Ionic bonds between atoms. Chemical bond. See what “Ionic chemical bond” is in other dictionaries

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Lesson Objectives:

• Form a concept of chemical bonds using the example of an ionic bond. To achieve an understanding of the formation of ionic bonds as an extreme case of polar ones.
• During the lesson, ensure the mastery of the following basic concepts: ions (cation, anion), ionic bond.
• To develop the mental activity of students through the creation of a problem situation when learning new material.

Tasks:

• teach to recognize types of chemical bonds;
• repeat the structure of an atom;
• explore the mechanism of formation of ionic chemical bonds;
• teach how to draw up formation schemes and electronic formulas of ionic compounds, reaction equations with the designation of electron transitions.

Equipment: computer, projector, multimedia resource, periodic table of chemical elements D.I. Periodic table " Ionic bond».

Lesson type: Formation of new knowledge.

Lesson type: Multimedia lesson.

X lesson od

I.Organizational moment.

II . Checking homework.

Teacher: How can atoms take on stable electronic configurations? What are the ways to form a covalent bond?

Student: Polar and nonpolar covalent bonds are formed by an exchange mechanism. The exchange mechanism includes cases when one electron from each atom participates in the formation of an electron pair. For example, hydrogen: (slide 2)

Bonding occurs through the formation of a shared electron pair by combining unpaired electrons. Each atom has one s electron. The H atoms are equivalent and the pairs belong equally to both atoms. Therefore, on the same principle, the formation of common electron pairs occurs (overlap p- electron clouds) upon the formation of an F 2 molecule. (slide 3)

Record H · means that a hydrogen atom has 1 electron in its outer electron layer. The recording shows that there are 7 electrons on the outer electron layer of the fluorine atom.

When the N 2 molecule is formed. 3 common electron pairs are formed. The p-orbitals overlap. (slide 4)

The bond is called non-polar.

Teacher: We have now looked at cases when molecules of a simple substance are formed. But around us there are many substances with complex structures. Let's take a hydrogen fluoride molecule. How does the connection form in this case?

Student: When a hydrogen fluoride molecule is formed, the orbital of the s-electron of hydrogen and the orbital of the p-electron overlap fluoride H-F. (slide 5)

The bonding electron pair is shifted to the fluorine atom, resulting in the formation dipole. Connection called polar.

III. Updating knowledge.

Teacher: A chemical bond arises as a result of changes that occur with the outer electron shells of the connecting atoms. This is possible because the outer electron layers are not complete in elements other than noble gases. Chemical bonding is explained by the desire of atoms to acquire a stable electronic configuration, similar to the configuration of the “closest” inert gas to them.

Teacher: Write down the diagram of the electronic structure of the sodium atom (at the board). (slide 6)

Student: To achieve stability of the electron shell, the sodium atom must either give up one electron or accept seven. Sodium will easily give up its electron, which is far from the nucleus and weakly bound to it.

Teacher: Make a diagram of electron release.

Na° - 1ē → Na+ = Ne

Teacher: Write down the diagram of the electronic structure of the fluorine atom (at the board).

Teacher: How to complete filling of the electronic layer?

Student: To achieve stability of the electron shell, the fluorine atom must either give up seven electrons or accept one. It is energetically more favorable for fluorine to accept an electron.

Teacher: Make a diagram for receiving an electron.

F° + 1ē → F- = Ne

IV. Learning new material.

The teacher asks a question to the class in which the task of the lesson is set:

Are there other possible ways in which atoms can take on stable electronic configurations? What are the ways to form such connections?

Today we will look at one type of bond - an ionic bond. Let us compare the structure of the electron shells of the already mentioned atoms and inert gases.

Conversation with the class.

Teacher: What charge did the sodium and fluorine atoms have before the reaction?

Student: The sodium and fluorine atoms are electrically neutral, because the charges of their nuclei are balanced by the electrons rotating around the nucleus.

Teacher: What happens between atoms when they give and take electrons?

Student: Atoms acquire charges.

The teacher gives explanations: In the formula of an ion, its charge is additionally written down. To do this, use the superscript. It indicates the amount of charge with a number (they do not write one), and then a sign (plus or minus). For example, a Sodium ion with a charge of +1 has the formula Na + (read “sodium-plus”), a Fluoride ion with a charge of -1 – F - (“fluorine-minus”), a hydroxide ion with a charge of -1 – OH - (“ o-ash-minus"), a carbonate ion with a charge -2 – CO 3 2- (“tse-o-three-two-minus”).

In the formulas of ionic compounds, positively charged ions are written first, without indicating charges, and then negatively charged ones. If the formula is correct, then the sum of the charges of all ions in it is zero.

Positively charged ion called a cation, and a negatively charged ion is an anion.

Teacher: We write down the definition in our workbooks:

Ion is a charged particle into which an atom turns as a result of accepting or losing electrons.

Teacher: How to determine the charge value of the calcium ion Ca 2+?

Student: An ion is an electrically charged particle formed as a result of the loss or gain of one or more electrons by an atom. Calcium has two electrons in its last electron level; ionization of a calcium atom occurs when two electrons are lost. Ca 2+ is a doubly charged cation.

Teacher: What happens to the radii of these ions?

When transitioning When an electrically neutral atom is transformed into an ionic state, the particle size changes greatly. The atom, giving up its valence electrons, turns into a more compact particle - a cation. For example, when a sodium atom transforms into a Na+ cation, which, as indicated above, has the structure of neon, the radius of the particle decreases greatly. The radius of an anion is always greater than the radius of the corresponding electrically neutral atom.

Teacher: What happens to differently charged particles?

Student: Oppositely charged sodium and fluorine ions, resulting from the transfer of an electron from a sodium atom to a fluorine atom, are mutually attracted and form sodium fluoride. (slide 7)

Na + + F - = NaF

The scheme for the formation of ions that we have considered shows how a chemical bond, which is called ionic.

Ionic bond– a chemical bond formed by the electrostatic attraction of oppositely charged ions to each other.

The compounds that are formed in this case are called ionic compounds.

V. Consolidation of new material.

Assignments to consolidate knowledge and skills

1. Compare the structure of the electronic shells of a calcium atom and a calcium cation, a chlorine atom and a chloride anion:

Comment on the formation of ionic bonds in calcium chloride:

2. To complete this task, you need to divide into groups of 3-4 people. Each group member considers one example and presents the results to the whole group.

Student response:

1. Calcium is an element main subgroup Group II, metal. It is easier for its atom to give away two outer electrons than to accept the missing six:

2. Chlorine is an element of the main subgroup of group VII, a non-metal. It is easier for its atom to accept one electron, which it lacks to complete the outer level, than to give up seven electrons from the outer level:

3. First, let's find the least common multiple between the charges of the resulting ions, it is equal to 2 (2x1). Then we determine how many calcium atoms need to be taken so that they give up two electrons, that is, we need to take one Ca atom and two CI atoms.

4. Schematically, the formation of an ionic bond between calcium and chlorine atoms can be written: (slide 8)

Ca 2+ + 2CI - → CaCI 2

Self-control tasks

1. Based on the formation scheme of a chemical compound, create an equation chemical reaction: (slide 9)

2. Based on the scheme for the formation of a chemical compound, create an equation for the chemical reaction: (slide 10)

3. A scheme for the formation of a chemical compound is given: (slide 11)

Select a pair of chemical elements whose atoms can interact in accordance with this scheme:

A) Na And O;
b) Li And F;
V) K And O;
G) Na And F

An ionic chemical bond is a bond that is formed between atoms of chemical elements (positively or negatively charged ions). So what is an ionic bond, and how does it form?

General characteristics of ionic chemical bonds

Ions are particles that have a charge into which atoms turn in the process of giving or accepting electrons. They are attracted to each other quite strongly, which is why substances with this type of bond have high boiling and melting points.

Rice. 1. Ions.

An ionic bond is a chemical bond between unlike ions due to their electrostatic attraction. It can be considered the limiting case of a covalent bond, when the difference in electronegativity of the bonded atoms is so great that complete separation of charges occurs.

Rice. 2. Ionic chemical bond.

It is generally believed that the bond becomes electronic if the EO is >1.7.

The difference in electronegativity value is greater the further the elements are located from each other in the periodic table by period. This bond is characteristic of metals and non-metals, especially those located in the most distant groups, for example, I and VII.

Example: table salt, sodium chloride NaCl:

Rice. 3. Diagram of the ionic chemical bond of sodium chloride.

An ionic bond exists in crystals; it is strong and long, but not saturated and not directed. Ionic bonding is characteristic only for complex substances, such as salts, alkalis, some metal oxides. In the gaseous state, such substances exist in the form ionic molecules.

Ionic chemical bonds form between typical metals and nonmetals. Electrons are necessarily transferred from the metal to the non-metal, forming ions. The result is an electrostatic attraction called an ionic bond.

In fact, a completely ionic bond does not occur. The so-called ionic bond is partly ionic and partly covalent in nature. However, the bond of complex molecular ions can be considered ionic.

Examples of ionic bond formation

There are several examples of ionic bond formation:

• interaction between calcium and fluoride

Ca 0 (atom) -2e=Ca 2 + (ion)

– it is easier for calcium to give away two electrons than to gain the missing ones.

F 0 (atom)+1е= F- (ion)

– fluorine, on the contrary, is easier to accept one electron than to give up seven electrons.

Let's find the least common multiple between the charges of the resulting ions. It is equal to 2. Let us determine the number of fluorine atoms that will accept two electrons from a calcium atom: 2: 1 = 2. 4.

Let's create the formula for an ionic chemical bond:

Ca 0 +2F 0 →Ca 2 +F−2.

• interaction of sodium and oxygen

4.3. Total ratings received: 281.

All chemical compounds are formed through the formation of a chemical bond. And depending on the type of connecting particles, several types are distinguished. The most basic– these are covalent polar, covalent nonpolar, metallic and ionic. Today we'll talk about ionic.

What are ions

It is formed between two atoms - as a rule, provided that the difference in electronegativity between them is very large. The electronegativity of atoms and ions is assessed using the Paulling scale.

Therefore, in order to correctly consider the characteristics of compounds, the concept of ionicity was introduced. This characteristic allows you to determine what percentage of a particular bond is ionic.

The compound with the highest ionicity is cesium fluoride, in which it is approximately 97%. Ionic bonding is characteristic for substances formed by metal atoms located in the first and second groups of the D.I. table. Mendeleev, and atoms of non-metals located in the sixth and seventh groups of the same table.

Pay attention! It is worth noting that there is no compound in which the relationship is exclusively ionic. For open on at the moment elements, it is impossible to achieve such a large difference in electronegativity to obtain a 100% ionic compound. Therefore, the definition of an ionic bond is not entirely correct, since in reality compounds with partial ionic interaction are considered.

Why was this term introduced if such a phenomenon does not really exist? The fact is that this approach helped explain many of the nuances in the properties of salts, oxides and other substances. For example, why are they highly soluble in water, and why are they solutions are capable of conducting electric current . This cannot be explained from any other perspective.

Education mechanism

The formation of an ionic bond is possible only if two conditions are met: if the metal atom participating in the reaction is able to easily give up electrons located in the last energy level, and the non-metal atom is able to accept these electrons. Metal atoms by their nature are reducing agents, that is, they are capable of electron donation.

This is due to the fact that the last energy level in a metal can contain from one to three electrons, and the radius of the particle itself is quite large. Therefore, the force of interaction between the nucleus and electrons at the last level is so small that they can easily leave it. The situation with nonmetals is completely different. They have small radius, and the number of own electrons at the last level can be from three to seven.

And the interaction between them and the positive nucleus is quite strong, but any atom strives to complete the energy level, so non-metal atoms strive to obtain the missing electrons.

And when two atoms - a metal and a non-metal - meet, electrons transfer from the metal atom to the non-metal atom, and a chemical interaction is formed.

Connection diagram

The figure clearly shows how exactly the formation of an ionic bond occurs. Initially, there are neutrally charged sodium and chlorine atoms.

The first has one electron at the last energy level, the second seven. Next, an electron transfers from sodium to chlorine and the formation of two ions. Which combine with each other to form a substance. What is an ion? An ion is a charged particle in which the number of protons is not equal to the number of electrons.

Differences from covalent type

Due to its specificity, an ionic bond has no directionality. This is due to the fact that the electric field of the ion is a sphere, and it decreases or increases in one direction uniformly, obeying the same law.

Unlike covalent, which is formed due to the overlap of electron clouds.

The second difference is that covalent bond is saturated. What does it mean? The number of electronic clouds that can take part in interaction is limited.

And in the ionic one, due to the fact that the electric field has a spherical shape, it can connect with an unlimited number of ions. This means that we can say that it is not saturated.

It can also be characterized by several other properties:

1. Communication energy is quantitative characteristic, and it depends on the amount of energy that must be expended to break it. It depends on two criteria - bond length and ion charge involved in its education. The stronger the bond, the shorter its length and the greater the charges of the ions that form it.
2. Length - this criterion has already been mentioned in the previous paragraph. It depends solely on the radius of the particles involved in the formation of the compound. The radius of atoms changes as follows: it decreases over the period with increasing atomic number and increases in the group.

Substances with ionic bonds

It is typical for a significant number chemical compounds. This is a large part of all salts, including the well-known table salt. It occurs in all connections where there is a direct contact between metal and non-metal. Here are some examples of substances with ionic bonds:

• sodium and potassium chlorides,
• cesium fluoride,
• magnesium oxide.

It can also manifest itself in complex compounds.

For example, magnesium sulfate.

Here is the formula of a substance with ionic and covalent bonds:

An ionic bond will form between oxygen and magnesium ions, but sulfur is connected to each other using a polar covalent bond.

From which we can conclude that ionic bonds are characteristic of complex chemical compounds.

What is an ionic bond in chemistry

Types of chemical bonds - ionic, covalent, metallic

Conclusion

Properties directly depend on the device crystal lattice. Therefore, all compounds with ionic bonds are highly soluble in water and other polar solvents, conduct and are dielectrics. At the same time, they are quite refractory and fragile. The properties of these substances are often used in the design of electrical devices.

Ionic bond- a chemical bond formed as a result of mutual electrostatic attraction of oppositely charged ions, in which a stable state is achieved by complete transfer of the total electron density to an atom of a more electronegative element.

A purely ionic bond is an extreme case of a covalent bond.

In practice, the complete transfer of electrons from one atom to another atom through a bond is not realized, since each element has a greater or lesser (but not zero) EO, and any chemical bond will be covalent to some extent.

Such a bond occurs in the case of a large difference in the EO of atoms, for example, between cations s-metals of the first and second groups of the periodic system and anions of non-metals of groups VIА and VIIА (LiF, NaCl, CsF, etc.).

Unlike a covalent bond, ionic bond has no directionality . This is explained by the fact that the electric field of the ion has spherical symmetry, i.e. decreases with distance according to the same law in any direction. Therefore, the interaction between ions is independent of direction.

The interaction of two ions of opposite sign cannot lead to complete mutual compensation of their force fields. Because of this, they retain the ability to attract ions of the opposite sign in other directions. Therefore, unlike a covalent bond, ionic bonding is also characterized by unsaturation .

The lack of directionality and saturation in ionic bonds determines the tendency of ionic molecules to associate. All ionic compounds in the solid state have an ionic crystal lattice, in which each ion is surrounded by several ions of the opposite sign. In this case, all bonds of a given ion with neighboring ions are equivalent.

Metal connection

Metals are characterized by a number of special properties: electrical and thermal conductivity, a characteristic metallic luster, malleability, high ductility, and great strength. These specific properties of metals can be explained by a special type of chemical bond called metal .

A metallic bond is the result of overlapping delocalized orbitals of atoms approaching each other in the crystal lattice of a metal.

Most metals have a significant number of vacant orbitals and a small number of electrons in their outer electronic level.

Therefore, it is energetically more favorable for the electrons not to be localized, but to belong to the entire metal atom. At the lattice nodes of the metal there are positively charged ions that are immersed in an electron “gas” distributed throughout the metal:

Me ↔ Me n + + n .

There is an electrostatic interaction between positively charged metal ions (Me n +) and non-localized electrons (n), which ensures the stability of the substance. The energy of this interaction is intermediate between the energies of covalent and molecular crystals. Therefore, elements with a purely metallic bond ( s-, And p-elements) are characterized by relatively high melting points and hardness.

The presence of electrons, which can freely move throughout the volume of the crystal, provides the specific properties of the metal

Hydrogen bond

Hydrogen bond – a special type of intermolecular interaction. Hydrogen atoms that are covalently bonded to an atom of an element that has a high electronegativity value (most often F, O, N, but also Cl, S, and C) carry a relatively high effective charge. As a result, such hydrogen atoms can interact electrostatically with atoms of these elements.

Thus, the H d + atom of one water molecule is oriented and interacts accordingly (as shown by three dots) with the O d - atom of another water molecule:

Bonds formed by an H atom located between two atoms of electronegative elements are called hydrogen:

d- d+ d-

A − H ××× B

The energy of a hydrogen bond is significantly less than the energy of a conventional covalent bond (150–400 kJ/mol), but this energy is sufficient to cause aggregation of the molecules of the corresponding compounds into liquid state, for example, in liquid hydrogen fluoride HF (Fig. 2.14). For fluorine compounds it reaches about 40 kJ/mol.

Rice. 2.14. Aggregation of HF molecules due to hydrogen bonds

The length of a hydrogen bond is also shorter than the length of a covalent bond. Thus, in polymer (HF) n, the bond length is F−H = 0.092 nm, and the bond length is F∙∙∙H = 0.14 nm. For water, the bond length is O−H=0.096 nm, and the bond length O∙∙∙H=0.177 nm.

The formation of intermolecular hydrogen bonds leads to a significant change in the properties of substances: an increase in viscosity, dielectric constant, boiling and melting points.

STRUCTURE OF SUBSTANCE.

CHEMICAL BOND.

3.1. Ionic chemical bond

In the Periodic Table of Elements, the noble gases stand apart. These are unique chemical elements because even in the form of a simple substance they exist in the form of individual atoms not bonded to each other.

Some chemists still find it difficult to answer the question of how to consider particles of noble gases in a simple substance: as free atoms or as monatomic molecules. There is no clear opinion about what type of crystal lattice is characteristic of simple substances of these elements. In terms of physical properties, these are substances with molecular crystal lattices, and in terms of composition - ...? After all, the forces of intermolecular interaction that hold particles in crystals act between atoms.

Such an “indifferent” attitude towards the formation of chemical bonds is the “ultimate dream” for the atoms of all other chemical elements, which are very rarely found in the form of free atoms, only under extreme conditions.

Why are noble gas atoms so self-sufficient? After analyzing their position in the Periodic Table of Elements, you yourself can name the reason. The thing is that noble gas atoms contain a complete outer electron layer, on which the helium atom has two electrons, and the rest have eight. The atoms of all other elements strive to acquire just such a stable electronic configuration and often achieve this either as a result of the addition of electrons from other atoms (this process in chemistry is called reduction), or as a result of giving up their outer electrons other atoms (oxidation process). Atoms that have acquired foreign electrons turn into negative ions, or anions. Atoms that donate their electrons become positive ions, or cations.

It is clear that electrostatic attraction forces arise between oppositely charged cations and anions, which will hold the ions near each other, thereby realizing an ionic chemical bond.

An ionic chemical bond is a bond formed between cations and anions due to their electrostatic attraction.

Since cations are formed predominantly by metal atoms, and anions by non-metal atoms, it is logical to conclude that this type of bond is characteristic of binary (two-element) compounds formed by typical metals (alkali and alkaline earth) and typical non-metals (halogens, oxygen). Classic example substances with ionic bonds are halides and oxides of alkali and alkaline earth metals (Fig. 3.1).

The formation of an ionic bond between sodium and chlorine atoms can be represented as follows:

Two oppositely charged ions bound by forces of mutual attraction do not lose the ability to interact with other oppositely charged ions. As a result, crystalline compounds are formed, consisting of a huge number of ions.

Crystalline substances characterized by the correct arrangement of those particles (in our case, ions) of which they consist, at strictly defined points in space. When these points are connected by straight lines, a spatial framework is formed, which is called a crystal lattice. The points at which crystal particles are located are called lattice nodes. It is clear that substances with an ionic type of bond have ionic crystal lattices (color insert, Fig. 4).

Such compounds are solid, durable, non-volatile substances with high melting points. Under normal conditions, crystals of such substances do not conduct electric current, and solutions and melts of most ionic compounds are excellent electrolytes.

Substances with ionic crystal lattices are fragile. If you try to deform such a crystal lattice, one of its layers will move relative to the other until the equally charged ions are opposite each other. These ions will immediately begin to repel each other, and the lattice will collapse. Hence the fragility of ionic compounds.

Ca) (Ca 2^) + 2e

Ionic compounds are not only binary compounds of alkali and alkaline earth metals. These are also compounds formed by three or more elements. You can easily name them. These are all salts (color insert, Fig. 5), as well as hydroxides of alkali and alkaline earth metals.

In conclusion, we present a classification of ions according to various criteria:

1) according to composition, simple (Na +, Cl -, Ca 2+) and complex (OH -, S0 4 2-, NH 4 +) ions are distinguished;

2) according to the sign of the charge, positive ions, or cations (M n +, NH 4 +, H +), and negative ions, or anions (OH -, anions of acid residues) are distinguished;

3) based on the presence of a hydration shell, they distinguish between hydrated (for example, blue Cu 2+. 4H 2 0 ions) and non-hydrated (for example, uncolored Cu 2+ ions).

Everything in our world is relative. The same can be said about ionic bonds. The number of compounds with an ionic type of bond is very limited, but even in them a purely ionic bond is not observed. For example, there are no “pure” sodium and chloride ions with charges +1 and -1, respectively. The true charge of these ions is +0.8 and -0.8. Consequently, even in compounds that are considered ionic, the covalent nature of the bond appears to some extent. And finally, it is a relative truth that ionic bonding is the result of the interaction of the most common metals with the most common nonmetals. For example, ammonium salts, formed due to ionic bonds between ammonium cations and anions of the acid residue (for example, NH 4 Cl, NH 4 NO 3), having an ionic bond, consist exclusively of non-metals.

1. Why noble gases were previously classified as group zero Periodic table? Why are they now classified as group eight? By analogy, what metals are called noble? Why?

2. Write the electronic configuration of the outer layer of atoms of noble gases, halogens, and alkali metals.

3. Define the concept of “cation”. What groups of cations do you know?

4. Define the concept of “anion”. What groups of anions do you know?

5. Based on the concepts of “cation” and “anion”, give another definition of an ionic bond.

6. Draw up schemes for the formation of ionic bonds for substances: CaF 2, Li 2 0, KCl.

7. Define the concepts “crystal lattice”, “ionic crystal lattice”.

8. What physical properties are characterized by substances with ionic crystal lattices?

9. Among listed substances: KCl, AlCl 3, BaO, Fe 2 O 3, Fe 2 (S0 4) 3, H 2 S0 4, Si0 2, NH 3 - identify compounds with an ionic crystal lattice.

10. Without making calculations, determine which of the compounds: NaCl, KCl, LiCl, CaCl 2 has a higher mass fraction of chlorine. Confirm your conclusion with calculations.

11. Determine the formula of the ionic compound, mass fractions elements in which are: calcium 24.39%, nitrogen 17.07%, oxygen 58.54%.

3.2. Covalent chemical bond.

An alternative way to construct a stable configuration of eight (for hydrogen - two) electrons is their socialization, i.e. provision for joint use. As a result of this process, common electron pairs are formed, which play the role of “ connecting thread"between atoms forming a chemical bond.

A chemical bond between atoms that arises by sharing electrons to form common electron pairs is called covalent.

The formation of a common electron pair can occur in two ways.

When two atoms with unpaired electrons approach each other, the corresponding electron orbitals interpenetrate and overlap. At the point of overlap, the so-called electron density is formed, i.e. a region of space where the probability of finding an electron increases significantly. The overlap region is conventionally considered to be the common electron pair of two atoms. This mechanism of covalent bond formation is called exchange.

The exchange mechanism, for example, is realized when a chemical bond is formed in the hydrogen molecule H2. Hydrogen atoms share their only unpaired electrons with each other, thereby obtaining a complete energy level of two electrons, similar to the noble gas atom of helium. The resulting electron pair in equally belongs to both atoms:

N. + . N → N: N or N-N

Chlorine atoms also contain one unpaired electron. Due to their pairing, a chemical bond is formed, i.e. common electron pair in a chlorine molecule C1 2:

In both examples given, atoms of the same element are connected by a covalent bond. The shared electron pair belongs equally to both atoms.

A covalent bond formed between atoms of the same element is called non-polar.

Atoms of different elements can share electrons to form a covalent bond. In this case, it is necessary to take into account such a property of a chemical element as electronegativity.

Electronegativity is the property of the atoms of an element to attract shared electron pairs.

The most important non-metal elements can be arranged in the following order according to their increasing electronegativity:

Let us consider the formation of a covalent bond in an ammonia molecule. The nitrogen atom contains five electrons on the outer energy level in full accordance with the group number, of which three are unpaired electrons (to determine the number of unpaired electrons, you need to subtract the number of outer electrons from the cherished eight, in our case: 8 - 5 = 3).

Chemical bonds in the ammonia molecule are formed due to the formation of three electron pairs between three hydrogen atoms and one nitrogen atom:

The nitrogen atom is much more electronegative than hydrogen, so it attracts shared electron pairs to a greater extent. As a result of this displacement, the nitrogen atom acquires a partial negative charge δ-, and the hydrogen atoms acquire a partial positive charge δ+.

A covalent chemical bond between atoms with different electronegativity is called polar.

In all of the examples above, the chemical bond occurs through one shared pair of electrons. However, atoms are also capable of forming two or three common electron pairs, for example in molecules of carbon monoxide (IV) or nitrogen:

In the ammonia molecule, each atom complements its electron shell to the noble gas configuration: the nitrogen atom received eight electrons, the hydrogen atoms received two electrons. At the same time, the nitrogen atom still has a lone pair of electrons, due to which it can form a fourth chemical bond with the hydrogen cation, i.e. a particle completely devoid of electrons. At the same time, the mechanism of occurrence of the fourth N-H bonds other. A nitrogen atom that provides a pair of electrons to form a bond is called donor, and the hydrogen cation offering an empty orbital is acceptor. The resulting particle carries a positive charge and is called an ammonium cation:

This mechanism of covalent bond formation is called donor-acceptor. All four N-H bonds in the ammonium cation are absolutely equivalent; it is impossible to distinguish which of them is formed by the donor-acceptor mechanism and which by the exchange mechanism.

Substances with a covalent type of bond in the solid state form crystal lattices of two types: atomic and molecular.

Crystal lattices in which atoms are located at the nodes are called atomic. Substances with an atomic crystal lattice are characterized by great strength and hardness, a high melting point, they are non-volatile, and practically do not dissolve in any solvent without chemical interaction. Examples of such substances are diamond, quartz Si0 2, aluminum oxide, carborundum SiC.

Crystal lattices, in the nodes of which molecules of a substance are located, are called molecular. Intramolecular covalent bonds are quite strong, but individual molecules are connected to each other by rather weak intermolecular forces. Therefore, the molecular lattice is the weakest among all types of lattice. Such substances have low hardness, relatively low temperatures melting; they are volatile. Examples of compounds with a molecular crystal lattice include water, iodine, carbon dioxide, acetic acid, and sucrose.

All allotropic modifications of carbon, including the most famous - diamond and graphite, have atomic crystal lattices (color insert, Fig. 6, 7).

? 1. Define a covalent bond. What two mechanisms of its formation do you know? Give examples, write diagrams.

2. Define a covalent nonpolar bond. Give examples, write diagrams.

3. Define a polar covalent bond. Give examples, write schemes for the formation of a covalent bond according to the exchange and donor-acceptor mechanisms.

4. What types of bonds are characteristic of the following substances: Br 2, NVg, KBr? Write schemes for their formation.

5. How are covalent bonds distinguished by their multiplicity? What bonds are formed in the following compounds: SO 2, H 2 S, HCN? Write the structural formulas of these substances.

6. Without making calculations, indicate which of the sulfur oxides: SO 2 and SO 3 has the maximum sulfur content. Confirm your conclusion with calculations.

7. When burning 24 g of carbon, 33.6 liters are obtained carbon dioxide. What is the mass fraction of impurities in the carbon sample?

8. Can an ionic bond be considered a covalent bond? Why?

3.3. Metal chemical bond

Metal atoms are characterized by three distinctive features.

Firstly, they have 1-3 electrons in the outer energy level. However, tin and lead atoms have four valence electrons, antimony and bismuth have five, and polonium have six. Why are these elements metals? Obviously, the second one is beginning to take its toll distinctive feature structure of metal atoms - their relatively large radius. Finally, thirdly, metal atoms have large number free orbitals. Thus, the sodium atom, for example, has one outer valence electron located at the third energy level, which has nine orbitals (one s-, three p- and five d- orbitals).

When metal atoms come closer, their free orbitals can overlap, and valence electrons have the opportunity to move from the orbital of one atom to the free orbital of neighboring atoms that is similar in energy. An atom from which an electron has “left” turns into an ion. As a result, a collection of free electrons is formed in a metal product or piece of metal, which continuously move between ions. At the same time, being attracted to the positive ions of the metal, the electrons again turn them into atoms, then break away again, turning the latter into ions, and this happens endlessly. Therefore, in simple substances In metals, an endless process of atom-ion transformation is realized, which is carried out by valence electrons, and the particles that make up metals are called atom-ions:

The same picture is observed in metal alloys.

A metallic bond is a bond in metals and alloys between metal atoms carried out by a set of valence electrons.

The metallic bond also determines the special crystalline structure of metals and alloys - metal crystal lattice, at the nodes of which atom ions are located.

The metal crystal lattice and the metal bond determine all the most characteristic properties metals: malleability, ductility, ductility, electrical and thermal conductivity, metallic luster, ability to form alloys.

Plasticity explains the ability of metals to be flattened upon impact or drawn into wires under the influence of force. This is the most important mechanical property metals forms the basis of the blacksmith profession, respected by most peoples of the world. It is not without reason that among the gods of different beliefs, almost the only working god was the god of fire, the patron of the blacksmith's craft: among the Greeks - Hephaestus, among the Romans - Vulcan, among the Slavs - Svarog. The plasticity of the metal is explained by the fact that under external influence the layers of atom ions in crystals easily shift, as if sliding relative to each other without breaking the bond between them. A simple experience can give you some idea of ​​this. If you place a few drops of water between two flat glass plates, for example between mirrors, the mirrors will easily slide over each other, but it will be quite difficult to separate them. In our experiment, water plays the role of a collection of metal electrons.

The most ductile are gold, silver and copper. The thinnest foil with a thickness of only 0.003 mm can be made from gold. Such thin sheets of foil are used for gilding products, such as wood carvings. Artistic items made of gold, created thanks to its plasticity, have reached us through millennia (color insert, Fig. 8).

The high electrical conductivity of metals is precisely due to the presence in them of a set of mobile electrons, which, under the influence of electric field acquire directional movement. The best conductors of electric current are silver and copper. Aluminum is slightly inferior to them. However, in most countries, more and more often, wires are made not from copper, but from cheaper aluminum. The worst conductors of electric current are manganese, lead and mercury, as well as tungsten and some similar refractory metals. The electrical resistance of tungsten is so high that it begins to glow when current passes through it, which is used to make filaments of incandescent lamps (Fig. 3.2).

Similar to electrical conductivity, the thermal conductivity of metals also changes, which is also explained by the high mobility of electrons, which, colliding with metal ions vibrating at lattice sites, exchange thermal energy with them. As the temperature rises, these vibrations of the ions are transmitted to other ions with the help of electrons, and the temperature of the metal quickly equalizes. You can judge the practical significance of this property by looking at metal kitchen utensils.

The smooth surface of a metal or metal product is characterized by a metallic luster, which is the result of the reflection of light rays. Metals that have high light reflectivity include mercury (the famous Venetian mirrors were previously made from it), silver, palladium and aluminum. The last three metals are used to make mirrors, spotlights and headlights.

In powder, metals lose their shine, acquiring a black or gray color, and only magnesium and aluminum retain it. Therefore, a decorative coating is made from aluminum dust - silver paint. The light reflected by the surface of metals determines the silvery-white color of most metals, since they scatter equally all rays of the visible part of the spectrum. But gold and copper absorb to a greater extent rays with a short wavelength, close to violet, while reflecting long-wave rays, and therefore are colored yellow (red) or red-yellow (copper), respectively. Look at fig. 9 on a colored insert, which shows fancy metal nuggets made by nature, having the appropriate color.

Even in ancient times, people noticed that alloys have different, often more useful properties than their constituent pure metals. Therefore, metals are rarely used in their pure form. Their alloys are most often used. Tens of thousands of different alloys have been obtained from just over 80 known metals. For example, the first alloy produced by man, bronze, has a higher strength than its constituents, copper and tin. Steel and cast iron are stronger than pure iron. Pure aluminum is a very soft metal, relatively weak in tensile strength. But an alloy consisting of aluminum, magnesium, manganese, copper, nickel, called duralumin, is 4 times tensile strength than aluminum, and therefore it is figuratively called “winged metal” and is used for the manufacture of aircraft structures (Fig. 3.3).

In addition to greater strength, alloys also have higher corrosion resistance and hardness, and better casting properties than pure metals. Thus, pure copper is very difficult to cast, while tin bronze has excellent casting qualities - artistic products that require fine detailing are cast from it. Cast iron, an alloy of iron and carbon, is also an excellent casting material.

In addition to high mechanical properties, alloys have properties that pure metals do not have. For example, stainless steel, an iron-based alloy, has high heat resistance and corrosion resistance even in aggressive environments.

The scientific and technological revolution that began about 100 years ago affected both industry and social sphere, is also closely related to the production of metals and alloys.

Based on tungsten, molybdenum, titanium and other metals, they began to create corrosion-resistant, super-hard and refractory alloys, the use of which has significantly expanded the capabilities of mechanical engineering. In nuclear and space technology (Fig. 3.4), parts operating at temperatures up to 3000 0 C are made from an alloy of tungsten and rhenium.

In medicine, surgical instruments and implants made of tantalum and platinum alloys are used.

I. What features characterize the structure of metal atoms? What position do metals occupy in the periodic table?

2. Define metal connection. What does it have in common with ionic and covalent bonds?

3. What structure does the metal crystal lattice have? Compare it with ionic and atomic crystal lattices.

4. What physical properties metals are determined by their crystal structure?

5. Name a metal that is liquid under normal conditions. What safety rules must be followed when working with objects containing this metal?

6. Bronze is composed of 20% tin and 80% copper. Calculate the mass of each component of bronze required to make a sculpture weighing 200 kg. How much of each metal was required for this?

7. The density of metallic gold is 19 g/cm3. Determine the area of ​​a gold film 0.003 mm thick that can be made from a piece of metal weighing 3 g.

8. To obtain metallic copper, two of its natural oxides are used, containing 89 and 80% metal, respectively. Determine the formulas of the oxides.

9. In Wood’s famous low-melting alloy with a melting point of just 62 0 C, the mass fraction of bismuth is twice that of lead; the mass fraction of lead is twice that of tin; and the mass fraction of cadmium is equal to mass fraction tin. Calculate the mass fractions of metals in the alloy.

3.4. Hydrogen chemical bond

By considering the hydrogen bond, we complete our acquaintance with the types of chemical bonds. And this is no coincidence.

Firstly, the hydrogen bond is the subject of endless discussions between physicists and chemists, who consider this type of bond from various points of view. Physicists say that this is a type of intermolecular interaction that has physical nature, and argue that the energy of such a bond is only 4 -40 kJ/mol. Most chemists hold a different point of view, which will be outlined below.

Secondly, consideration of the hydrogen bond will allow us to compare this bond with other types and thereby generalize our knowledge about the nature of chemical bonds in general.

Thirdly, this is the most significant chemical bond on our planet, because it determines the structure of those compounds that are carriers of life on Earth and are responsible for storage and reproduction hereditary information living organisms.

All previously studied types of chemical bonds have names based on generalized chemical concepts: ions, atoms, metals. And “hydrogen bond” is a specific term associated with a specific chemical element - hydrogen. Obviously, this is due to the specific structure of the hydrogen atom, which has one valence electron. By participating in the formation of a chemical bond, this electron exposes the tiny nucleus of the hydrogen atom, which is nothing more than a proton.

The chemical bond between the hydrogen atoms of one molecule and the atoms of electronegative elements (fluorine, oxygen, nitrogen) of another molecule is called hydrogen.

Intermolecular hydrogen bonding explains the fact that substances with small relative molecular weights under normal conditions are liquids (water, alcohols - methyl, ethyl, carboxylic acids- formic, acetic) or easily liquefied gases (ammonia, hydrogen fluoride).

The mechanism of hydrogen bond formation is of a dual nature. On the one hand, it consists of an electrostatic attraction between a hydrogen atom with a partial positive charge and an oxygen atom (fluorine or nitrogen) with a partial negative charge. On the other hand, the donor-acceptor interaction between the almost free orbital of the hydrogen atom and the lone electron pair of the oxygen atom (fluorine or nitrogen) also contributes to the formation of a hydrogen bond. For example, water is associated into a liquid due to hydrogen bonds that arise between dipole molecules:

In liquid water, many hydrogen bonds form between molecules.

The ability of some gases, due to the formation of hydrogen bonds, to easily liquefy and return to a gaseous state with the absorption of heat allows them to be used as refrigerants in industrial refrigeration units. Ammonia is most widely used in this role:

It is hydrogen bonds that explain the abnormally high boiling temperatures (100°C) and melting temperatures (0 0 C) of water. At the same time, unlike most other liquids, the density of water when transitioning to a solid state (ice, snow) does not increase, but decreases. This explains the fact that ice is lighter than water and does not sink in it, and therefore reservoirs do not freeze to the bottom in winter, thereby preserving the life of aquatic inhabitants.

Hydrogen bonds contribute greatly to the formation of crystals in the form of an endless variety of snowflakes.

All the examples discussed above concerned a type of hydrogen bond called intermolecular hydrogen bond. However, it is even more important in organizing the structures of vital molecules intramolecular hydrogen bond. This bond determines the secondary structure of protein molecules.

Protein molecule represents a long polymer chain twisted into a spiral. The turns of this helix are kept from unwinding due to hydrogen bonds between the hydrogen and oxygen atoms of sections of the primary structure of the protein molecule.

Being very delicate and vulnerable, hydrogen bonds in proteins can easily break - proteins denature. Such denaturation can be reversible or irreversible.

Reversible denaturation of protein molecules has social significance. Thus, mechanical influences (road service workers, miners, miners and other specialists using vibrating tools) can serve as denaturing factors for the proteins of the human body. high temperatures(hot shop workers - metallurgists (Fig. 3.5), glass makers, etc.), electromagnetic radiation (radiologists, nuclear power plant workers), chemical exposure (chemical production workers). Therefore, all of the listed categories of workers, to compensate for the harmful effects of working conditions on the body, use the provisions provided for by law. Russian Federation benefits: they have shorter working hours, longer paid vacations, special meals, earlier retirement, higher wages.

Irreversible denaturation is well known to you from the procedure of boiling eggs or cooking meat, fish and other protein products.

How denaturing factors lead to the destruction of the natural structure of protein molecules can be judged from simple experiments. If you add a little egg white to a solution ethyl alcohol or salt heavy metal(copper sulfate, lead(II) nitrate), you will notice the formation of a precipitate due to protein denaturation. Nicotine, very strong tea and coffee have a similar effect. Maybe these experiences will help you understand how harmful bad habits such as smoking, drinking alcohol, etc. are.

1. Define hydrogen bond. Which point of view - physicists or chemists - do you share on the issue of its nature?

2. What is the mechanism for the formation of a hydrogen bond? What types of hydrogen bonds do you know?

3. What special properties do substances with intermolecular hydrogen bonds have?

4. What role does intermolecular hydrogen bonding play in the practical life of humans and in nature?

5. What role does intramolecular hydrogen bonding play in organizing the structure of proteins?

6. What do you think is the social role of hydrogen bonds? Illustrate your answer with examples from the practical life of a person.

7. Prepare a message about the chemical nature of the negative effects of smoking and drinking alcohol on the human body.