A metallic bond is a multicenter bond that exists in metals and their alloys between positively charged ions and valence electrons, which are common to all ions and move freely through the crystal.

They have a small number of valence electrons and low ionization. These electrons, due to the large radii of metal atoms, are rather weakly bound to their nuclei and can easily break away from them and become common to the entire metal crystal. As a result, positively charged metal ions and an electron gas appear in the crystal lattice of the metal - a set of mobile electrons that freely move around the metal crystal.

As a result, the metal is a series of positive ions localized in certain positions, and a large number of electrons that move relatively freely in the field of positive centers. The spatial structure of metals is a crystal, which can be represented as a cell with positively charged ions at the nodes, immersed in a negatively charged electron gas. All atoms donate their valence electrons to the formation of an electron gas; they move freely inside the crystal without breaking the chemical bond.

The theory of the free movement of electrons in the crystal lattice of metals was experimentally confirmed by the experience of Tolman and Stewart (in 1916): during a sharp deceleration of a previously untwisted coil with a wound wire, free electrons continued to move by inertia for some time, and at this time the ammeter included in the circuit coils, registered the impulse of electric current.

Varieties of metal bond models

Signs of a metallic bond are the following characteristics:

1. Multielectronicity, since all valence electrons participate in the formation of a metallic bond;
2. Multicenter, or delocalization - a bond connects simultaneously a large number of atoms contained in a metal crystal;
3. Isotropy, or non-directionality - due to the unimpeded movement of the electron gas in all directions simultaneously, the metallic bond is spherically symmetrical.

Metal crystals form mainly three types of crystal lattices, however, some metals, depending on temperature, may have different structures.

Crystal lattices of metals: a) cubic face-centered (Cu, Au, Ag, Al); b) cubic body-centered (Li, Na, Ba, Mo, W, V); c) hexagonal (Mg, Zn, Ti, Cd, Cr)

A metallic bond exists in crystals and melts of all metals and alloys. In its pure form, it is characteristic of alkali and alkaline earth metals. In transition d-metals, the bond between atoms is partially covalent.

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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 valency, 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 of the 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 of T and II groups of the periodic system and anions of non-metallic elements of groups VI and VII (16 and 17 subgroups - respectively, chalcogens and halogens). The bonds 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 that do not contain -OH groups are insoluble, in particular, triethylamine N (C 2 H 5) 3); soluble bases are called alkalis.

Aqueous solutions of acids 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 of organic bases that do not have 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. A strong acid has a weak conjugate base, and a weak acid has 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 of 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 of bonds are two limiting cases of many existing types of chemical bonds, and in practice most of the bonds are intermediate.

Compounds of two elements located at opposite ends of the same or different periods of the Mendeleev system 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 transition 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-sided, 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 a brief review of the types of bonds, one detail emerges: 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

The lesson will consider several types of chemical bonds: metallic, hydrogen and van der Waals, and you will also learn how physical and chemical properties depend on different types of chemical bonds in a substance.

Topic: Types of chemical bond

Lesson: Metallic and hydrogen chemical bonds

metal connection it is a type of bond in metals and their alloys between metal atoms or ions and relatively free electrons (electron gas) in a crystal lattice.

Metals are chemical elements with low electronegativity, so they donate their valence electrons easily. If there is a non-metal next to a metal element, then the electrons from the metal atom pass to the non-metal. This type of connection is called ionic(Fig. 1).

Rice. 1. Education

When simple substances metals or their alloys, the situation is changing.

During the formation of molecules, the electron orbitals of metals do not remain unchanged. They interact with each other, forming a new molecular orbital. Depending on the composition and structure of the compound, molecular orbitals can either be close to the totality of atomic orbitals or differ significantly from them. When the electron orbitals of metal atoms interact, molecular orbitals are formed. Such that the valence electrons of the metal atom can move freely along these molecular orbitals. There is no complete separation, charge, i.e. metal is not a collection of cations and electrons floating around. But this is not a collection of atoms, which sometimes turn into a cationic form and transfer their electron to another cation. The real situation is a combination of these two extreme options.

Rice. 2

The essence of the formation of a metallic bond consists in the following: metal atoms donate outer electrons, and some of them turn into positively charged ions. Broken from the atoms e electrons move relatively freely between emerging positivemetal ions. A metallic bond arises between these particles, i.e., the electrons, as it were, cement the positive ions in the metal lattice (Fig. 2).

The presence of a metallic bond determines the physical properties of metals:

High plasticity

Heat and electrical conductivity

Metallic sheen

Plastic is the ability of a material to easily deform under mechanical loading. A metallic bond is realized between all metal atoms simultaneously, therefore, during mechanical action on a metal, specific bonds are not broken, but only the position of the atom changes. Metal atoms that are not rigidly bound to each other can, as it were, slide over a layer of electron gas, as happens when one glass slides over another with a layer of water between them. Due to this, metals can be easily deformed or rolled into thin foil. The most ductile metals are pure gold, silver and copper. All these metals occur naturally in nature in varying degrees of purity. Rice. 3.

Rice. 3. Metals found in nature in native form

From them, especially from gold, various ornaments are made. Due to its amazing plasticity, gold is used in the decoration of palaces. From it you can roll out the foil with a thickness of only 3. 10 -3 mm. It is called gold leaf, applied to plaster, moldings or other objects.

Thermal and electrical conductivity . The best conductors of electricity are copper, silver, gold and aluminum. But since gold and silver are expensive metals, cheaper copper and aluminum are used to make cables. The worst electrical conductors are manganese, lead, mercury and tungsten. Tungsten has such a high electrical resistance that it glows when an electric current is passed through it. This property is used in the manufacture of incandescent lamps.

Body temperature is a measure of the energy of its constituent atoms or molecules. The electron gas of a metal can quite quickly transfer excess energy from one ion or atom to another. The temperature of the metal quickly equalizes throughout the volume, even if the heating comes from one side. This is observed, for example, if you lower a metal spoon into tea.

Metallic sheen. Luster is the ability of the body to reflect light rays. Silver, aluminum and palladium have high light reflectivity. Therefore, it is these metals that are applied in a thin layer on the glass surface in the manufacture of headlights, projectors and mirrors.

hydrogen bond

Consider the boiling and melting points of hydrogen compounds of chalcogens: oxygen, sulfur, selenium and tellurium. Rice. four.

Rice. four

If we mentally extrapolate the direct boiling and melting points of the hydrogen compounds of sulfur, selenium and tellurium, we will see that the melting point of water should be approximately -100 0 C, and the boiling point should be approximately -80 0 C. This happens because there is a interaction - hydrogen bond, which brings together water molecules to the association . Additional energy is required to destroy these associates.

A hydrogen bond is formed between a highly polarized, highly positively charged hydrogen atom and another atom with a very high electronegativity: fluorine, oxygen, or nitrogen . Examples of substances capable of forming a hydrogen bond are shown in fig. 5.

Rice. 5

Consider the formation of hydrogen bonds between water molecules. The hydrogen bond is represented by three dots. The occurrence of a hydrogen bond is due to the unique feature of the hydrogen atom. Since the hydrogen atom contains only one electron, when the common electron pair is pulled away by another atom, the nucleus of the hydrogen atom is exposed, the positive charge of which acts on the electronegative elements in the molecules of substances.

Compare Properties ethyl alcohol and dimethyl ether. Based on the structure of these substances, it follows that ethyl alcohol can form intermolecular hydrogen bonds. This is due to the presence of a hydroxo group. Dimethyl ether cannot form intermolecular hydrogen bonds.

Let's compare their properties in Table 1.

Tab. one

T bp., T pl, solubility in water is higher for ethyl alcohol. This is a general pattern for substances between the molecules of which a hydrogen bond is formed. These substances are characterized by higher T bp., T pl, solubility in water and lower volatility.

Physical Properties compounds also depend on the molecular weight of the substance. Therefore, it is legitimate to compare the physical properties of substances with hydrogen bonds only for substances with similar molecular weights.

Energy one hydrogen bond about 10 times less covalent bond energy. If organic molecules of complex composition have several functional groups capable of forming a hydrogen bond, then intramolecular hydrogen bonds (proteins, DNA, amino acids, orthonitrophenol, etc.) can form in them. Due to the hydrogen bond, the secondary structure of proteins, the double helix of DNA, is formed.

Van der Waals connection.

Consider the noble gases. Helium compounds have not yet been obtained. It is incapable of forming conventional chemical bonds.

At very negative temperatures, liquid and even solid helium can be obtained. In the liquid state, helium atoms are held together by the forces of electrostatic attraction. There are three options for these forces:

orientation forces. This is the interaction between two dipoles (HCl)

Inductive attraction. This is the attraction of a dipole and a non-polar molecule.

dispersive attraction. This is an interaction between two non-polar molecules (He). It arises due to the uneven movement of electrons around the nucleus.

Summing up the lesson

The lesson discusses three types of chemical bonds: metallic, hydrogen and van der Waals. The dependence of physical and chemical properties on different types of chemical bonds in a substance was explained.

Bibliography

1. Rudzitis G.E. Chemistry. Fundamentals of General Chemistry. Grade 11: textbook for educational institutions: basic level / G.E. Rudzitis, F.G. Feldman. - 14th ed. - M.: Education, 2012.

2. Popel P.P. Chemistry: 8th grade: a textbook for general educational institutions / P.P. Popel, L.S. Krivlya. - K .: Information Center "Academy", 2008. - 240 p.: ill.

3. Gabrielyan O.S. Chemistry. Grade 11. A basic level of. 2nd ed., ster. - M.: Bustard, 2007. - 220 p.

Homework

1. No. 2, 4, 6 (p. 41) Rudzitis G.E. Chemistry. Fundamentals of General Chemistry. Grade 11: textbook for educational institutions: basic level / G.E. Rudzitis, F.G. Feldman. - 14th ed. - M.: Education, 2012.

2. Why is tungsten used to make the hairs of incandescent lamps?

3. What explains the absence of a hydrogen bond in aldehyde molecules?

All currently known chemical elements located in the periodic table are conditionally divided into two large groups: metals and non-metals. In order for them to become not just elements, but compounds, chemicals, to be able to interact with each other, they must exist in the form of simple and complex substances.

It is for this that some electrons are trying to accept, while others - to give. Replenishing each other in this way, the elements form various chemical molecules. But what keeps them together? Why are there substances of such strength that even the most serious tools cannot destroy? And others, on the contrary, are destroyed by the slightest impact. All this is explained by the formation of various types of chemical bonds between atoms in molecules, the formation of a crystal lattice of a certain structure.

Types of chemical bonds in compounds

In total, 4 main types of chemical bonds can be distinguished.

1. Covalent non-polar. It is formed between two identical non-metals due to the socialization of electrons, the formation of common electron pairs. Valence unpaired particles take part in its formation. Examples: halogens, oxygen, hydrogen, nitrogen, sulfur, phosphorus.
2. covalent polar. It is formed between two different non-metals or between a metal that is very weak in properties and a non-metal that is weak in electronegativity. It is also based on common electron pairs and their pulling towards oneself by that atom, whose electron affinity is higher. Examples: NH 3, SiC, P 2 O 5 and others.
3. Hydrogen bond. The most unstable and weak, it is formed between a strongly electronegative atom of one molecule and a positive one of another. Most often this happens when substances are dissolved in water (alcohol, ammonia, and so on). Thanks to this connection, macromolecules of proteins, nucleic acids, complex carbohydrates, and so on can exist.
4. Ionic bond. It is formed due to the forces of electrostatic attraction of differently charged ions of metals and non-metals. The stronger the difference in this indicator, the more pronounced is the ionic nature of the interaction. Examples of compounds: binary salts, complex compounds - bases, salts.
5. A metallic bond, the mechanism of formation of which, as well as properties, will be discussed further. It is formed in metals, their alloys of various kinds.

There is such a thing as the unity of a chemical bond. It just says that it is impossible to consider every chemical bond as a reference. They are all just nominal units. After all, all interactions are based on a single principle - electron static interaction. Therefore, ionic, metallic, covalent bonds and hydrogen bonds have a single chemical nature and are only boundary cases of each other.

Metals and their physical properties

Metals are in the vast majority among all chemical elements. This is due to their special properties. A significant part of them was obtained by man by nuclear reactions in the laboratory, they are radioactive with a short half-life.

However, the majority are natural elements that form whole rocks and ores and are part of most important compounds. It was from them that people learned to cast alloys and make a lot of beautiful and important products. These are such as copper, iron, aluminum, silver, gold, chromium, manganese, nickel, zinc, lead and many others.

For all metals, general physical properties can be distinguished, which are explained by the scheme for the formation of a metallic bond. What are these properties?

1. malleability and plasticity. It is known that many metals can be rolled even to the state of foil (gold, aluminum). From others, wire, metal flexible sheets, products that can deform under physical impact, but immediately restore their shape after its termination, are obtained. It is these qualities of metals that are called malleability and ductility. The reason for this feature is the metallic type of connection. Ions and electrons in a crystal slide relative to each other without breaking, which makes it possible to maintain the integrity of the entire structure.
2. Metallic sheen. It also explains the metallic bond, the mechanism of formation, its characteristics and features. So, not all particles are able to absorb or reflect light waves of the same wavelength. The atoms of most metals reflect short-wavelength rays and acquire almost the same color of silver, white, pale bluish. The exceptions are copper and gold, their color is reddish-red and yellow, respectively. They are able to reflect longer wavelength radiation.
3. Thermal and electrical conductivity. These properties are also explained by the structure of the crystal lattice and the fact that a metallic type of bond is realized in its formation. Due to the "electron gas" moving inside the crystal, electric current and heat are instantly and evenly distributed among all atoms and ions and conducted through the metal.
4. Solid state of aggregation under normal conditions. The only exception here is mercury. All other metals are necessarily strong, solid compounds, as well as their alloys. It is also a result of the presence of a metallic bond in metals. The mechanism of formation of this type of particle binding fully confirms the properties.

These are the main physical characteristics for metals, which are explained and determined by the scheme of formation of a metallic bond. This method of connecting atoms is relevant specifically for elements of metals, their alloys. That is, for them in the solid and liquid state.

Metal type chemical bond

What is its peculiarity? The thing is that such a bond is formed not due to differently charged ions and their electrostatic attraction, and not due to the difference in electronegativity and the presence of free electron pairs. That is, ionic, metallic, covalent bonds have a slightly different nature and distinctive features of the particles being bound.

All metals have the following characteristics:

• a small number of electrons per (except for some exceptions, which may have 6.7 and 8);
• large 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.

Thus, 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". It explains most of the 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 the electronic structure of a given element, one 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!

Metal bond: formation mechanism

If we consider it in general terms, then we have already outlined the main points above. The presence of free electrons and those easily detached from the nucleus due to the low ionization energy are the main conditions for the formation of this type of bond. Thus, it turns out that it is implemented between the following particles:

• atoms in the nodes of the crystal lattice;
• free electrons, which were valence in the metal;
• ions at the sites of the crystal lattice.

The end result is a metallic bond. The mechanism of formation in general terms 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.

The crystals themselves can have a different shape. It depends on the specific substance we are dealing with.

Types of metal crystals

This structure of a metal or its alloy is characterized by a very dense packing of particles. It is provided by ions at the nodes of the crystal. Lattices themselves can be of different geometric shapes in space.

1. Volume-centric cubic lattice - alkali metals.
2. Hexagonal compact structure - all alkaline earths except barium.
3. Face-centric cubic - aluminum, copper, zinc, many transition metals.
4. Rhombohedral structure - in mercury.
5. Tetragonal - indium.

The lower it is located in the periodic system, the more complex its packing and the spatial organization of the crystal. In this case, the metallic chemical bond, examples of which can be given for each existing metal, is decisive in the construction of a crystal. Alloys have a very diverse organization in space, some of which are still not fully understood.

Communication characteristics: non-directional

Covalent and metallic bonds have one very pronounced distinguishing feature. Unlike the first, the metallic bond is not directional. What does it mean? That is, the electron cloud inside the crystal moves completely freely within its limits in different directions, each of the electrons is able to join absolutely any ion at the nodes of the structure. That is, the interaction is carried out in different directions. Hence, they say that the metallic bond is non-directional.

The mechanism of covalent bonding involves the formation of common electron pairs, that is, clouds of overlapping atoms. Moreover, it occurs strictly along a certain line connecting their centers. Therefore, they talk about the direction of such a connection.

Saturability

This characteristic reflects the ability of atoms to have limited or unlimited interaction with others. So, the covalent and metallic bonds in this indicator are again opposites.

The first one is saturable. The atoms participating in its formation have a strictly defined number of valence outer electrons that are directly involved in the formation of the compound. More than it is, it will not have electrons. Therefore, the number of formed bonds is limited by valency. Hence the saturation of the connection. Due to this characteristic, most compounds have a constant chemical composition.

Metallic and hydrogen bonds, on the other hand, are unsaturable. This is due to the presence of numerous free electrons and orbitals inside the crystal. Ions also play a role in the nodes of the crystal lattice, each of which can become an atom and again an ion at any time.

Another characteristic of a metallic bond is the delocalization of the internal electron cloud. It manifests itself in the ability of a small number of common electrons to bind together many atomic nuclei of metals. That is, the density seems to be delocalized, distributed evenly between all links of the crystal.

Examples of bond formation in metals

Let's look at a few specific options that illustrate how a metallic bond is formed. Examples of substances are as follows:

• zinc;
• aluminum;
• potassium;
• chromium.

Formation of a metallic bond between zinc atoms: Zn 0 - 2e - ↔ Zn 2+. The zinc atom has four energy levels. Free orbitals, based on the electronic structure, it has 15 - 3 in p-orbitals, 5 in 4d and 7 in 4f. The electronic structure is as follows: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 0 4d 0 4f 0, there are 30 electrons in the atom. That is, two free valence negative particles are able to move within 15 spacious and unoccupied orbitals. And so it is with every atom. As a result - a huge common space, consisting of empty orbitals, and a small number of electrons that bind the entire structure together.

Metal bond between aluminum atoms: AL 0 - e - ↔ AL 3+. The thirteen electrons of an aluminum atom are located on three energy levels, which they obviously have in excess. Electronic structure: 1s 2 2s 2 2p 6 3s 2 3p 1 3d 0 . Free orbitals - 7 pieces. Obviously, the electron cloud will be small compared to the total internal free space in the crystal.

Chromium metal bond. This element is special in its electronic structure. Indeed, to stabilize the system, the electron falls from 4s to the 3d orbital: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 5 4p 0 4d 0 4f 0 . There are 24 electrons in total, of which six are valence. It is they who go into the common electronic space to form a chemical bond. There are 15 free orbitals, which is still much more than is required to fill. Therefore, chromium is also a typical example of a metal with a corresponding bond in the molecule.

One of the most active metals, reacting even with ordinary water with ignition, is potassium. What explains these properties? Again, in many ways - a metallic type of connection. This element has only 19 electrons, but they are already located at 4 energy levels. That is, on 30 orbitals of different sublevels. Electronic structure: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 0 4p 0 4d 0 4f 0 . Just two with very low ionization energy. Freely come off and go into the common electronic space. There are 22 orbitals to move one atom, that is, a very large free space for the "electron gas".

Similarities and differences with other types of relationships

In general, this issue has already been discussed above. We can only generalize and draw a conclusion. The main distinguishing features of metal crystals from all other types of communication are:

• several types of particles involved in the binding process (atoms, ions or atom-ions, electrons);
• different spatial geometric structure of crystals.

With hydrogen and ionic bonds, the metal bond is unsaturable and non-directional. With a covalent polar - a strong electrostatic attraction between the particles. Separately from the ionic - the type of particles in the nodes of the crystal lattice (ions). With covalent non-polar - atoms at the nodes of the crystal.

Types of bonds in metals of different state of aggregation

As we noted above, the metallic chemical bond, examples of which are given in the article, is formed in two states of aggregation of metals and their alloys: solid and liquid.

The question arises: what type of bond in metal vapors? Answer: covalent polar and non-polar. As in all compounds that are in the form of a gas. That is, with prolonged heating of the metal and its transfer from a solid state to a liquid, the bonds do not break and the crystalline structure is preserved. However, when it comes to transferring a liquid to a vapor state, the crystal is destroyed and the metallic bond is converted into a covalent one.

Material classification

Currently, all modern materials are accepted to be classified accordingly.

The most important in technology are classifications according to functional and structural signs of materials.

The main criterion for the classification of materials by structural features is the state of aggregation, depending on which they are divided into the following types: solid materials, liquids, gases, plasma.

Solid materials, in turn, are divided into crystalline and non-crystalline.

Crystalline materials can be divided according to the type of bond between particles: atomic (covalent), ionic, metallic, molecular (Fig. 2.1.).

Types of bonds between atoms (molecules) in crystals

An atom consists of a positively charged nucleus and electrons moving around it (negatively charged). An atom in a stationary state is electrically neutral. Distinguish between external (valence) electrons, the connection of which with the nucleus is insignificant and internal - firmly connected with the nucleus.

The formation of the crystal lattice occurs as follows. During the transition from the liquid to the crystalline state, the distance between the atoms decreases, and the interaction forces between them increase.

The connection between atoms is carried out by electrostatic forces, i.e. by nature, the connection is one - it has an electrical nature, but manifests itself differently in different crystals. There are the following types of bonds: ionic, covalent, polar, metallic.

Covalent type of bond

A covalent bond is formed due to common electron pairs that arise in the shells of the bonded atoms.

She may be formed by atoms of the same element and then it is non-polar; for example, such a covalent bond exists in the molecules of single-element gases H 2, O 2, N 2, Cl 2, etc.

The covalent bond can be formed by atoms of different elements, similar in chemical nature, and then it is polar; for example, such a covalent bond exists in H 2 O, NF 3 , CO 2 molecules.

A covalent bond is formed between atoms of elements that have an electronegative character.

With this type of bond, the socialization of free valence electrons of neighboring atoms is carried out. In an effort to acquire a stable valence shell consisting of 8 electrons, atoms combine into molecules, forming one or more pairs of electrons, which become common to the connecting atoms, i.e. are simultaneously part of the electron shells of two atoms.

Materials with a covalent bond are very brittle, but have high hardness (diamond). These are, as a rule, dielectrics or semiconductors (germanium, silicon). electric charges are interconnected, and there are no free electrons.

Atoms in the molecules of simple gases are connected by a covalent bond (H 2, Cl 2, etc.)

The only substance known to man with an example of a covalent bond between a metal and carbon is cyanocobalamin, known as vitamin B12.

Ionic crystals (NaCl)

Ionic bond is a chemical bond educated at the expense electrostatic attraction between cations and anions.

The formation of such crystals is formed by the transition of electrons of atoms of one type to atoms of another from Na to Cl. An atom that loses an electron becomes a positively charged ion, while an atom that gains an electron becomes a negative ion. The approach of ions of different signs occurs until the repulsive forces of the nucleus and electron shells balance the forces of attraction. Most mineral dielectrics and some organic materials have an ionic bond. (NaCl, CsCl, CaF2.)

Ionically bonded solids are in most cases mechanically strong, temperature-resistant, but often brittle. Materials with this type of connection are not used as structural materials.

Metal connection type

In metals, the bond between individual atoms is formed due to the interaction of positively charged nuclei and collectivized electrons, which move freely in interatomic spaces. 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. Such a bond is called a metallic bond.

The presence of free electrons leads to high electrical and thermal conductivity of the metal, and is also the reason for the brilliance of metals. The ductility of metals is explained by the movement and sliding of individual layers of atoms.

Practically in any material there is not one, but several types of bonds. The properties of materials are determined by the predominant types of chemical bonds of atoms and molecules of the substance of the material.

From atomic-crystalline materials, the structure of which is dominated by covalent bonds, polymorphic modifications of carbon and semiconductor materials based on elements of group IV of the periodic system of elements are of greatest importance in technology. Typical representatives of the former are diamond and graphite - the most common and stable modification of carbon with a layered structure in the earth's crust. Semiconductor crystalline germanium and silicon are the main materials of semiconductor electronics.

Of great interest are some compounds with a covalent bond, such as Fe 3 C, SiO, AlN - these compounds play an important role in technical alloys.

Into a vast collection ion-crystal materials that have a crystal structure with ionic bonds include metal oxides (compounds of metals with oxygen), which are components of the most important ores, technological additives in the smelting of metals, as well as chemical compounds of metals and non-metals (boron, carbon, nitrogen), which are used as alloy components.

The metallic type of bond is characteristic of more than 80 elements of the periodic table.

To crystalline solids materials with a structure can also be attributed molecular crystals, which is characteristic of many polymeric materials whose molecules consist of a large number of repeating units. These are biopolymers - high-molecular natural compounds and their derivatives (including wood); synthetic polymers derived from simple organic compounds whose molecules have inorganic main chains and do not contain organic side groups. Inorganic polymers include silicates and binders. Natural silicates are a class of the most important rock-forming minerals that make up about 80% of the mass of the earth's crust. Inorganic binders include cement, gypsum, lime, etc. Molecular crystals of inert gases - elements of group VIII of the periodic system - evaporate at low temperatures without passing into a liquid state. They find application in cryoelectronics, which is engaged in the creation of electronic devices based on the phenomena that take place in solids at cryogenic temperatures.

Rice. 1.2. Arrangement of atoms in crystalline (a) and amorphous (b) matter

The second class of materials are non-crystalline solid materials. They are divided on the basis of orderliness and structure stability into amorphous, glassy and non-glassy in a semi-disordered state.

Typical representatives of amorphous materials are amorphous semiconductors, amorphous metals and alloys.

To the group vitreous materials include: a number of organic polymers (polymethyl acrylate at temperatures below 105 ° C, polyvinyl chloride - below 82 ° C and others); many inorganic materials - inorganic glass based on oxides of silicon, boron, aluminum, phosphorus, etc.; many materials for stone casting - basalts and diabases with a glassy structure, metallurgical slags, natural carbonates with an island and chain structure (dolomite, marl, marble, etc.).

In a non-glassy, ​​semi-disordered state, there are jellies (structured polymer-solvent systems formed during the solidification of polymer solutions or swelling of solid polymers), many synthetic polymers in a highly elastic state, rubbers, most materials based on biopolymers, including textile and leather materials, and also organic binders - bitumen, tar, pitch, etc.

By function technical materials are divided into the following groups.

Construction materials - solid materials intended for the manufacture of products subjected to mechanical stress. They must have a set of mechanical properties that provide the required performance and service life of products when exposed to the working environment, temperature and other factors.

Rice. 1.1. Classification of solid crystalline materials by structural feature

At the same time, technological requirements are imposed on them, which determine the least laboriousness in the manufacture of parts and structures, and economic ones, relating to the cost and availability of the material, which is very important in mass production. Structural materials include metals, silicates and ceramics, polymers, rubber, wood, and many composite materials.

Electrical materials characterized by special electrical and magnetic properties and are intended for the manufacture of products used for the production, transmission, conversion and consumption of electricity. These include magnetic materials, conductors, semiconductors, as well as dielectrics in solid liquid and gaseous phases.

Tribological materials are intended for use in friction units in order to control friction and wear parameters to ensure the specified performance and resource of these units. The main types of such materials are lubricating, antifriction and friction. The former include lubricants in the solid (graphite, talc, molybdenum disulfide, etc.), liquid (lubricating oils) and gaseous phases (air, hydrocarbon vapors and other gases). The totality of antifriction materials includes non-ferrous metal alloys (babbits, bronzes, etc. ), gray cast iron, plastics (textolite, materials based on fluoroplastics, etc.), cermet composite materials (bronze graphite, iron graphite, etc.), some types of wood and wood-laminated plastics, rubber, many composites Friction materials have a high coefficient of friction and high wear resistance These include some types of plastics, cast irons, cermets and other composite materials.

Tool materials are distinguished by high hardness, wear resistance and strength, they are intended for the manufacture of cutting, measuring, metalwork and other tools. This includes materials such as tool steel and hard alloys, diamond and some types of ceramic materials, and many composite materials.

working bodies - gaseous and liquid materials, with the help of which energy is converted into mechanical work, cold, heat. The working fluids are water vapor in steam engines and turbines; ammonia, carbon dioxide, freon and other refrigerants in refrigerators; hydraulic oils; air in pneumatic motors; gaseous products of fossil fuel combustion in gas turbines, internal combustion engines.

Fuel - combustible materials, the main part of which is carbon, used to obtain thermal energy by burning them. By origin, fuel is divided into natural (oil, coal, natural gas, oil shale, peat, wood) and artificial (coke, motor fuels, generator gases, etc.); according to the type of machines in which it is burned - for rocket, motor, nuclear, turbine, etc.