Elements with a covalent non-polar bond. covalent bond. Basic properties of covalent bonds

There are four main types of chemical bonds:

1. covalent bond carried out by shared electron pairs. It is formed in the result of overlapping electron clouds (orbitals) of non-metal atoms. The greater the overlap of electron clouds, the stronger chemical bond. A covalent bond can be polar or non-polar. covalent non-polar connection occurs between atoms of the same species that have the same electronegativity. (Electronegativity is the property of atoms to attract electrons to themselves.) For example, the formation of a hydrogen molecule can be shown by the diagram:

H . + . h=h( : ) H H 2

or H . + . H=H-H

Similarly, molecules O 2, Cl 2, N 2, F 2, etc. are formed.

A non-polar covalent bond is symmetrical. An electron cloud formed by a common (shared) electron pair equally belongs to two atoms.

polar covalent connection occurs between atoms whose electronegativity differs, but only slightly. In this case, the common electron pair shifts towards a more electronegative element, for example, when a hydrogen chloride molecule is formed, the electron cloud of the bond is shifted towards the chlorine atom. Due to this displacement, the chlorine atom acquires a partial negative charge, and the hydrogen atom acquires a partial positive charge, and the resulting molecule is polar.

H + Cl = H Cl H → Cl HCl

Molecules HBr, HI, HF, H 2 O, CH 4, etc. are formed similarly.

covalent bonds there are single(carried out by one common electron pair), double(implemented by two common electron pairs), triple(implemented by three common electron pairs). For example, ethane has all single bonds, ethylene has a double bond, and acetylene has a triple bond.

Ethane: CH 3 -CH 3 Ethylene: CH 2 \u003d CH 2 Acetylene: CH ≡ CH

2. Ionic bond occurs in compounds formed by atoms of elements that differ greatly in electronegativity, that is, with sharply opposite properties (metal and non-metal atoms). Ions are charged particles into which atoms turn as a result of the recoil or attachment of electrons.

An ionic bond is formed due to the electrostatic attraction of oppositely charged ions. For example, a sodium atom, donating its electron, turns into a positively charged ion, and a chlorine atom, accepting this electron, turns into a negatively charged ion. Due to the electrostatic attraction between sodium and chloride ions, a ionic bond:

Na + Cl Na + + Cl – Na + Cl –

Sodium chloride molecules exist only in the vapor state. In the solid (crystalline) state, ionic compounds consist of regularly arranged positive and negative ions. There are no molecules in this case.

An ionic bond can be seen as an extreme case of a covalent bond.

3. Metal bondexists in metals and alloys. It is carried out due to the attraction between metal ions and socialized electrons (these are valence electrons that have left their orbits and move throughout the piece of metal between ions - "electron gas").

4. Hydrogen bond- this is a kind of bond that occurs between the hydrogen atom of one molecule, which has a partial positive charge, and the electronegative atom of another or the same molecule. The hydrogen bond can be intermolecular and intramolecular. HF…HF…HF. Denoted by dots. Weaker than covalent.

Data on ionization energy (EI), PEI and composition of stable molecules - their real values and comparisons - both free atoms and atoms bound into molecules, allow us to understand how atoms form molecules through the mechanism of covalent bonding.

COVALENT BOND- (from the Latin "co" together and "vales" having power) (homeopolar bond), a chemical bond between two atoms that occurs when the electrons belonging to these atoms are shared. Atoms in the molecules of simple gases are connected by a covalent bond. A bond in which there is one common pair of electrons is called single; there are also double and triple bonds.

Let's look at a few examples to see how we can use our rules to determine the number of covalent chemical bonds that an atom can form if we know the number of electrons in the outer shell of a given atom and the charge of its nucleus. The charge of the nucleus and the number of electrons in the outer shell are determined experimentally and are included in the table of elements.

Calculation of the possible number of covalent bonds

For example, let's count the number of covalent bonds that sodium can form ( Na), aluminum (Al), phosphorus (P) and chlorine ( Cl). sodium ( Na) and aluminum ( Al) have, respectively, 1 and 3 electrons on the outer shell, and, according to the first rule (for the mechanism of formation of a covalent bond, one electron on the outer shell is used), they can form: sodium (Na)- 1 and aluminum ( Al)- 3 covalent bonds. After the formation of bonds, the number of electrons on the outer shells of sodium ( Na) and aluminum ( Al) equals, respectively, 2 and 6; i.e., less than the maximum number (8) for these atoms. Phosphorus ( P) and chlorine ( Cl) have, respectively, 5 and 7 electrons on the outer shell and, according to the second of the above regularities, they could form 5 and 7 covalent bonds. In accordance with the fourth regularity, the formation of a covalent bond, the number of electrons in the outer shell of these atoms increases by 1. According to the sixth regularity, when a covalent bond is formed, the number of electrons in the outer shell of the bonded atoms cannot be more than 8. That is, phosphorus ( P) can only form 3 bonds (8-5 = 3), while chlorine ( Cl) can form only one (8-7 = 1).

Example: based on the analysis, we found that a certain substance consists of sodium atoms (Na) and chlorine ( Cl). Knowing the regularities of the mechanism of formation of covalent bonds, we can say that sodium ( Na) can form only 1 covalent bond. Thus, we can assume that each sodium atom ( Na) bonded to the chlorine atom ( Cl) through a covalent bond in this substance, and that this substance is composed of the molecules of an atom NaCl. The structure formula for this molecule is: Na-Cl. Here, a dash (-) means a covalent bond. The electronic formula of this molecule can be shown as follows:
. .
Na:Cl:
. .
In accordance with the electronic formula, on the outer shell of the sodium atom ( Na) in NaCl there are 2 electrons, and on the outer shell of the chlorine atom ( Cl) there are 8 electrons. In this formula, electrons (dots) between sodium atoms ( Na) and chlorine (Cl) are bonding electrons. Since PEI in chlorine ( Cl) equal to 13 eV, and for sodium (Na) it is equal to 5.14 eV, the bonding pair of electrons is much closer to the atom Cl than to an atom Na. If the ionization energies of the atoms that form the molecule are very different, then the bond formed will be polar covalent bond.

Let's consider another case. Based on the analysis, we found that a certain substance consists of aluminum atoms ( Al) and chlorine atoms ( Cl). For aluminum ( Al) there are 3 electrons in the outer shell; thus it can form 3 covalent chemical bonds while chlorine (Cl), as in the previous case, can form only 1 bond. This substance is presented as AlCl 3, and its electronic formula can be illustrated as follows:

Figure 3.1. Electronic formulaAlCl 3

whose formula is:
Cl - Al - Cl
Cl

This electronic formula shows that AlCl 3 on the outer shell of chlorine atoms ( Cl) there are 8 electrons, while on the outer shell of the aluminum atom ( Al) There are 6 of them. According to the mechanism of formation of a covalent bond, both binding electrons (one from each atom) enter the outer shells of the bound atoms.

Multiple covalent bonds

Atoms that have more than one electron in the outer shell can form not one, but several covalent bonds with each other. Such connections are called multiple (more often multiples) connections. Examples of such bonds are bonds of nitrogen molecules ( N= N) and oxygen ( O=O).

The bond formed when single atoms combine is called homoatomic covalent bond, e If the atoms are different, then the bond is called heteroatomic covalent bond[Greek prefixes "homo" and "hetero" respectively mean the same and different].

Imagine what a molecule with paired atoms actually looks like. The simplest molecule with paired atoms is the hydrogen molecule.

covalent bond(from the Latin "with" jointly and "vales" valid) is carried out by an electron pair belonging to both atoms. Formed between atoms of non-metals.

The electronegativity of non-metals is quite high, so that at chemical interaction two non-metal atoms, the complete transfer of electrons from one to the other (as in the case) is impossible. In this case, electron pooling is necessary to perform.

As an example, let's discuss the interaction of hydrogen and chlorine atoms:

H 1s 1 - one electron

Cl 1s 2 2s 2 2 p6 3 s2 3 p5 - seven electrons in the outer level

Each of the two atoms lacks one electron in order to have a complete outer electron shell. And each of the atoms allocates “for common use” one electron. Thus, the octet rule is satisfied. The best way to represent this is with the Lewis formulas:

Formation of a covalent bond

The shared electrons now belong to both atoms. The hydrogen atom has two electrons (its own and the shared electron of the chlorine atom), and the chlorine atom has eight electrons (its own plus the shared electron of the hydrogen atom). These two shared electrons form a covalent bond between the hydrogen and chlorine atoms. The particle formed when two atoms bond is called molecule.

Non-polar covalent bond

A covalent bond can form between two the same atoms. For example:

This diagram explains why hydrogen and chlorine exist as diatomic molecules. Thanks to the pairing and socialization of two electrons, it is possible to fulfill the octet rule for both atoms.

In addition to single bonds, a double or triple covalent bond can be formed, as, for example, in oxygen O 2 or nitrogen N 2 molecules. Nitrogen atoms each have five valence electrons, so three more electrons are required to complete the shell. This is achieved by sharing three pairs of electrons, as shown below:

Covalent compounds are usually gases, liquids, or relatively low-melting solids. One of the rare exceptions is diamond, which melts above 3,500°C. This is due to the structure of diamond, which is a continuous lattice of covalently bonded carbon atoms, and not a collection of individual molecules. In fact, any diamond crystal, regardless of its size, is one huge molecule.

A covalent bond occurs when the electrons of two nonmetal atoms join together. The resulting structure is called a molecule.

Polar covalent bond

In most cases, two covalently bonded atoms have different electronegativity and shared electrons do not belong equally to two atoms. Most of the time they are closer to one atom than to another. In a molecule of hydrogen chloride, for example, the electrons that form a covalent bond are located closer to the chlorine atom, since its electronegativity is higher than that of hydrogen. However, the difference in the ability to attract electrons is not so great that there is a complete transfer of an electron from a hydrogen atom to a chlorine atom. Therefore, the bond between hydrogen and chlorine atoms can be viewed as a cross between an ionic bond (complete electron transfer) and a non-polar covalent bond (symmetrical arrangement of a pair of electrons between two atoms). The partial charge on atoms is denoted by the Greek letter δ. Such a connection is called polar covalent bond, and the hydrogen chloride molecule is said to be polar, that is, it has a positively charged end (hydrogen atom) and a negatively charged end (chlorine atom).

The table below lists the main types of bonds and examples of substances:

Exchange and donor-acceptor mechanism of covalent bond formation

1) Exchange mechanism. Each atom contributes one unpaired electron to a common electron pair.

2) Donor-acceptor mechanism. One atom (donor) provides an electron pair, and another atom (acceptor) provides an empty orbital for this pair.

7.11. The structure of substances with a covalent bond

Substances in which only a covalent bond is present of all types of chemical bond are divided into two unequal groups: molecular (very much) and non-molecular (much less).
solid crystals molecular substances consist of molecules weakly interconnected by forces of intermolecular interaction. Such crystals do not have high strength and hardness (remember ice or sugar). They also have low melting and boiling points (see table 22).

Table 22. Melting and boiling points of some molecular substances

Substance			Substance
H2	– 259	– 253	Br2	– 7	58
N 2	– 210	– 196	H2O	0	100
HCl	– 112	– 85	P4	44	257
NH3	– 78	– 33	C 10 H 8 (naphthalene)	80	218
SO2	– 75	– 10	S8	119

Unlike their molecular counterparts, non-molecular substances with a covalent bond form very hard crystals. Diamond crystals (the hardest substance) are of this type.
In a diamond crystal (Fig. 7.5), each carbon atom is connected to four other carbon atoms by simple covalent bonds (sp 3 hybridization). The carbon atoms form a three-dimensional framework. Essentially, the entire diamond crystal is one huge and very strong molecule.
Silicon crystals, which are widely used in radio electronics and electronic engineering, have the same structure.
If you replace half of the carbon atoms in diamond with silicon atoms, without disturbing the skeletal structure of the crystal, you get a crystal of silicon carbide SiC - also a very hard substance used as an abrasive material. Ordinary quartz sand (silicon dioxide) also belongs to this type of crystalline substances. Quartz is very solid; called emery, it is also used as an abrasive. The structure of quartz is easy to obtain if oxygen atoms are inserted between every two silicon atoms in a silicon crystal. In this case, each silicon atom will be associated with four oxygen atoms, and each oxygen atom with two silicon atoms.

Crystals of diamond, silicon, quartz and similar in structure are called atomic crystals.
An atomic crystal is a crystal consisting of atoms of one or more elements linked by chemical bonds.
The chemical bond in an atomic crystal can be covalent or metallic.
As you already know, any atomic crystal, like an ionic one, is a huge "supermolecule". The structural formula of such a "supermolecule" cannot be written down - one can only show its fragment, for example:

Unlike molecular substances, substances that form atomic crystals are among the most refractory (see table 23.).

Table 23. Melting and boiling points of some non-molecular substances with covalent bonds

Such high melting points are quite understandable, if we remember that during the melting of these substances, not weak intermolecular, but strong chemical bonds are broken. For the same reason, many substances that form atomic crystals do not melt when heated, but decompose or immediately pass into a vapor state (sublimate), for example, graphite sublimates at 3700 o C.

Silicon - Si. Very hard, brittle silicon crystals look like metal, but it is not a metal. According to the type of electrical conductivity, this substance belongs to semiconductors, which determines its enormous importance in the modern world. Silicon is the most important semiconductor material. Radio receivers, televisions, computers, modern telephones, electronic clocks, solar panels and many other household and industrial devices contain transistors, microcircuits and photocells made of high-purity silicon monocrystals as the most important structural elements. Technical silicon is used in steel production and non-ferrous metallurgy. According to its chemical properties, silicon is a rather inert substance, it reacts only at high temperatures.

Silicon dioxide - SiO 2. Another name for this substance is silica. Silicon dioxide occurs naturally in two forms: crystalline and amorphous. Many semi-precious and ornamental stones are varieties of crystalline silicon dioxide (quartz): rock crystal, jasper, chalcedony, agate. and opal is an amorphous form of silica. Quartz is very widespread in nature, because dunes in deserts, and sandbanks of rivers and seas are all quartz sand. Quartz is a colorless crystalline very hard and refractory substance. In terms of hardness, it is inferior to diamond and corundum, but, nevertheless, it is widely used as an abrasive material. Quartz sand is widely used in construction and building materials industry. Quartz glass is used to make laboratory glassware and scientific instruments because it does not crack when abrupt change temperature. By their own chemical properties silica - acid oxide, but reacts with alkalis only when fused. At high temperatures, silicon carbide, carborundum, is obtained from silicon dioxide and graphite. Carborundum is the second hardest substance after diamond, it is also used to make grinding wheels and sandpaper.

7.12. Polarity of a covalent bond. Electronegativity

Recall that isolated atoms of different elements have different propensities for both donating and accepting electrons. These differences persist even after the formation of a covalent bond. That is, the atoms of some elements tend to attract the electron pair of a covalent bond to themselves more strongly than the atoms of other elements.

Consider a molecule HCl.
In this example, let's see how we can estimate the displacement of the electron bond cloud using molar ionization energies and electron means. 1312 kJ/mol, and 1251 kJ/mol - the difference is insignificant, about 5%. 73 kJ / mol, and 349 kJ / mol - here the difference is much larger: the electron affinity energy of the chlorine atom is almost five times greater than that of the hydrogen atom. From this we can conclude that the electron pair of the covalent bond in the hydrogen chloride molecule is largely shifted towards the chlorine atom. In other words, the bond electrons spend more time near the chlorine atom than near the hydrogen atom. Such an uneven distribution of the electron density leads to a redistribution of electric charges within the molecule. Partial (excess) charges arise on the atoms; on the hydrogen atom it is positive, and on the chlorine atom it is negative.

In this case, the bond is said to be polarized, and the bond itself is called a polar covalent bond.
If the electron pair of a covalent bond is not shifted to any of the bonded atoms, that is, the bond electrons equally belong to the bonded atoms, then such a bond is called a nonpolar covalent bond.
The concept of "formal charge" in the case of a covalent bond is also applicable. Only in the definition we should not talk about ions, but about atoms. AT general case the following definition can be given.

In molecules in which covalent bonds are formed only by the exchange mechanism, the formal charges of atoms are equal to zero. So, in the HCl molecule, the formal charges on the atoms of both chlorine and hydrogen are equal to zero. Therefore, in this molecule, the real (effective) charges on the chlorine and hydrogen atoms are equal to the partial (excess) charges.
It is far from always easy to determine the sign of the partial charge on the atom of one or another element in the molecule by molar ionization energies and affinity to the electrode, that is, to estimate in which direction the electron pairs of bonds are shifted. Usually, for these purposes, another energy characteristic of an atom is used - electronegativity.

Currently, there is no single, generally accepted designation for electronegativity. You can designate it with the letters E / O. Also, there is no single, generally accepted method for calculating electronegativity. Simplified, it can be represented as half the sum of molar ionization energies and electron affinity - this was one of the first ways to calculate it.
The absolute values of the electronegativity of atoms of various elements are used very rarely. More often, relative electronegativity is used, denoted by the letter c. Initially, this value was defined as the ratio of the electronegativity of an atom of a given element to the electronegativity of a lithium atom. Subsequently, the methods of its calculation have changed somewhat.
Relative electronegativity is a dimensionless quantity. Its values are given in Appendix 10.

Since the relative electronegativity depends primarily on the ionization energy of the atom (the electron affinity energy is always much lower), then in the system chemical elements it changes approximately the same as the ionization energy, that is, it increases diagonally from cesium (0.86) to fluorine (4.10). The values of the relative electronegativity of helium and neon given in the table are of no practical importance, since these elements do not form compounds.

Using the table of electronegativity, one can easily determine in the direction of which of the two atoms the electrons that bind these atoms are displaced, and, consequently, the signs of the partial charges arising on these atoms.

H2O			Communication is polar
H2	Atoms are the same	H--H	Communication is non-polar
CO2			Communication is polar
Cl2	Atoms are the same	Cl--Cl	Communication is non-polar
H 2 S			Communication is polar

Thus, in the case of the formation of a covalent bond between atoms of different elements, such a bond will always be polar, and in the case of the formation of a covalent bond between atoms of one element (in simple substances), the bond is in most cases non-polar.

The greater the difference in the electronegativity of the bonded atoms, the more polar is the covalent bond between these atoms.

Hydrogen sulfide H 2 S- a colorless gas with a characteristic odor characteristic of rotten eggs; poisonous. It is thermally unstable and decomposes when heated. Hydrogen sulfide is slightly soluble in water water solution called hydrosulphuric acid. Hydrogen sulfide provokes (catalyses) the corrosion of metals, it is this gas that is "guilty" of the darkening of silver.
In nature, it is found in some mineral waters. In the process of life, it is formed by some bacteria. Hydrogen sulfide is destructive to all living things. The hydrogen sulfide layer was discovered in the depths of the Black Sea and inspires concern to scientists: the life of marine life there is under constant threat.

POLAR COVALENT BOND, NON-POLAR COVALENT BOND, ABSOLUTE ELECTRO-NEGATIVITY, RELATIVE ELECTRO-NEGATIVITY.
1. Experiments and subsequent calculations showed that the effective charge of silicon in silicon tetrafluoride is +1.64 e, and xenon in xenon hexafluoride +2.3 e. Determine the values of partial charges on fluorine atoms in these compounds. 2. Compose the structural formulas of the following substances and, using the notation " " and "", characterize the polarity of covalent bonds in the molecules of these compounds: a) CH 4 , CCl 4 , SiCl 4 ; b) H 2 O, H 2 S, H 2 Se, H 2 Te; c) NH 3 , NF 3 , NCl 3 ; d) SO 2, Cl 2 O, OF 2.
3. Using the electronegativity table, indicate in which of the compounds the bond is more polar: a) CCl 4 or SiCl 4; b) H 2 S or H 2 O; c) NF 3 or NCl 3 ; d) Cl 2 O or OF 2.

7.13. Donor-acceptor bond formation mechanism

In the previous paragraphs, you learned in detail about two types of bonds: ionic and covalent. Recall that an ionic bond is formed when an electron is completely transferred from one atom to another. Covalent - with the socialization of unpaired electrons of bonded atoms.

In addition, there is another mechanism for the formation of bonds. Consider it using the example of the interaction of an ammonia molecule with a boron trifluoride molecule:

As a result, both covalent and ionic bonds appear between nitrogen and boron atoms. In this case, the nitrogen atom is donor electron pair ("gives" it to form a bond), and the boron atom - acceptor("accepts" it when the connection is formed). Hence the name of the mechanism for the formation of such a connection - " donor-acceptor.

When a bond is formed by the donor-acceptor mechanism, both a covalent bond and an ionic bond are formed simultaneously.
Of course, after the formation of a bond due to the difference in the electronegativity of the bonded atoms, the bond is polarized, and partial charges arise that reduce the effective (real) charges of the atoms.

Let's look at other examples.

If a strongly polar hydrogen chloride molecule appears next to the ammonia molecule, in which there is a significant partial charge on the hydrogen atom, then in this case the hydrogen atom will play the role of an electron pair acceptor. His 1 s-AO, although not completely empty, like that of the boron atom in the previous example, the electron density in the cloud of this orbital is significantly reduced.

The spatial structure of the resulting cation, ammonium ion NH 4 , similar to the structure of the methane molecule, that is, all four N-H bonds are exactly the same.
The formation of ionic crystals of ammonium chloride NH 4 Cl can be observed by mixing gaseous ammonia with gaseous hydrogen chloride:

NH 3 (g) + HCl (g) \u003d NH 4 Cl (cr)

The donor of an electron pair can be not only a nitrogen atom. It can be, for example, the oxygen atom of a water molecule. With the same hydrogen chloride, a water molecule will interact as follows:

The resulting H 3 O cation is called oxonium ion and, as you will soon learn, is of great importance in chemistry.
In conclusion, consider the electronic structure of the molecule carbon monoxide(carbon monoxide) CO:

In it, in addition to three covalent bonds (triple bond), there is also an ionic bond.
Conditions for bond formation by the donor-acceptor mechanism:
1) the presence of an unshared pair of valence electrons in one of the atoms;
2) the presence of a free orbital on the valence sublevel of another atom.
The donor-acceptor mechanism of bond formation is quite widespread. It is especially common in the formation of compounds d-elements. atoms of almost all d-elements have many free valence orbitals. Therefore, they are active acceptors of electron pairs.

DONOR-ACCEPTOR BOND FORMATION MECHANISM, AMMONIUM ION, OXONIUM ION, CONDITIONS FOR FORMATION OF BOND BY DONOR-ACCEPTOR MECHANISM.
1. Make up reaction equations and formation schemes
a) ammonium bromide NH 4 Br from ammonia and hydrogen bromide;
b) ammonium sulfate (NH 4) 2 SO 4 from ammonia and sulfuric acid.
2. Make the reaction equations and interaction schemes a) water with hydrogen bromide; b) water with sulfuric acid.
3. Which atoms in the four previous reactions are electron pair donors, and which are acceptors? Why? Explain your answer with diagrams of valence sublevels.
4. Structural formula of nitric acid The angles between O–N–O bonds are close to 120 o . Define:
a) the type of hybridization of the nitrogen atom;
b) which AO of the nitrogen atom takes part in the formation of -bonds;
c) which AO of the nitrogen atom takes part in the formation of the -bond by the donor-acceptor mechanism.
What do you think the angle between the H–O–N bonds in this molecule is approximately equal to? 5.Compose the structural formula of the cyanide ion CN (negative charge - on the carbon atom). It is known that cyanides (compounds containing such an ion) and carbon monoxide CO are strong poisons, and their biological effect is very close. Suggest your explanation of the proximity of their biological action.

7.14. Metal connection. Metals

A covalent bond is formed between atoms that are close in their propensity to donate and gain electrons only when the sizes of the bonded atoms are small. In this case, the electron density in the region of overlapping electron clouds is significant, and the atoms are strongly bound, as, for example, in the HF molecule. If at least one of the bonded atoms has a large radius, the formation of a covalent bond becomes less favorable, since the electron density in the region of overlapping electron clouds for large atoms is much less than for small ones. An example of such a molecule with a weaker bond is the HI molecule (using Table 21, compare the atomization energies of HF and HI molecules).

And yet between large atoms ( r o > 1.1) a chemical bond arises, but in this case it is formed due to the socialization of all (or part) of the valence electrons of all bonded atoms. For example, in the case of sodium atoms, all 3 s- electrons of these atoms, in this case a single electron cloud is formed:

Atoms form a crystal with metallic connection.
In this way, both atoms of one element and atoms of different elements can bind to each other. In the first case, simple substances are formed, called metals, and in the second - complex substances called intermetallic compounds.

Of all the substances with a metallic bond between atoms in the school, you will only publish metals. What is the spatial structure of metals? The metal crystal is made up of atomic cores, remaining after the socialization of valence electrons, and the electron cloud of socialized electrons. Atomic cores usually form the closest packing, and the electron cloud occupies the entire remaining free volume of the crystal.

The main types of densest packings are cubic closest packing(KPU) and hexagonal close packing(GPU). The names of these packings are associated with the symmetry of the crystals in which they are realized. Some metals form loosely packed crystals - body-centered cubic(BCC). Volumetric and spherical models of these packages are shown in Figure 7.6.
The cubic closest packing is formed by atoms of Cu, Al, Pb, Au, and some other elements. Hexagonal close packing - atoms of Be, Zn, Cd, Sc and a number of others. Body-centered cubic packing of atoms is present in crystals alkali metals, elements of VB and VIB groups. Some metals at different temperatures may have a different structure. The reasons for such differences and structural features of metals have not yet been fully elucidated.
When melted, metal crystals turn into metallic liquids. The type of chemical bond between atoms does not change.
The metallic bond does not have directionality and saturation. In this respect, it is similar to an ionic bond.
In the case of intermetallic compounds, one can also speak of the polarizability of a metallic bond.
characteristic physical properties metals:
1) high electrical conductivity;
2) high thermal conductivity;
3) high plasticity.

The melting points of different metals are very different from each other: the lowest melting point for mercury (-39 o C), and the highest for tungsten (3410 o C).

Beryllium Be- light gray light enough hard, but usually brittle metal. Melting point 1287 o C. In air, it is covered with an oxide film. Beryllium is a fairly rare metal, living organisms in the course of their evolution practically did not come into contact with it, therefore it is not surprising that it is poisonous to the animal world. It is used in nuclear technology.

Zinc Zn is a white soft metal with a bluish tint. Melting point 420 o C. In air and in water it is covered with a thin dense film zinc oxide to prevent further oxidation. In production, it is used for galvanizing sheets, pipes, wire, protecting iron from corrosion.
Zinc is part of many alloys, such as cupronickel and nickel silver; coins are minted from its alloys. Zinc is an integral part of brass, widely used in mechanical engineering. Alloys containing zinc are used for casting typographic fonts.

Wolfram W. It is the most refractory of all metals: the melting point of tungsten is 3387 o C. Usually, tungsten is quite brittle, but after thorough cleaning it becomes plastic, which makes it possible to draw a thin wire from it, from which the filaments of electric light bulbs are made. However, most of the resulting tungsten goes to the production of hard and wear-resistant alloys that can retain these properties when heated even to 1000 o C.

METAL, INTERMETALLIC COMPOUND, METAL BOND, TIGHT PACKING.
1. To characterize different packages, the concept of "space filling factor" is used, that is, the ratio of the volume of atoms to the volume of the crystal

where Va- atom volume,
Z is the number of atoms in a unit cell,
V i is the volume of the elementary cell.
Atoms in this case are represented by rigid balls of radius R that are in contact with each other. Ball volume V w = (4/3) R 3 .
Determine the space fill factor for KPU and BCC packaging.
2. Using the values of metal radii (Appendix 9), calculate the unit cell size of a) copper (KPU), b) aluminum (KPU) and c) cesium (BCC).

Covalent, ionic, and metallic are the three main types of chemical bonds.

Let's get to know more about covalent chemical bond. Let's consider the mechanism of its occurrence. Let's take the formation of a hydrogen molecule as an example:

A spherically symmetric cloud formed by a 1s electron surrounds the nucleus of a free hydrogen atom. When atoms approach up to a certain distance, their orbitals partially overlap (see Fig.), as a result, a molecular two-electron cloud appears between the centers of both nuclei, which has a maximum electron density in the space between the nuclei. With an increase in the density of the negative charge, there is a strong increase in the forces of attraction between the molecular cloud and the nuclei.

So, we see that a covalent bond is formed by overlapping electron clouds of atoms, which is accompanied by the release of energy. If the distance between the nuclei of the atoms approaching to touch is 0.106 nm, then after the overlap of the electron clouds it will be 0.074 nm. The greater the overlap of electron orbitals, the stronger the chemical bond.

covalent called chemical bonding carried out by electron pairs. Compounds with a covalent bond are called homeopolar or atomic.

Exist two types of covalent bond: polar and non-polar.

With non-polar covalent bond formed by a common pair of electrons, the electron cloud is distributed symmetrically with respect to the nuclei of both atoms. An example can be diatomic molecules that consist of one element: Cl 2, N 2, H 2, F 2, O 2 and others, in which the electron pair belongs to both atoms equally.

At polar In a covalent bond, the electron cloud is displaced towards the atom with a higher relative electronegativity. For example, volatile molecules inorganic compounds such as H 2 S, HCl, H 2 O and others.

The formation of the HCl molecule can be represented as follows:

Because the relative electronegativity of the chlorine atom (2.83) is greater than that of the hydrogen atom (2.1), the electron pair shifts towards the chlorine atom.

In addition to the exchange mechanism for the formation of a covalent bond - due to overlap, there is also donor-acceptor the mechanism of its formation. This is a mechanism in which the formation of a covalent bond occurs due to a two-electron cloud of one atom (donor) and a free orbital of another atom (acceptor). Let's look at an example of the mechanism for the formation of ammonium NH 4 +. In the ammonia molecule, the nitrogen atom has a two-electron cloud:

The hydrogen ion has a free 1s orbital, let's denote it as .

In the process of formation of the ammonium ion, the two-electron cloud of nitrogen becomes common for nitrogen and hydrogen atoms, which means it is converted into a molecular electron cloud. Therefore, a fourth covalent bond appears. The process of ammonium formation can be represented as follows: