms: Mg, α-Ti, Cd, Re, Os, Ru, Zn, β-Co, Be, β-Ca, etc.
Other properties
Freely moving electrons cause high electrical and thermal conductivity. Substances with a metallic bond often combine strength with ductility, since when atoms are displaced relative to each other, bonds do not break. Another important property is metallic aromaticity.
Metals conduct heat and electricity well, they are strong enough, they can be deformed without breaking. Some metals are malleable (they can be forged), some are malleable (they can be drawn into wire). These unique properties are explained by a special type of chemical bond that connects metal atoms to each other - a metallic bond.
Metals in the solid state exist in the form of crystals of positive ions, as if “floating” in a sea of electrons freely moving between them.
The metallic bond explains the properties of metals, in particular their strength. Under the action of a deforming force, the metal lattice can change its shape without cracking, unlike ionic crystals.
The high thermal conductivity of metals is explained by the fact that if you heat a piece of metal on one side, then the kinetic energy of the electrons will increase. This increase in energy will propagate in the "electronic sea" throughout the sample at great speed.
The electrical conductivity of metals also becomes clear. If a potential difference is applied to the ends of a metal sample, then the cloud of delocalized electrons will shift in the direction of the positive potential: this stream of electrons moving in the same direction is the familiar electric current.
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Each atom has a certain number of electrons.
Entering into chemical reactions, atoms donate, acquire, or socialize electrons, reaching the most stable electronic configuration. The configuration with the lowest energy is the most stable (as in noble gas atoms). This pattern is called the "octet rule" (Fig. 1).
Rice. one.
This rule applies to all connection types. Electronic bonds between atoms allow them to form stable structures, from the simplest crystals to complex biomolecules that eventually form living systems. They differ from crystals in their continuous metabolism. However, many chemical reactions proceed according to the mechanisms electronic transfer, which play an important role in the energy processes in the body.
A chemical bond is a force that holds together two or more atoms, ions, molecules, or any combination of them..
The nature of the 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 in 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, those located in the most high-energy orbitals. Accordingly, the outer shell of an atom containing these orbitals is called valence shell. At present, 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 theory of valency, 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 opposite sign a chemical bond is formed, called Kossel " electrovalent(now called ionic).
In this case, anions and cations form a stable electronic configuration with a filled outer electron shell. Typical ionic bonds are formed from cations T and II groups periodic system and anions of non-metallic elements of groups VI and VII (16 and 17 subgroups - respectively, chalcogens and halogens). The bonds in ionic compounds are unsaturated and non-directional, so they retain the possibility of electrostatic interaction with other ions. On fig. 2 and 3 show examples of ionic bonds corresponding to the Kossel electron transfer model.
Rice. 2.
Rice. 3. Ionic bond in the sodium chloride (NaCl) molecule
Here it is appropriate to recall some of the properties that explain the behavior of substances in nature, in particular, to consider the concept of acids and grounds.
Aqueous solutions of all these substances are electrolytes. They change color in different ways. indicators. The mechanism of action of indicators was discovered by F.V. Ostwald. He showed that the indicators are weak acids or bases, the color of which in the undissociated and dissociated states is different.
Bases can neutralize acids. Not all bases are soluble in water (for example, some organic compounds, not containing ‑ OH groups, in particular, triethylamine N (C 2 H 5) 3); soluble bases are called alkalis.
Aqueous solutions of acids enter into 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 H 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 H+ , while the base forms ions HE- . This theory does not take into account the existence organic bases without hydroxyl groups.
In line with proton Bronsted and Lowry's theory, an acid is a substance containing molecules or ions that donate protons ( donors protons), and the base is a substance consisting of molecules or ions that accept protons ( acceptors protons). Note that in aqueous solutions, hydrogen ions exist in a hydrated form, that is, in the form of hydronium ions H3O+ . This theory describes reactions 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 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 conjugated pairs of acids and bases:
1)NH 4+ and NH 3
2) HCl and Cl ‑
Here, in each conjugated pair, the acid and base differ by one proton. Every acid has a conjugate base. strong acid corresponds to a weak conjugate base, and weak acid is a strong conjugate base.
The Bronsted-Lowry theory makes it possible to 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 acetic acid water is a base, and with aqueous solutions of ammonia it is an acid.
1) CH 3 COOH + H 2 O ↔ H 3 O + + CH 3 SOO- . Here the acetic acid molecule donates a proton to the water molecule;
2) NH3 + H 2 O ↔ NH4 + + HE- . Here the ammonia molecule accepts a proton from the water molecule.
Thus, water can form two conjugated pairs:
1) H 2 O(acid) and HE- (conjugate base)
2) H 3 O+ (acid) and H 2 O(conjugate base).
In the first case, water donates a proton, and in the second, it accepts it.
Such a property is called amphiprotonity. Substances that can react as both acids and bases are called amphoteric. Such substances are often found in nature. For example, amino acids can form salts with both acids and bases. Therefore, peptides readily form coordination compounds with the metal ions present.
Thus, the characteristic property of an ionic bond is the complete displacement of a bunch of binding electrons to one of the nuclei. This means that there is a region between the ions 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 each atom. In this case, the socialized bond electrons are distributed equally among the atoms. An example of a covalent bond is homonuclear diatomic H molecules 2 , N 2 , F 2. Allotropes have the same type of bond. 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 FROM 2 H 5 HE, sulfur hexafluoride SF 6, acetylene FROM 2 H 2. All these molecules have the same common electrons, and their bonds are saturated and directed in the same way (Fig. 4).
For biologists, it is important that the covalent radii of atoms in double and triple bonds are reduced compared to a single bond.
Rice. four. Covalent bond in the Cl 2 molecule.
Ionic and covalent types bonds are two limiting cases of many existing types of chemical bonds, and in practice most of the bonds are intermediate.
Connections of two elements located at opposite ends of one or different periods systems of Mendeleev, predominantly form ionic bonds. As the elements approach each other within a period, the ionic nature of their compounds decreases, while the covalent character increases. For example, the halides and oxides of the elements on the left side of the periodic table form predominantly ionic bonds ( NaCl, AgBr, BaSO 4 , CaCO 3 , KNO 3 , CaO, NaOH), and the same compounds of the 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 another 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 socializes 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 most important d-elements for metabolism is largely described by coordination bonds.
Pic. 5.
As a rule, in a complex compound, a metal atom acts as an electron pair acceptor; 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 Bronsted-Lowry theory. The Lewis theory explains the nature of the formation of complex ions and the participation of substances in nucleophilic substitution reactions, that is, in the formation of CS.
According to Lewis, an acid is a substance capable of forming a covalent bond by accepting an electron pair from a base. A Lewis base is a substance that has a lone pair of electrons, which, by donating electrons, forms a covalent bond with Lewis acid.
That is, the Lewis 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 able to accept an electron pair.
Therefore, according to this theory, cations are Lewis acids and anions are Lewis bases. The following reactions are examples:
It was noted above that the subdivision of substances into ionic and covalent ones is relative, since there is no complete transfer of an electron from metal atoms to acceptor atoms in covalent molecules. In compounds with an ionic bond, 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; it is higher for anions than for cations. The highest polarizability among cations is for cations of larger charge and smaller size, for example, for Hg 2+ , Cd 2+ , Pb 2+ , Al 3+ , Tl 3+. Has a strong polarizing effect H+ . Since the effect of ion polarization is two-way, it significantly changes the properties of the compounds they form.
The third type of connection -dipole-dipole connection
In addition to the listed types of communication, there are also dipole-dipole intermolecular interactions, also known as 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 ( dispersion 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 debye(1D \u003d 3.338 × 10 -30 coulomb meters - C × m).
In biochemistry, another type of bond is distinguished - hydrogen connection, which 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 a similar electronegativity (for example, with chlorine and sulfur), the hydrogen bond is much weaker. The hydrogen atom is distinguished by one essential feature: when the binding electrons are pulled away, its nucleus - the proton - is exposed and ceases to be screened 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, for stabilizing the structure of proteins in the form of an α-helix, or for the formation of a DNA double helix (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. one.
Table 1. Energy of intermolecular forces
Note: The degree of intermolecular interactions reflect the enthalpy of melting and evaporation (boiling). Ionic compounds require much more energy to separate ions than to separate molecules. The melting enthalpies of ionic compounds are much higher than those of molecular compounds.
The fourth type of connection -metallic bond
Finally, there is another type of intermolecular bonds - metal: connection of positive ions of the lattice of metals with free electrons. This type of connection does not occur in biological objects.
From overview types of bonds, one detail is clarified: an important parameter of an atom or ion of a metal - an electron donor, as well as an atom - an electron acceptor is its the size.
Without going into details, we note that the covalent radii of atoms, the ionic radii of metals, and the van der Waals radii of interacting molecules increase as their atomic number in the groups of the periodic system increases. 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.
The most important for biologists and physicians are coordination(donor-acceptor) bonds considered by coordination chemistry.
Medical bioinorganics. G.K. Barashkov
