What is a non-polar bond. Types of chemical bonds: ionic, metallic, covalent (polar, nonpolar), hydrogen. Scheme of formation of a polar covalent bond

It is extremely rare that chemical substances consist of individual, unrelated atoms of chemical elements. Under normal conditions, only a small number of gases called noble gases have this structure: helium, neon, argon, krypton, xenon and radon. Most often, chemical substances do not consist of isolated atoms, but of their combinations into various groups. Such associations of atoms can number a few, hundreds, thousands, or even more atoms. The force that holds these atoms in such groups is called chemical bond.

In other words, we can say that a chemical bond is an interaction that provides the connection of individual atoms into more complex structures (molecules, ions, radicals, crystals, etc.).

The reason for the formation of a chemical bond is that the energy of more complex structures is less than the total energy of the individual atoms that form it.

So, in particular, if the interaction of atoms X and Y produces a molecule XY, this means that the internal energy of the molecules of this substance is lower than the internal energy of the individual atoms from which it was formed:

E(XY)< E(X) + E(Y)

For this reason, when chemical bonds are formed between individual atoms, energy is released.

Electrons of the outer electron layer with the lowest binding energy with the nucleus, called valence. For example, in boron these are electrons of the 2nd energy level - 2 electrons per 2 s- orbitals and 1 by 2 p-orbitals:

When a chemical bond is formed, each atom tends to obtain the electronic configuration of noble gas atoms, i.e. so that there are 8 electrons in its outer electron layer (2 for elements of the first period). This phenomenon is called the octet rule.

It is possible for atoms to achieve the electron configuration of a noble gas if initially single atoms share some of their valence electrons with other atoms. In this case, common electron pairs are formed.

Depending on the degree of electron sharing, covalent, ionic and metallic bonds can be distinguished.

Covalent bond

Covalent bonds most often occur between atoms of nonmetal elements. If the nonmetal atoms forming a covalent bond belong to different chemical elements, such a bond is called a polar covalent bond. The reason for this name lies in the fact that atoms of different elements also have different abilities to attract a common electron pair. Obviously, this leads to a displacement of the common electron pair towards one of the atoms, as a result of which a partial negative charge is formed on it. In turn, a partial positive charge is formed on the other atom. For example, in a hydrogen chloride molecule the electron pair is shifted from the hydrogen atom to the chlorine atom:

Examples of substances with polar covalent bonds:

CCl 4, H 2 S, CO 2, NH 3, SiO 2, etc.

A covalent nonpolar bond is formed between nonmetal atoms of the same chemical element. Since the atoms are identical, their ability to attract shared electrons is also the same. In this regard, no displacement of the electron pair is observed:

The above mechanism for the formation of a covalent bond, when both atoms provide electrons to form common electron pairs, is called exchange.

There is also a donor-acceptor mechanism.

When a covalent bond is formed by the donor-acceptor mechanism, a shared electron pair is formed due to the filled orbital of one atom (with two electrons) and the empty orbital of another atom. An atom that provides a lone pair of electrons is called a donor, and an atom with a vacant orbital is called an acceptor. Atoms that have paired electrons, for example N, O, P, S, act as donors of electron pairs.

For example, according to the donor-acceptor mechanism, the fourth covalent N-H bond is formed in the ammonium cation NH 4 +:

In addition to polarity, covalent bonds are also characterized by energy. Bond energy is the minimum energy required to break a bond between atoms.

The binding energy decreases with increasing radii of bonded atoms. Since we know that atomic radii increase down the subgroups, we can, for example, conclude that the strength of the halogen-hydrogen bond increases in the series:

HI< HBr < HCl < HF

Also, the bond energy depends on its multiplicity - the greater the bond multiplicity, the greater its energy. Bond multiplicity refers to the number of shared electron pairs between two atoms.

Ionic bond

An ionic bond can be considered as an extreme case of a polar covalent bond. If in a covalent-polar bond the common electron pair is partially shifted to one of the pair of atoms, then in an ionic bond it is almost completely “given” to one of the atoms. The atom that donates electron(s) acquires a positive charge and becomes cation, and the atom that has taken electrons from it acquires a negative charge and becomes anion.

Thus, an ionic bond is a bond formed by the electrostatic attraction of cations to anions.

The formation of this type of bond is typical during the interaction of atoms of typical metals and typical non-metals.

For example, potassium fluoride. The potassium cation is formed by the removal of one electron from a neutral atom, and the fluorine ion is formed by the addition of one electron to the fluorine atom:

An electrostatic attraction force arises between the resulting ions, resulting in the formation of an ionic compound.

When a chemical bond was formed, electrons from the sodium atom passed to the chlorine atom and oppositely charged ions were formed, which have a completed external energy level.

It has been established that electrons from the metal atom are not completely detached, but are only shifted towards the chlorine atom, as in a covalent bond.

Most binary compounds that contain metal atoms are ionic. For example, oxides, halides, sulfides, nitrides.

Ionic bonding also occurs between simple cations and simple anions (F −, Cl −, S 2-), as well as between simple cations and complex anions (NO 3 −, SO 4 2-, PO 4 3-, OH −). Therefore, ionic compounds include salts and bases (Na 2 SO 4, Cu(NO 3) 2, (NH 4) 2 SO 4), Ca(OH) 2, NaOH)

Metal connection

This type of bond is formed in metals.

Atoms of all metals have electrons in their outer electron layer that have a low binding energy with the nucleus of the atom. For most metals, the process of losing outer electrons is energetically favorable.

Due to such a weak interaction with the nucleus, these electrons in metals are very mobile and the following process continuously occurs in each metal crystal:

М 0 — ne − = M n + ,

where M 0 is a neutral metal atom, and M n + a cation of the same metal. The figure below provides an illustration of the processes taking place.

That is, electrons “rush” across a metal crystal, detaching from one metal atom, forming a cation from it, joining another cation, forming a neutral atom. This phenomenon was called “electron wind,” and the collection of free electrons in a crystal of a nonmetal atom was called “electron gas.” This type of interaction between metal atoms is called a metallic bond.

Hydrogen bond

If a hydrogen atom in a substance is bonded to an element with high electronegativity (nitrogen, oxygen, or fluorine), that substance is characterized by a phenomenon called hydrogen bonding.

Since a hydrogen atom is bonded to an electronegative atom, a partial positive charge is formed on the hydrogen atom, and a partial negative charge is formed on the atom of the electronegative element. In this regard, electrostatic attraction becomes possible between a partially positively charged hydrogen atom of one molecule and an electronegative atom of another. For example, hydrogen bonding is observed for water molecules:

It is the hydrogen bond that explains the abnormally high melting point of water. In addition to water, strong hydrogen bonds are also formed in substances such as hydrogen fluoride, ammonia, oxygen-containing acids, phenols, alcohols, and amines.

It's no secret that chemistry is a rather complex and also diverse science. Many different reactions, reagents, chemicals and other complex and confusing terms - they all interact with each other. But the main thing is that we deal with chemistry every day, no matter whether we listen to the teacher in class and learn new material or make tea, which in general is also a chemical process.

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It can be concluded that you just need to know chemistry, understanding it and knowing how our world or some of its parts work is interesting, and, moreover, useful.

Now we have to deal with such a term as a covalent bond, which, by the way, can be either polar or non-polar. By the way, the word “covalent” itself is derived from the Latin “co” - together and “vales” - having force.

Appearances of the term

Let's start with the fact that The term “covalent” was first introduced in 1919 by Irving Langmuir - Nobel Prize Laureate. The concept of "covalent" implies a chemical bond in which both atoms share electrons, which is called shared possession. Thus, it differs, for example, from a metallic one, in which electrons are free, or from an ionic one, where one completely gives electrons to another. It should be noted that it is formed between non-metals.

Based on the above, we can draw a small conclusion about what this process is like. It arises between atoms due to the formation of common electron pairs, and these pairs arise on the external and pre-external sublevels of electrons.

Examples, substances with polarity:

Types of covalent bond

There are also two types: polar and, accordingly, nonpolar bonds. We will analyze the features of each of them separately.

Covalent polar - formation

What does the term “polar” mean?

What usually happens is that two atoms have different electronegativity, therefore the electrons they share do not belong equally, but are always closer to one than to the other. For example, a hydrogen chloride molecule, in which the electrons of the covalent bond are located closer to the chlorine atom, since its electronegativity is higher than that of hydrogen. However, in reality, the difference in electron attraction is small enough for complete electron transfer from hydrogen to chlorine to occur.

As a result, when polar, the electron density shifts to a more electronegative one, and a partial negative charge appears on it. In turn, the nucleus whose electronegativity is lower develops, accordingly, a partial positive charge.

We conclude: polar occurs between different nonmetals that differ in their electronegativity values, and the electrons are located closer to the nucleus with greater electronegativity.

Electronegativity is the ability of some atoms to attract electrons from others, thereby forming a chemical reaction.

Examples of covalent polar, substances with a polar covalent bond:

Formula of a substance with a polar covalent bond

Covalent nonpolar, difference between polar and nonpolar

And finally, non-polar, we will soon find out what it is.

The main difference between non-polar and polar- this is symmetry. If in the case of a polar bond the electrons were located closer to one atom, then in a non-polar bond the electrons were located symmetrically, that is, equally relative to both.

It is noteworthy that non-polar occurs between non-metal atoms of one chemical element.

Eg, substances with non-polar covalent bonds:

Also, a collection of electrons is often called simply an electron cloud, based on this we conclude that the electronic cloud of communication, which forms a common pair of electrons, is distributed in space symmetrically, or evenly in relation to the nuclei of both.

Examples of a covalent nonpolar bond and a scheme for the formation of a covalent nonpolar bond

But it is also useful to know how to distinguish between covalent polar and nonpolar.

Covalent nonpolar- these are always atoms of the same substance. H2. CL2.

This article has come to an end, now we know what this chemical process is, we know how to define it and its varieties, we know the formulas for the formation of substances, and in general a little more about our complex world, successes in chemistry and the formation of new formulas.

A covalent bond occurs due to the sharing of electrons belonging to both atoms participating in the interaction. The electronegativity of nonmetals is large enough that no electron transfer occurs.

Electrons in overlapping electron orbitals are shared. This creates a situation in which the outer electronic levels of the atoms are filled, that is, an 8- or 2-electron outer shell is formed.

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The state in which the electron shell is completely filled is characterized by the lowest energy and, accordingly, maximum stability.

There are two mechanisms of formation:

1. donor-acceptor;
2. exchange.

In the first case, one of the atoms provides its pair of electrons, and the second provides a free electron orbital.

In the second, one electron from each participant in the interaction comes into the common pair.

Depending on what type they are- atomic or molecular, compounds with a similar type of bond can vary significantly in physicochemical characteristics.

Molecular substances most often gases, liquids or solids with low melting and boiling points, non-electrically conductive, and low strength. These include: hydrogen (H 2), oxygen (O 2), nitrogen (N 2), chlorine (Cl 2), bromine (Br 2), orthorhombic sulfur (S 8), white phosphorus (P 4) and others simple substances; carbon dioxide (CO 2), sulfur dioxide (SO 2), nitrogen oxide V (N 2 O 5), water (H 2 O), hydrogen chloride (HCl), hydrogen fluoride (HF), ammonia (NH 3), methane (CH 4), ethyl alcohol (C 2 H 5 OH), organic polymers and others.

Atomic substances exist in the form of durable crystals with high boiling and melting points, are insoluble in water and other solvents, and many do not conduct electric current. An example is diamond, which has exceptional strength. This is explained by the fact that diamond is a crystal consisting of carbon atoms connected by covalent bonds. There are no individual molecules in a diamond. Also, substances such as graphite, silicon (Si), silicon dioxide (SiO 2), silicon carbide (SiC) and others have an atomic structure.

Covalent bonds can be not only single (as in the chlorine molecule Cl2), but also double, as in the oxygen molecule O2, or triple, as, for example, in the nitrogen molecule N2. At the same time, triple ones have more energy and are more durable than double and single ones.

A covalent bond can be formed both between two atoms of the same element (non-polar) and between atoms of different chemical elements (polar).

It is not difficult to indicate the formula of a compound with a covalent polar bond if you compare the electronegativity values ​​of the atoms that make up the molecules. No difference in electronegativity will determine non-polarity. If there is a difference, then the molecule will be polar.

Don't miss: mechanism of education, specific examples.

Covalent nonpolar chemical bond

Characteristic of simple substances, non-metals. The electrons belong to the atoms equally, and there is no shift in the electron density.

Examples include the following molecules:

H2, O2, O3, N2, F2, Cl2.

The exception is inert gases. Their outer energy level is completely filled, and the formation of molecules is energetically unfavorable for them, and therefore they exist in the form of individual atoms.

Also, an example of substances with a non-polar covalent bond would be, for example, PH3. Despite the fact that the substance consists of different elements, the electronegativities of the elements do not actually differ, which means that the electron pair will not shift.

Covalent polar chemical bond

Considering a covalent polar bond, many examples can be given: HCl, H2O, H2S, NH3, CH4, CO2, SO3, CCl4, SiO2, CO.

Scheme of formation of a polar covalent bond

Depending on the mechanism of formation, they can become common electrons of one or both atoms.

The picture clearly shows the interaction in the hydrochloric acid molecule.

A pair of electrons belongs to both one atom and the second, both of them, thus, the outer levels are filled. But the more electronegative chlorine attracts a pair of electrons a little closer to itself (while it remains shared). The difference in electronegativity is not large enough for a pair of electrons to go completely to one of the atoms. As a result, a partial negative charge appears on chlorine and a partial positive charge on hydrogen. The HCl molecule is a polar molecule.

Physico-chemical properties of the bond

The connection can be characterized by the following properties: directivity, polarity, polarizability and saturability.

Definition

A covalent bond is a chemical bond formed by atoms sharing their valence electrons. A prerequisite for the formation of a covalent bond is the overlap of atomic orbitals (AO) in which the valence electrons are located. In the simplest case, the overlap of two AOs leads to the formation of two molecular orbitals (MO): a bonding MO and an antibonding (antibonding) MO. The shared electrons are located on the lower energy bonding MO:

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Covalent bond (atomic bond, homeopolar bond) - a bond between two atoms due to electron sharing of two electrons - one from each atom:

A. + B. -> A: B

For this reason, the homeopolar relationship is directional. The pair of electrons that perform the bond belongs simultaneously to both bonded atoms, for example:

Types of covalent bond

There are three types of covalent chemical bonds, differing in the mechanism of their formation:

1. Simple covalent bond. For its formation, each atom provides one unpaired electron. When a simple covalent bond is formed, the formal charges of the atoms remain unchanged. If the atoms forming a simple covalent bond are the same, then the true charges of the atoms in the molecule are also the same, since the atoms forming the bond equally own a shared electron pair, such a bond is called a non-polar covalent bond. If the atoms are different, then the degree of possession of a shared pair of electrons is determined by the difference in the electronegativity of the atoms, an atom with a higher electronegativity has a pair of bonding electrons to a greater extent, and therefore its true charge has a negative sign, an atom with a lower electronegativity acquires the same charge, but with a positive sign.

Sigma (σ)-, pi (π)-bonds are an approximate description of the types of covalent bonds in molecules of organic compounds; the σ-bond is characterized by the fact that the density of the electron cloud is maximum along the axis connecting the nuclei of atoms. When a π bond is formed, the so-called lateral overlap of electron clouds occurs, and the density of the electron cloud is maximum “above” and “below” the σ bond plane. For example, take ethylene, acetylene and benzene.

In the ethylene molecule C 2 H 4 there is a double bond CH 2 = CH 2, its electronic formula: H:C::C:H. The nuclei of all ethylene atoms are located in the same plane. The three electron clouds of each carbon atom form three covalent bonds with other atoms in the same plane (with angles between them of approximately 120°). The cloud of the fourth valence electron of the carbon atom is located above and below the plane of the molecule. Such electron clouds of both carbon atoms, partially overlapping above and below the plane of the molecule, form a second bond between the carbon atoms. The first, stronger covalent bond between carbon atoms is called a σ bond; the second, weaker covalent bond is called a π bond.

In a linear acetylene molecule

N-S≡S-N (N: S::: S: N)

there are σ bonds between carbon and hydrogen atoms, one σ bond between two carbon atoms, and two π bonds between the same carbon atoms. Two π-bonds are located above the sphere of action of the σ-bond in two mutually perpendicular planes.

All six carbon atoms of the cyclic benzene molecule C 6 H 6 lie in the same plane. There are σ bonds between carbon atoms in the plane of the ring; Each carbon atom has the same bonds with hydrogen atoms. Carbon atoms spend three electrons to make these bonds. Clouds of fourth valence electrons of carbon atoms, shaped like figures of eight, are located perpendicular to the plane of the benzene molecule. Each such cloud overlaps equally with the electron clouds of neighboring carbon atoms. In a benzene molecule, not three separate π bonds are formed, but a single π electron system of six electrons, common to all carbon atoms. The bonds between the carbon atoms in the benzene molecule are exactly the same.

A covalent bond is formed as a result of the sharing of electrons (to form common electron pairs), which occurs during the overlap of electron clouds. The formation of a covalent bond involves the electron clouds of two atoms. There are two main types of covalent bonds:

• A covalent nonpolar bond is formed between nonmetal atoms of the same chemical element. Simple substances, for example O 2, have such a connection; N 2; C 12.
• A polar covalent bond is formed between atoms of different nonmetals.

see also

Literature

• “Chemical Encyclopedic Dictionary”, M., “Soviet Encyclopedia”, 1983, p.264.

Wikimedia Foundation. 2010.

• Big Polytechnic Encyclopedia

CHEMICAL BONDING, the mechanism by which atoms join together to form molecules. There are several types of such bonds, based either on the attraction of opposite charges, or on the formation of stable configurations through the exchange of electrons.... ... Scientific and technical encyclopedic dictionary

Chemical bond- CHEMICAL BOND, the interaction of atoms, causing their combination into molecules and crystals. The forces acting during the formation of a chemical bond are mainly electrical in nature. The formation of a chemical bond is accompanied by a restructuring... ... Illustrated Encyclopedic Dictionary

The mutual attraction of atoms, leading to the formation of molecules and crystals. It is customary to say that in a molecule or in a crystal there are chemical structures between neighboring atoms. The valence of an atom (which is discussed in more detail below) shows the number of bonds... Great Soviet Encyclopedia

chemical bond- mutual attraction of atoms, leading to the formation of molecules and crystals. The valence of an atom shows the number of bonds formed by a given atom with neighboring ones. The term “chemical structure” was introduced by Academician A. M. Butlerov in... ... Encyclopedic Dictionary of Metallurgy

An ionic bond is a strong chemical bond formed between atoms with a large difference in electronegativity, in which the shared electron pair is completely transferred to the atom with a higher electronegativity. An example is the compound CsF... Wikipedia

Chemical bonding is a phenomenon of interaction of atoms caused by the overlap of electron clouds of bonding particles, which is accompanied by a decrease in the total energy of the system. The term “chemical structure” was first introduced by A. M. Butlerov in 1861... ... Wikipedia

7.11. The structure of substances with covalent bonds

Substances in which, of all types of chemical bonds, only a covalent one is present, are divided into two unequal groups: molecular (very many) and non-molecular (much less).
Crystals of solid molecular substances consist of molecules weakly bound together by the forces of intermolecular interaction of molecules. Such crystals do not have high strength and hardness (think ice or sugar). Their melting and boiling points are also low (see Table 22).

Table 22. Melting and boiling points of some molecular substances

Unlike their molecular counterparts, non-molecular substances with covalent bonds form very hard crystals. Diamond crystals (the hardest substance) belong to 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, 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 framework structure of the crystal, you will 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 substance. Quartz is a very hard substance; Under the name "emery" it is also used as an abrasive material. The quartz structure is easily obtained by inserting oxygen atoms 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 structures are called atomic crystals.
An atomic crystal is a crystal consisting of atoms of one or more elements linked by chemical bonds.
A chemical bond in an atomic crystal can be covalent or metallic.
As you already know, any atomic crystal, like an ionic crystal, is a huge “supermolecule”. The structural formula of such a “supermolecule” cannot be written down - you 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 temperatures are quite understandable if we remember that when these substances melt, it is not weak intermolecular bonds that are broken, but strong chemical bonds. For the same reason, many substances that form atomic crystals do not melt when heated, but decompose or immediately transform into a vapor state (sublimate), for example, graphite sublimes at 3700 o C.

7.12. Polarity of a covalent bond. Electronegativity

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

Consider a molecule HCl.
Using this example, let's see how we can estimate the displacement of the electron communication cloud using molar ionization energies and means to the electron. 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 greater: the electron affinity energy of the chlorine atom is almost five times greater than that for 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 bonding electrons spend more time near the chlorine atom than near the hydrogen atom. This uneven distribution of electron density leads to a redistribution of electrical charges inside 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 displaced 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 be talking about ions, but about atoms. In general, the following definition can be given.

In molecules in which covalent bonds are formed only by an exchange mechanism, the formal charges of the atoms are equal to zero. Thus, in the HCl molecule, the formal charges on both the chlorine and hydrogen atoms are zero. Consequently, in this molecule the real (effective) charges on the chlorine and hydrogen atoms are equal to the partial (excess) charges.
It is not always easy to determine the sign of the partial charge on an atom of one or another element in a molecule based on the molar ionization energies and affinity for 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. It can be denoted by the letters E/O. There is also no single, generally accepted method for calculating electronegativity. In a simplified way, it can be represented as half the sum of the molar ionization energies and electron affinity - this was one of the first ways to calculate it.
The absolute values ​​of electronegativity of atoms of various elements are used very rarely. The most commonly used is relative electronegativity, denoted by 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 changed somewhat.
Relative electronegativity is a dimensionless quantity. Its values ​​are given in Appendix 10.

Since relative electronegativity depends primarily on the ionization energy of the atom (electron affinity energy is always much lower), then in a system of 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 have no practical significance, since these elements do not form compounds.

Using the electronegativity table, you can easily determine towards which of the two atoms the electrons connecting these atoms are shifted, and, therefore, the signs of the partial charges arising on these atoms.

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 the same element (in simple substances), the bond is in most cases non-polar.

The greater the difference in electronegativity of the bonded atoms, the more polar the covalent bond between these atoms turns out to be.

POLAR COVALENT BOND, NON-POLAR COVALENT BOND, ABSOLUTE ELECTRONEGATIVITY, RELATIVE ELECTRONEGATIVITY.
1. Experiments and subsequent calculations showed that the effective charge of silicon in silicon tetrafluoride is +1.64 e, and of xenon in xenon hexafluoride +2.3 e. Determine the values ​​of the partial charges on the fluorine atoms in these compounds. 2. Make up the structural formulas of the following substances and, using the notations " " 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 mechanism of bond formation

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 - when sharing unpaired electrons of bonded atoms.

In addition, there is another mechanism for bond formation. Let's 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 arise between the nitrogen and boron atoms. In this case, the nitrogen atom is donor electron pair ("gives" it for the formation of a bond), and the boron atom - acceptor(“accepts” it when forming a connection). Hence the name of the mechanism for the formation of such a connection - “ donor-acceptor".

When a bond is formed using 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, polarization of the bond occurs, and partial charges arise, reducing the effective (real) charges of the atoms.

Let's look at other examples.

If there is a highly polar hydrogen chloride molecule next to the ammonia molecule, in which there is a significant partial charge on the hydrogen atom, then in this case the role of the electron pair acceptor will be played by the hydrogen atom. Its 1 s-AO, although not completely empty, like 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 is ammonium ion NH 4 is 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 ammonia gas with hydrogen chloride gas:

NH 3 (g) + HCl (g) = NH 4 Cl (cr)

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

The resulting H3O cation is called oxonium ion and, as you will soon learn, is of great importance in chemistry.
In conclusion, let us consider the electronic structure of the carbon monoxide (carbon monoxide) CO molecule:

In addition to three covalent bonds (triple bond), it also contains an ionic bond.
Conditions for bond formation according to the donor-acceptor mechanism:
1) the presence of a lone 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 occurs especially often during the formation of compounds d-elements. Almost everyone's atoms d-elements have many empty valence orbitals. Therefore, they are active acceptors of electron pairs.

DONOR-ACCEPTOR MECHANISM OF BOND FORMATION, AMMONIUM ION, OXONIUM ION, CONDITIONS FOR BOND FORMATION BY DONOR-ACCEPTOR MECHANISM.
1.Make 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. Create reaction equations and interaction schemes for a) water with hydrogen bromide; b) water with sulfuric acid.
3.Which atoms in the four previous reactions are donors of an electron pair, 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) type of hybridization of the nitrogen atom;
b) which AO of the nitrogen atom takes part in the formation of the -bond;
c) which AO of the nitrogen atom takes part in the formation of an -bond according to the donor-acceptor mechanism.
What do you think the angle between the H–O–N bonds in this molecule is approximately equal to? 5.Create 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 similar. Offer your explanation of the proximity of their biological action.

7.14. Metal connection. Metals

A covalent bond is formed between atoms that are similar in their propensity to give up 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 turn out to be tightly 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 advantageous, 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 occurs, but in this case it is formed due to the sharing 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, and a single electron cloud is formed:

Atoms form a crystal with metal communication
In this way, both atoms of the same element and atoms of different elements can bond with each other. In the first case, simple substances called metals, and in the second - complex substances called intermetallic compounds.

Of all the substances with metallic bonds between atoms, you will only learn about metals in school. What is the spatial structure of metals? The metal crystal consists of atomic skeletons, remaining after the socialization of valence electrons, and the electron cloud of socialized electrons. The atomic cores usually form a very close packing, and the electron cloud occupies the entire remaining free volume of the crystal.

The main types of dense packaging are cubic closest packing(KPU) and hexagonal close packing(GPU). The names of these packages are associated with the symmetry of the crystals in which they are realized. Some metals form loosely packed crystals - body-centered cubic(OTSK). Volume and ball-and-stick models of these packages are shown in Figure 7.6.
Cubic close 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 of alkali metals, elements of VB and VIB groups. Some metals may have different structures at different temperatures. The reasons for such differences and structural features of metals are still not fully understood.
When melted, metal crystals turn into metal liquids. The type of chemical bond between atoms does not change.
The metal bond does not have directionality and saturation. In this respect it is similar to an ionic bond.
In the case of intermetallic compounds, we can also talk about the polarizability of the metallic bond.
Characteristic physical properties of metals:
1) high electrical conductivity;
2) high thermal conductivity;
3) high ductility.

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

METAL, INTERMETALLIC COMPOUND, METALLIC BOND, DENSE PACKING.
1. To characterize various packages, the concept of “space filling coefficient” is used, that is, the ratio of the volume of atoms to the volume of the crystal

Where V a - volume of an atom,
Z is the number of atoms in a unit cell,
V i- volume of the unit cell.
Atoms in this case are represented by rigid balls of radius R, touching each other. Ball volume V w = (4/3) R 3 .
Determine the space filling factor for bulk and bcc packaging.
2. Using the values ​​of metal radii (Appendix 9), calculate the unit cell size of a) copper (CPU), b) aluminum (CPU) and c) cesium (BCC).