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Metal connection. Properties of metallic bond.

A metallic bond is a chemical bond caused by the presence of relatively free electrons. Characteristic of both pure metals and their alloys and intermetallic compounds.

Metal link mechanism

Positive metal ions are located at all nodes of the crystal lattice. Between them, valence electrons move randomly, like gas molecules, detached from the atoms during the formation of ions. These electrons act as cement, holding the positive ions together; otherwise, the lattice would disintegrate under the influence of repulsive forces between the ions. At the same time, electrons are held by ions within the crystal lattice and cannot leave it. The coupling forces are not localized or directed. For this reason, in most cases high coordination numbers appear (for example, 12 or 8). When two metal atoms come close together, the orbitals in their outer shells overlap to form molecular orbitals. If a third atom approaches, its orbital overlaps with the orbitals of the first two atoms, resulting in another molecular orbital. When there are many atoms, a huge number of three-dimensional molecular orbitals arise, extending in all directions. Due to multiple overlapping orbitals, the valence electrons of each atom are influenced by many atoms.

Characteristic crystal lattices

Most metals form one of the following highly symmetrical lattices with close packing of atoms: body-centered cubic, face-centered cubic, and hexagonal.

In a body-centered cubic (bcc) lattice, the atoms are located at the vertices of the cube and one atom is at the center of the cube volume. 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 (fcc) lattice, the 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, the atoms are located at the vertices and center of the hexagonal bases of the prism, and three atoms are located in the middle plane of the prism. Metals have this 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 that have a metallic bond often combine strength with plasticity, since when atoms are displaced relative to each other, the bonds do not break. Another important property is metallic aromaticity.

Metals conduct heat and electricity well, they are strong enough, and can be deformed without destruction. Some metals are malleable (they can be forged), some are malleable (you can draw wire from them). 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.

Metallic bond explains the properties of metals, in particular their strength. Under the influence of a deforming force, a metal lattice can change its shape without cracking, unlike ionic crystals.

The high thermal conductivity of metals is explained by the fact that if a piece of metal is heated on one side, the kinetic energy of the electrons will increase. This increase in energy will spread in the “electron sea” throughout the sample at high speed.

The electrical conductivity of metals also becomes clear. If a potential difference is applied to the ends of a metal sample, the cloud of delocalized electrons will shift in the direction of the positive potential: this flow of electrons moving in one direction represents the familiar electric current.

Metal connection. Properties of metallic bond. - concept and types. Classification and features of the category "Metallic bond. Properties of metallic bond." 2017, 2018.

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

This is why some electrons try to accept, while others try to give away. By replenishing each other in this way, the elements form various chemical molecules. But what keeps them together? Why do there exist substances of such strength that even the most serious instruments cannot be destroyed? 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, there are 4 main types of chemical bonds.

1. Covalent non-polar. It is formed between two identical non-metals due to the sharing 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. Formed between two different non-metals or between a metal with very weak properties and a non-metal with weak electronegativity. It is also based on common electron pairs and the pulling of them toward itself by the atom whose electron affinity is higher. Examples: NH 3, SiC, P 2 O 5 and others.
3. Hydrogen bond. The most unstable and weakest, it is formed between a highly electronegative atom of one molecule and a positive atom of another. Most often this happens when substances are dissolved in water (alcohol, ammonia, etc.). 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 metal and non-metal ions. The stronger the difference in this indicator, the more clearly the ionic nature of the interaction is expressed. Examples of compounds: binary salts, complex compounds - bases, salts.
5. A metal bond, the formation mechanism of which, as well as its properties, will be discussed further. It is formed in metals and 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 standard. They are all just conventionally designated units. After all, all interactions are based on a single principle - electron-static interaction. Therefore, ionic, metallic, covalent and hydrogen bonds have the same chemical nature and are only borderline cases of each other.

Metals and their physical properties

Metals are found in the overwhelming majority of all chemical elements. This is due to their special properties. A significant part of them was obtained by humans through nuclear reactions in laboratory conditions; they are radioactive with a short half-life.

However, the majority are natural elements that form entire 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 copper, iron, aluminum, silver, gold, chromium, manganese, nickel, zinc, lead and many others.

For all metals, common physical properties can be identified, which are explained by the formation of a metallic bond. What are these properties?

1. Malleability and ductility. It is known that many metals can be rolled even to the state of foil (gold, aluminum). Others produce wire, flexible metal sheets, and products that can be deformed during physical impact, but immediately restore their shape after it stops. It is these qualities of metals that are called malleability and ductility. The reason for this feature is the metal type of connection. The ions and electrons in the crystal slide relative to each other without breaking, which allows maintaining the integrity of the entire structure.
2. Metallic shine. It also explains the metallic bond, the formation mechanism, its characteristics and features. Thus, not all particles are able to absorb or reflect light waves of the same wavelength. The atoms of most metals reflect short-wave rays and acquire almost the same color of silver, white, and pale bluish tint. The exceptions are copper and gold, their colors are red-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 the 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 between all atoms and ions and are 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. This is also a result of metallic bonding being present in metals. The mechanism of formation of this type of particle binding fully confirms the properties.

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

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 slightly different natures and distinctive features of the particles being bonded.

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 outer unpaired electrons from the nucleus. At the same time, the atom has a lot of free orbitals. The diagram of the formation of a metallic bond will precisely 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 through different parts of the lattice. Periodically, each of them attaches to an ion at a site in the crystal and turns it into an atom, then detaches again to form an ion.

Thus, a metallic bond is the bond between atoms, ions and free electrons in a common metal crystal. An electron cloud moving freely within a structure is called an “electron gas.” This is what explains most metals and their alloys.

How exactly does a metal chemical bond realize itself? Various examples can be given. Let's try to look at it on a piece of lithium. Even if you take it the size of a pea, there will be thousands of atoms. So let’s imagine that each of these thousands of atoms gives up its single valence electron to the common crystalline space. At the same time, knowing the electronic structure of a 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 metal bond is always strong. After all, electron gas does not allow the crystal to collapse, but only displaces the layers and immediately restores them. It shines, has a certain density (most often high), fusibility, malleability and plasticity.

Where else is metal bonding sold? Examples of substances:

• metals in the form of simple structures;
• all metal alloys with each other;
• all metals and their alloys in liquid and solid states.

There are simply an incredible number of specific examples, since there are more than 80 metals in the periodic table!

Metal bond: mechanism of formation

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

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

The result is a metal bond. The mechanism of formation is generally expressed by the following notation: Me 0 - e - ↔ Me n+. From the diagram it is obvious what particles are present in the metal crystal.

The crystals themselves can have different shapes. 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 in the crystal nodes. The lattices themselves can have different geometric shapes in space.

1. Body-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. Mercury has a rhombohedral structure.
5. Tetragonal - indium.

The lower and lower it is located in the periodic system, the more complex its packaging and 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 the crystal. Alloys have very diverse organizations in space, some of which have not yet been fully studied.

Communication characteristics: non-directional

Covalent and metallic bonds have one very pronounced distinctive 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 boundaries in different directions, each electron is capable of attaching to absolutely any ion at the nodes of the structure. That is, 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 shared 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. Thus, covalent and metallic bonds are again opposites according to this indicator.

The first is saturable. The atoms taking part in its formation have a strictly defined number of valence external electrons, which are directly involved in the formation of the compound. It will not have more electrons than it has. Therefore, the number of bonds formed is limited by valence. Hence the saturation of the connection. Due to this characteristic, most compounds have a constant chemical composition.

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

Another characteristic of metallic bonding is the delocalization of the internal electron cloud. It manifests itself in the ability of a small number of shared electrons to bind together many atomic nuclei of metals. That is, the density is, as it were, delocalized, distributed evenly between all parts 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:

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

Formation of a metallic bond between zinc atoms: Zn 0 - 2e - ↔ Zn 2+. The zinc atom has four energy levels. Based on the electronic structure, it has 15 free orbitals - 3 in p-orbitals, 5 in 4 d 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, a total of 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 for every atom. The result is a huge common space consisting of empty orbitals and a small number of electrons that bind the entire structure together.

Metallic bond between aluminum atoms: AL 0 - e - ↔ AL 3+. The thirteen electrons of an aluminum atom are located at three energy levels, which they clearly have in abundance. 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.

Chrome metal bond. This element is special in its electronic structure. Indeed, to stabilize the system, the electron falls from the 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 electrons. They are the ones who go into the common electronic space to form a chemical bond. There are 15 free orbitals, which is still much more than 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 that reacts even with ordinary water with fire is potassium. What explains these properties? Again, in many ways - by a metal type of connection. This element has only 19 electrons, but they are located at 4 energy levels. That is, in 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 . Only two with very low ionization energy. They break away freely and go into the common electronic space. There are 22 orbitals for movement per atom, that is, a very large free space for “electron gas”.

Similarities and differences with other types of connections

In general, this issue has already been discussed above. One can only generalize and draw a conclusion. The main features of metal crystals that distinguish them from all other types of connections are:

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

Metallic bonds have in common with hydrogen and ionic bonds unsaturation and non-directionality. With covalent polar - strong electrostatic attraction between particles. Separately from ionic - a type of particles at the nodes of a crystal lattice (ions). With covalent nonpolar - atoms in the nodes of the crystal.

Types of bonds in metals of different states of aggregation

As we noted above, a 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 is in metal vapors? Answer: covalent polar and non-polar. As with all compounds that are in the form of a gas. That is, when the metal is heated for a long time and transferred from a solid to a liquid state, the bonds do not break and the crystalline structure is preserved. However, when it comes to transferring the liquid into a vapor state, the crystal is destroyed and the metallic bond is converted into a covalent one.

The lesson will cover 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 bonds

Lesson: Metal and Hydrogen Chemical Bonds

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

Metals are chemical elements with low electronegativity, so they easily give up their valence electrons. If there is a non-metal next to a metal element, then electrons from the metal atom go 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.

When molecules are formed, 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 be either close to the totality of atomic orbitals or significantly different 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 through these molecular orbitals. Complete separation of charge does not occur, i.e. metal- this is not a collection of cations and electrons floating around. But this is not a collection of atoms that sometimes transform 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 metal bond formation consists of as follows: metal atoms donate outer electrons, and some of them turn into positively charged ions. Torn away from atoms electrons move relatively freely between emerging positivemetal ions. A metallic bond arises between these particles, i.e., electrons seem to cement positive ions in the metal lattice (Fig. 2).

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

High ductility

Heat and electrical conductivity

Metallic shine

Plastic - this is the ability of a material to easily deform under mechanical load. A metallic bond is realized between all metal atoms simultaneously, therefore, when a metal is subjected to mechanical action, specific bonds are not broken, but only the position of the atom changes. Metal atoms, not connected by rigid bonds to each other, can, as it were, slide along a layer of electron gas, as happens when one glass slides over another with a layer of water between them. Thanks 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 are found in nature in native form in varying degrees of purity. Rice. 3.

Rice. 3. Metals found in nature in native form

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

Thermal and electrical conductivity . Copper, silver, gold and aluminum conduct electricity best. 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 high electrical resistance that when an electric current passes through it, it begins to glow. 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 transfer excess energy quite quickly from one ion or atom to another. The temperature of the metal quickly equalizes throughout the entire volume, even if heating occurs on one side. This is observed, for example, if you dip a metal spoon into tea.

Metallic shine. Gloss is the ability of a 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 to the glass surface in the manufacture of headlights, spotlights and mirrors.

Hydrogen bond

Let's consider the boiling and melting temperatures of hydrogen compounds of chalcogens: oxygen, sulfur, selenium and tellurium. Rice. 4.

Rice. 4

If we mentally extrapolate the direct boiling and melting temperatures of 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 - approximately -80 0 C. This happens because there is a gap between water molecules interaction - hydrogen bond, which unites 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 very high electronegativity: fluorine, oxygen or nitrogen . Examples of substances capable of forming hydrogen bonds are shown in Fig. 5.

Rice. 5

Consider the formation of hydrogen bonds between water molecules. A 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 a 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.

Let's compare the 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.

Table 1

Boiling point, mp., solubility in water is higher for ethyl alcohol. This is a general pattern for substances whose molecules form hydrogen bonds. These substances are characterized by higher boiling point, melting temperature, 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 masses.

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

Van der Waals connection.

Let's remember the noble gases. Helium compounds have not yet been obtained. It is not capable of forming ordinary chemical bonds.

At very low 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 variants of these powers:

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

· inductive attraction. This is the attraction between a dipole and a nonpolar molecule.

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

Summing up the lesson

The lesson covers 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. 11th grade: textbook for general education institutions: basic level / G.E. Rudzitis, F.G. Feldman. - 14th ed. - M.: Education, 2012.

2. Popel P.P. Chemistry: 8th grade: textbook for general education institutions / P.P. Popel, L.S. Krivlya. - K.: IC "Academy", 2008. - 240 pp.: ill.

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

Homework

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

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

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

As already indicated in paragraph 4.2.2.1, metal connection- electronic connection of atomic nuclei with minimal localization of shared electrons both on individual (as opposed to an ionic bond) nuclei, and on individual (as opposed to a covalent bond) bonds. The result is an electron-deficient multicenter chemical bond in which shared electrons (in the form of “electron gas”) provide bonding to the maximum possible number of nuclei (cations) that form the structure of liquid or solid metallic substances. Therefore, the metallic bond as a whole is non-directional and saturated; it should be considered as limiting case of delocalization of a covalent bond. Let us recall that in pure metals the metallic bond appears primarily homonuclear, i.e. cannot have an ionic component. As a result, a typical picture of the electron density distribution in metals is spherically symmetrical cores (cations) in a uniformly distributed electron gas (Fig. 5.10).

Consequently, the final structure of compounds with a predominantly metallic type of bond is determined primarily by the steric factor and packing density in the crystal lattice of these cations (high CN). The BC method cannot interpret metallic bonds. According to MMO, a metallic bond is characterized by a deficiency of electrons compared to a covalent bond. Strict application of MMO to metallic bonds and connections leads to band theory(electronic model of a metal), according to which in the atoms included in the crystal lattice of a metal, there is an interaction of almost free valence electrons located in external electron orbits with the (electric) periodic field of the crystal lattice. As a result, the energy levels of electrons split and form a more or less wide band. According to Fermi statistics, the highest energy band is populated by free electrons up to complete filling, especially if the energy terms of an individual atom correspond to two electrons with antiparallel spins. However, it can be partially filled, which provides the opportunity for electrons to move to higher energy levels. Then

this zone is called the conduction zone. There are several basic types of relative arrangement of energy bands, corresponding to an insulator, a monovalent metal, a divalent metal, a semiconductor with intrinsic conductivity, a -type semiconductor and an impurity semiconductor/b-type. The ratio of energy bands also determines the type of conductivity of a solid.

However, this theory does not allow quantitative characterization of various metal compounds and has not led to a solution to the problem of the origin of real crystal structures of metal phases. The specific nature of chemical bonds in homonuclear metals, metal alloys and intermetallic heterocompounds is considered by N.V. Ageev)