Chemical bond.

determination of chemical bond;

types of chemical bonds;

valence bond method;

basic characteristics of covalent bonds;

mechanisms of covalent bond formation;

complex compounds;

molecular orbital method;

intermolecular interactions.

DEFINITION OF CHEMICAL BOND

Chemical bond called the interaction between atoms, leading to the formation of molecules or ions and the strong holding of atoms near each other.

A chemical bond is of an electronic nature, that is, it is carried out due to the interaction of valence electrons. Depending on the distribution of valence electrons in the molecule, the following types of bonds are distinguished: ionic, covalent, metallic, etc. An ionic bond can be considered as an extreme case of a covalent bond between atoms that differ sharply in nature.

TYPES OF CHEMICAL BOND

Ionic bond.

Basic provisions of the modern theory of ionic bonding.

An ionic bond is formed during the interaction of elements that differ sharply from each other in properties, that is, between metals and non-metals.

The formation of a chemical bond is explained by the desire of atoms to achieve a stable eight-electron outer shell (s 2 p 6).

Ca: 1s 2 2s 2 p 6 3s 2 p 6 4s 2

Ca 2+ : 1s 2 2s 2 p 6 3s 2 p 6

Cl: 1s 2 2s 2 p 6 3s 2 p 5

Cl – : 1s 2 2s 2 p 6 3s 2 p 6

The resulting oppositely charged ions are held near each other due to electrostatic attraction.

The ionic bond is not directional.

There is no purely ionic bond. Since the ionization energy is greater than the electron affinity energy, a complete electron transfer does not occur even in the case of a pair of atoms with a large difference in electronegativity. Therefore, we can talk about the fraction of ionicity of the bond. The highest ionicity of the bond occurs in fluorides and chlorides of s-elements. Thus, in RbCl, KCl, NaCl and NaF crystals it is 99, 98, 90 and 97%, respectively.

Covalent bond.

Basic provisions of the modern theory of covalent bonds.

A covalent bond is formed between elements with similar properties, that is, nonmetals.

Each element provides 1 electron for the formation of bonds, and the spins of the electrons must be antiparallel.

If a covalent bond is formed by atoms of the same element, then this bond is not polar, that is, the common electron pair is not shifted to any of the atoms. If a covalent bond is formed by two different atoms, then the common electron pair is shifted to the most electronegative atom, this polar covalent bond.

When a covalent bond is formed, the electron clouds of interacting atoms overlap; as a result, a zone of increased electron density appears in the space between the atoms, attracting the positively charged nuclei of interacting atoms and holding them near each other. As a result, the energy of the system decreases (Fig. 14). However, when the atoms are very close together, the repulsion of nuclei increases. Therefore, there is an optimal distance between the cores ( link length,l sv), at which the system has minimal energy. In this state, energy is released, called binding energy - E St.

Rice. 14. Dependence of the energy of systems of two hydrogen atoms with parallel (1) and antiparallel (2) spins on the distance between the nuclei (E is the energy of the system, E is the binding energy, r is the distance between the nuclei, l– communication length).

To describe a covalent bond, two methods are used: the valence bond (VB) method and the molecular orbital method (MMO).

VALENCE BONDS METHOD.

The BC method is based on the following provisions:

1. A covalent chemical bond is formed by two electrons with opposite spins, and this electron pair belongs to two atoms. Combinations of such two-electron two-center bonds, reflecting the electronic structure of the molecule, are called valence schemes.

2. The stronger the covalent bond, the more the interacting electron clouds overlap.

To visually depict valence schemes, the following method is usually used: electrons located in the outer electronic layer are designated by dots located around the chemical symbol of the atom. Electrons shared by two atoms are shown by dots placed between their chemical symbols; a double or triple bond is indicated by two or three pairs of common points, respectively:

N: 1s 2 2s 2 p 3 ;

C: 1s 2 2s 2 p 4

From the above diagrams it is clear that each pair of electrons connecting two atoms corresponds to one line depicting a covalent bond in the structural formulas:

The number of common electron pairs connecting an atom of a given element with other atoms, or, in other words, the number of covalent bonds formed by an atom, is called covalency according to the BC method. Thus, the covalency of hydrogen is 1, that of nitrogen is 3.

According to the method of overlapping electron clouds, connections are of two types:  - connection and  - connection.

 - a bond occurs when two electron clouds overlap along the axis connecting the nuclei of atoms.

Rice. 15. Scheme of formation of  - connections.

 - a bond is formed when electron clouds overlap on either side of the line connecting the nuclei of interacting atoms.

Rice. 16. Scheme of formation of  - connections.

BASIC CHARACTERISTICS OF COVALENT BONDING.

1. Link length, ℓ. This is the minimum distance between the nuclei of interacting atoms, which corresponds to the most stable state of the system.

2. Bond energy, E min - this is the amount of energy that must be expended to break a chemical bond and to remove atoms beyond the interaction limits.

3. Dipole moment of connection, ,=qℓ. The dipole moment serves as a quantitative measure of the polarity of a molecule. For non-polar molecules, the dipole moment is 0, for non-polar molecules it is not equal to 0. The dipole moment of a polyatomic molecule is equal to the vector sum of the dipoles of individual bonds:

4. A covalent bond is characterized by directionality. The direction of a covalent bond is determined by the need for maximum overlap in space of electron clouds of interacting atoms, which lead to the formation of the strongest bonds.

Since these -bonds are strictly oriented in space, depending on the composition of the molecule, they can be at a certain angle to each other - such an angle is called valence.

Diatomic molecules have a linear structure. Polyatomic molecules have a more complex configuration. Let us consider the geometry of various molecules using the example of the formation of hydrides.

1. VI group, main subgroup (except oxygen), H 2 S, H 2 Se, H 2 Te.

S1s 2 2s 2 r 6 3s 2 r 4

For hydrogen, an electron with an s-AO participates in the formation of a bond, for sulfur – 3p y and 3p z. The H2S molecule has a flat structure with an angle between bonds of 90 0. .

Figure 17. Structure of the H 2 E molecule

2. Hydrides of elements of group V, the main subgroup: PH 3, AsH 3, SbH 3.

Р 1s 2 2s 2 р 6 3s 2 р 3 .

Participating in the formation of bonds are: for hydrogen s-AO, for phosphorus - p y, p x and p z AO.

The PH 3 molecule has the shape of a trigonal pyramid (at the base there is a triangle).

Figure 18. Structure of the EN 3 molecule

5. Saturability covalent bond is the number of covalent bonds that an atom can form. It is limited because an element has a limited number of valence electrons. The maximum number of covalent bonds that a given atom can form in the ground or excited state is called its covalency.

Example: hydrogen is monocovalent, oxygen is bicovalent, nitrogen is tricovalent, etc.

Some atoms can increase their covalency in the excited state by dissociating paired electrons.

Example. Be 0 1s 2 2s 2

A beryllium atom in an excited state has one valence electron on the 2p-AO and one electron on the 2s-AO, that is, covalency Be 0 = 0 and covalency Be* = 2. During the interaction, hybridization of orbitals occurs.

Hybridization- this is the equalization of the energy of various AOs as a result of mixing before chemical interaction. Hybridization is a conditional technique that allows one to predict the structure of a molecule using a combination of AOs. Those AOs whose energies are close can take part in hybridization.

Each type of hybridization corresponds to a certain geometric shape of the molecules.

In the case of hydrides of Group II elements of the main subgroup, two identical sp-hybrid orbitals participate in the formation of the bond. This type of connection is called sp-hybridization.

Figure 19. Molecule BeH 2 .sp-Hybridization.

sp-Hybrid orbitals have an asymmetrical shape; the elongated parts of the AO are directed towards hydrogen with a bond angle of 180 o. Therefore, the BeH 2 molecule has a linear structure (Fig.).

Let us consider the structure of molecules of hydrides of elements of group III of the main subgroup using the example of the formation of the BH 3 molecule.

B 0 1s 2 2s 2 p 1

Covalency B 0 = 1, covalency B* = 3.

Three sp-hybrid orbitals take part in the formation of bonds, which are formed as a result of the redistribution of electron densities of s-AO and two p-AO. This type of connection is called sp 2 - hybridization. The bond angle at sp 2 - hybridization is equal to 120 0, therefore the BH 3 molecule has a flat triangular structure.

Fig.20. BH 3 molecule. sp 2 -Hybridization.

Using the example of the formation of the CH 4 molecule, let us consider the structure of the molecules of hydrides of elements of group IV of the main subgroup.

C 0 1s 2 2s 2 p 2

Covalency C0 = 2, covalency C* = 4.

In carbon, four sp-hybrid orbitals, formed as a result of the redistribution of electron densities between the s-AO and three p-AO, participate in the formation of a chemical bond. The shape of the CH 4 molecule is a tetrahedron, the bond angle is 109°28`.

Rice. 21. Molecule CH 4 .sp 3 -Hybridization.

Exceptions to the general rule are the molecules H 2 O and NH 3.

In a water molecule, the angles between bonds are 104.5 degrees. Unlike hydrides of other elements in this group, water has special properties: it is polar and diamagnetic. All this is explained by the fact that the type of bond in a water molecule is sp 3. That is, four sp - hybrid orbitals participate in the formation of a chemical bond. Two orbitals contain one electron each, these orbitals interact with hydrogen, and the other two orbitals contain a pair of electrons. The presence of these two orbitals explains the unique properties of water.

In the ammonia molecule, the angles between the bonds are approximately 107.3 o, that is, the shape of the ammonia molecule is a tetrahedron, the type of bond is sp 3. Four hybrid sp 3 orbitals take part in the formation of a bond on a nitrogen molecule. Three orbitals contain one electron each; these orbitals are associated with hydrogen; the fourth AO contains a lone pair of electrons, which determines the uniqueness of the ammonia molecule.

MECHANISMS OF COVALENT BOND FORMATION.

MBC allows one to distinguish three mechanisms of covalent bond formation: exchange, donor-acceptor, and dative.

Exchange mechanism. It includes those cases of the formation of a chemical bond when each of the two bonded atoms allocates one electron for sharing, as if exchanging them. To bind the nuclei of two atoms, electrons must be in the space between the nuclei. This region in the molecule is called the binding region (the region where an electron pair is most likely to reside in the molecule). In order for the exchange of unpaired electrons between atoms to occur, the atomic orbitals must overlap (Fig. 10,11). This is the action of the exchange mechanism for the formation of a covalent chemical bond. Atomic orbitals can overlap only if they have the same symmetry properties relative to the internuclear axis (Fig. 10, 11, 22).

Rice. 22. Overlapping of AO, which does not lead to the formation of a chemical bond.

Donor-acceptor and dative mechanisms.

The donor-acceptor mechanism involves the transfer of a lone pair of electrons from one atom to a vacant atomic orbital of another atom. For example, the formation of the ion - :

The vacant p-AO in the boron atom in the BF 3 molecule accepts a pair of electrons from the fluoride ion (donor). In the resulting anion, four covalent B-F bonds are equal in length and energy. In the original molecule, all three B-F bonds were formed by the exchange mechanism.

Atoms whose outer shell consists only of s- or p-electrons can be either donors or acceptors of a lone pair of electrons. Atoms whose valence electrons are located above the d-AO can simultaneously act as both donors and acceptors. To distinguish between these two mechanisms, the concepts of the dative mechanism of bond formation were introduced.

The simplest example of a dative mechanism is the interaction of two chlorine atoms.

Two chlorine atoms in a chlorine molecule form a covalent bond by an exchange mechanism, combining their unpaired 3p electrons. In addition, the Cl- 1 atom transfers a lone pair of electrons 3p 5 - AO to the Cl- 2 atom to the vacant 3d-AO, and the Cl- 2 atom transfers the same pair of electrons to the vacant 3d-AO of the Cl- 1 atom. Each atom simultaneously performs the functions of an acceptor and donor. This is the dative mechanism. The action of the dative mechanism increases the bond strength, so the chlorine molecule is stronger than the fluorine molecule.

COMPLEX CONNECTIONS.

According to the principle of the donor-acceptor mechanism, a huge class of complex chemical compounds is formed - complex compounds.

Complex compounds are compounds containing complex ions capable of existing both in crystalline form and in solution, including a central ion or atom associated with negatively charged ions or neutral molecules by covalent bonds formed by a donor-acceptor mechanism.

Structure of complex compounds according to Werner.

Complex compounds consist of an inner sphere (complex ion) and an outer sphere. The connection between the ions of the inner sphere occurs via a donor-acceptor mechanism. Acceptors are called complexing agents; they can often be positive metal ions (except for group IA metals) having vacant orbitals. The ability to form complexes increases as the charge of the ion increases and its size decreases.

Electron pair donors are called ligands or addends. Ligands are neutral molecules or negatively charged ions. The number of ligands is determined by the coordination number of the complexing agent, which, as a rule, is equal to twice the valence of the complexing ion. Ligands can be monodentant or polydentant. The dentency of a ligand is determined by the number of coordination sites that the ligand occupies in the coordination sphere of the complexing agent. For example, F - is a monodentate ligand, S 2 O 3 2- is a bidentate ligand. The charge of the inner sphere is equal to the algebraic sum of the charges of its constituent ions. If the inner sphere has a negative charge, it is an anionic complex; if it is positive, it is a cationic complex. Cationic complexes are called by the name of the complexing ion in Russian; in anionic complexes the complexing agent is called in Latin with the addition of the suffix - at. The connection between the outer and inner spheres in a complex compound is ionic.

Example: K 2 – potassium tetrahydroxozincate, anionic complex.

2- - inner sphere

2K+ - outer sphere

Zn 2+ - complexing agent

OH – - ligands

coordination number – 4

the connection between the outer and inner spheres is ionic:

K 2 = 2K + + 2- .

the bond between the Zn 2+ ion and hydroxyl groups is covalent, formed according to the donor-acceptor mechanism: OH - donors, Zn 2+ - acceptor.

Zn 0: … 3d 10 4s 2

Zn 2+ : … 3d 10 4s 0 p 0 d 0

Types of complex compounds:

1. Ammonia compounds are ligands of the ammonia molecule.

Cl 2 – tetraammine copper (II) chloride. Ammonia compounds are produced by the action of ammonia on compounds containing a complexing agent.

2. Hydroxo compounds - OH - ligands.

Na – sodium tetrahydroxyaluminate. Hydroxo complexes are obtained by the action of excess alkali on metal hydroxides, which have amphoteric properties.

3. Aqua complexes are ligands of water molecules.

Cl 3 – hexaaquachrome (III) chloride. Aqua complexes are obtained by reacting anhydrous salts with water.

4. Acid complexes - ligands acid anions - Cl - , F - , CN - , SO 3 2- , I – , NO 2 – , C 2 O 4 – etc.

K 4 – potassium hexacyanoferrate (II). Prepared by reacting an excess of a salt containing a ligand with a salt containing a complexing agent.

METHOD OF MOLECULAR ORBITALS.

MBC explains the formation and structure of many molecules quite well, but this method is not universal. For example, the valence bond method does not provide a satisfactory explanation for the existence of the ion
, although at the end of the 19th century the existence of a fairly strong molecular hydrogen ion was established
: The bond breaking energy here is 2.65 eV. However, no electron pair can be formed in this case, since the composition of the ion
only one electron is included.

The molecular orbital method (MMO) allows one to explain a number of contradictions that cannot be explained using the valence bond method.

Basic provisions of the MMO.

When two atomic orbitals interact, two molecular orbitals are formed. Accordingly, when n-atomic orbitals interact, n-molecular orbitals are formed.

The electrons in a molecule belong equally to all the nuclei of the molecule.

Of the two molecular orbitals formed, one has a lower energy than the original one, this is the bonding molecular orbital, the other has higher energy than the original one, this antibonding molecular orbital.

MMOs use energy diagrams that are not to scale.

When filling energy sublevels with electrons, the same rules are used as for atomic orbitals:

the principle of minimum energy, i.e. sublevels with lower energy are filled first;

Pauli principle: at each energy sublevel there cannot be more than two electrons with antiparallel spins;

Hund's rule: filling of energy sublevels occurs in such a way that the total spin is maximum.

Multiplicity of communication. Communication multiplicity in MMO is determined by the formula:

, when K p = 0, no bond is formed.

Examples.

1. Can an H2 molecule exist?

Rice. 23. Scheme of the formation of the hydrogen molecule H2.

Conclusion: the H2 molecule will exist, since the bond multiplicity Kp > 0.

2. Can a He 2 molecule exist?

Rice. 24. Scheme of the formation of a helium molecule He 2.

Conclusion: the He 2 molecule will not exist, since the bond multiplicity Kp = 0.

3. Can an H 2 + particle exist?

Rice. 25. Scheme of the formation of an H 2 + particle.

The H 2 + particle can exist, since the bond multiplicity Kp > 0.

4. Can an O2 molecule exist?

Rice. 26. Scheme of formation of the O 2 molecule.

The O 2 molecule exists. From Fig. 26 it follows that the oxygen molecule has two unpaired electrons. Due to these two electrons, the oxygen molecule is paramagnetic.

Thus, the molecular orbital method explains the magnetic properties of molecules.

INTERMOLECULAR INTERACTION.

All intermolecular interactions can be divided into two groups: universal And specific. Universal ones appear in all molecules without exception. These interactions are often called connection or van der Waals forces. Although these forces are weak (the energy does not exceed eight kJ/mol), they are the reason for the transition of most substances from a gaseous state to a liquid state, the adsorption of gases on the surfaces of solids and other phenomena. The nature of these forces is electrostatic.

Main interaction forces:

1). Dipole – dipole (orientation) interaction exists between polar molecules.

The greater the dipole moments, the smaller the distance between molecules, and the lower the temperature, the greater the orientational interaction. Therefore, the greater the energy of this interaction, the higher the temperature the substance must be heated in order for it to boil.

2). Inductive interaction is carried out if there is contact between polar and non-polar molecules in a substance. A dipole is induced in a nonpolar molecule as a result of interaction with a polar molecule.

Cl  + - Cl  - … Al  + Cl  - 3

The energy of this interaction increases with increasing molecular polarizability, that is, the ability of molecules to form a dipole under the influence of an electric field. The energy of inductive interaction is significantly less than the energy of dipole-dipole interaction.

3). Dispersion interaction- this is the interaction of non-polar molecules due to instantaneous dipoles arising due to fluctuations of electron density in atoms.

In a series of substances of the same type, dispersion interaction increases with increasing sizes of the atoms that make up the molecules of these substances.

4) Repulsive forces are caused by the interaction of electron clouds of molecules and appear as they approach further.

Specific intermolecular interactions include all types of interactions of a donor-acceptor nature, that is, associated with the transfer of electrons from one molecule to another. The intermolecular bond formed in this case has all the characteristic features of a covalent bond: saturation and directionality.

A chemical bond formed by a positively polarized hydrogen that is part of a polar group or molecule and an electronegative atom of another or the same molecule is called a hydrogen bond. For example, water molecules can be represented as follows:

Solid lines are covalent polar bonds inside water molecules between hydrogen and oxygen atoms; dots indicate hydrogen bonds. The reason for the formation of hydrogen bonds is that hydrogen atoms are practically devoid of electron shells: their only electrons are displaced to the oxygen atoms of their molecules. This allows protons, unlike other cations, to approach the nuclei of oxygen atoms of neighboring molecules without experiencing repulsion from the electron shells of oxygen atoms.

A hydrogen bond is characterized by a binding energy of 10 to 40 kJ/mol. However, this energy is enough to cause association of molecules, those. their association into dimers or polymers, which in some cases exist not only in the liquid state of the substance, but are also preserved when it passes into vapor.

For example, hydrogen fluoride in the gas phase exists in the form of a dimer.

In complex organic molecules, there are both intermolecular hydrogen bonds and intramolecular hydrogen bonds.

Molecules with intramolecular hydrogen bonds cannot form intermolecular hydrogen bonds. Therefore, substances with such bonds do not form associates, are more volatile, and have lower viscosities, melting and boiling points than their isomers capable of forming intermolecular hydrogen bonds.

CHEMICAL BOND

Chemical bond is the interaction of two atoms carried out by exchanging electrons. When a chemical bond is formed, atoms tend to acquire a stable eight-electron (or two-electron) outer shell, corresponding to the structure of the atom of the nearest inert gas. The following types of chemical bonds are distinguished: covalent(polar and nonpolar; exchange and donor-acceptor), ionic, hydrogen And metal.

COVALENT BOND

It is carried out due to the electron pair belonging to both atoms. There are exchange and donor-acceptor mechanisms for the formation of covalent bonds.

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 the other atom (acceptor) provides an empty orbital for that pair;

Two atoms can not socialize c how many pairs of electrons? In this case they talk about multiples connections:

If the electron density is located symmetrically between atoms, the covalent bond is called non-polar.

If the electron density is shifted towards one of the atoms, then the covalent bond is called polar.

The greater the difference in the electronegativity of the atoms, the greater the polarity of the bond.

Electronegativity is the ability of an atom to attract electron density from other atoms. The most electronegative element is fluorine, the most electropositive is francium.

IONIC BOND

Ions- these are charged particles into which atoms turn as a result of the loss or addition of electrons.

(sodium fluoride is made up of sodium ions Na+ and fluoride ions F - )

If the difference in the electronegativity of the atoms is large, then the electron pair performing the bond goes to one of the atoms, and both atoms turn into ions.

The chemical bond between ions due to electrostatic attraction is calledionic bond.

HYDROGEN BONDING

Hydrogen bond - This is a bond between a positively charged hydrogen atom of one molecule and a negatively charged atom of another molecule. The hydrogen bond is partly electrostatic and partly donor-acceptor in nature.

The presence of hydrogen bonds explains the high boiling temperatures of water, alcohols, and carboxylic acids.

METAL LINK

The valence electrons of metals are rather weakly bound to their nuclei and can easily be detached from them. Therefore, the metal contains a number of positive ions located at certain positions in the crystal lattice, and a large number of electrons moving freely throughout the crystal. Electrons in a metal provide bonds between all metal atoms.

ORBITAL HYBRIDISATION

Orbital hybridization is a change in the shape of some orbitals during the formation of a covalent bond to achieve more efficient orbital overlap.

A

sp 3 - Hybridization. One s orbital and three p - the orbitals turn into four identical “hybrid” orbitals, the angle between the axes of which is 109° 28".

sp 3 - hybridization, have tetrahedral geometry ( CH 4, NH 3).

B

sp 2 - Hybridization. One s-orbital and two p-orbitals turn into three identical “hybrid” orbitals, the angle between their axes is 120°.

Molecules in which it takes place sp

Two sp - orbitals can form two s - bonds (BeH 2, ZnCl 2). Two more p - connections can be formed if two p - orbitals not involved in hybridization contain electrons (acetylene C 2 H 2 ).

Molecules in which it takes place sp - hybridization, have linear geometry.

END OF SECTION

It is one of the cornerstones of an interesting science called chemistry. In this article we will analyze all aspects of chemical bonds, their importance in science, give examples and much more.

What is a chemical bond

In chemistry, a chemical bond is understood as the mutual adhesion of atoms in a molecule and, as a result of the force of attraction that exists between. It is thanks to chemical bonds that various chemical compounds are formed; this is the nature of a chemical bond.

Types of Chemical Bonds

The mechanism of formation of a chemical bond strongly depends on its type or type; in general, the following main types of chemical bonds differ:

• Covalent chemical bond (which in turn can be polar or non-polar)
• Ionic bond
• Chemical bond

like people.

As for, a separate article is devoted to it on our website, and you can read in more detail at the link. Next, we will examine in more detail all the other main types of chemical bonds.

Ionic chemical bond

The formation of an ionic chemical bond occurs due to the mutual electrical attraction of two ions having different charges. Ions in such chemical bonds are usually simple, consisting of one atom of the substance.

Scheme of ionic chemical bond.

A characteristic feature of the ionic type of chemical bond is its lack of saturation, and as a result, a very different number of oppositely charged ions can join an ion or even a whole group of ions. An example of an ionic chemical bond is the cesium fluoride compound CsF, in which the level of “ionicity” is almost 97%.

Hydrogen chemical bond

Long before the advent of the modern theory of chemical bonds in its modern form, chemists noticed that hydrogen compounds with non-metals have various amazing properties. Let's say the boiling point of water and together with hydrogen fluoride is much higher than it could be, here is a ready-made example of a hydrogen chemical bond.

The picture shows a diagram of the formation of a hydrogen chemical bond.

The nature and properties of a hydrogen chemical bond are determined by the ability of the hydrogen atom H to form another chemical bond, hence the name of this bond. The reason for the formation of such a connection is the properties of electrostatic forces. For example, the total electron cloud in a hydrogen fluoride molecule is so shifted towards fluorine that the space around an atom of this substance is saturated with a negative electric field. Around a hydrogen atom, especially one deprived of its only electron, everything is exactly the opposite; its electronic field is much weaker and, as a result, has a positive charge. And positive and negative charges, as you know, attract, and in this simple way a hydrogen bond arises.

Chemical bond of metals

What chemical bond is characteristic of metals? These substances have their own type of chemical bond - the atoms of all metals are not arranged anyhow, but in a certain way, the order of their arrangement is called a crystal lattice. Electrons of different atoms form a common electron cloud, and they weakly interact with each other.

This is what a metal chemical bond looks like.

An example of a metallic chemical bond can be any metal: sodium, iron, zinc, and so on.

How to determine the type of chemical bond

Depending on the substances taking part in it, if there is a metal and a non-metal, then the bond is ionic, if there are two metals, then it is metallic, if there are two non-metals, then it is covalent.

Properties of chemical bonds

To compare different chemical reactions, different quantitative characteristics are used, such as:

• length,
• energy,
• polarity,
• order of connections.

Let's look at them in more detail.

Bond length is the equilibrium distance between the nuclei of atoms that are connected by a chemical bond. Usually measured experimentally.

The energy of a chemical bond determines its strength. In this case, energy refers to the force required to break a chemical bond and separate atoms.

The polarity of a chemical bond shows how much electron density is shifted towards one of the atoms. The ability of atoms to shift electron density toward themselves, or in simple terms “to pull the blanket over themselves,” is called electronegativity in chemistry.

The order of a chemical bond (in other words, the multiplicity of a chemical bond) is the number of electron pairs that enter into a chemical bond. The order can be either whole or fractional; the higher it is, the greater the number of electrons that carry out the chemical bond and the more difficult it is to break it.

Chemical bond, video

And finally, an educational video about different types of chemical bonds.

Chemical bond

All interactions leading to the combination of chemical particles (atoms, molecules, ions, etc.) into substances are divided into chemical bonds and intermolecular bonds (intermolecular interactions).

Chemical bonds- bonds directly between atoms. There are ionic, covalent and metallic bonds.

Intermolecular bonds- connections between molecules. These are hydrogen bonds, ion-dipole bonds (due to the formation of this bond, for example, the formation of a hydration shell of ions occurs), dipole-dipole (due to the formation of this bond, molecules of polar substances are combined, for example, in liquid acetone), etc.

Ionic bond- a chemical bond formed due to the electrostatic attraction of oppositely charged ions. In binary compounds (compounds of two elements), it is formed when the sizes of the bonded atoms are very different from each other: some atoms are large, others are small - that is, some atoms easily give up electrons, while others tend to accept them (usually these are atoms of the elements that form typical metals and atoms of elements forming typical nonmetals); the electronegativity of such atoms is also very different.
Ionic bonding is non-directional and non-saturable.

Covalent bond- a chemical bond that occurs due to the formation of a common pair of electrons. A covalent bond is formed between small atoms with the same or similar radii. A necessary condition is the presence of unpaired electrons in both bonded atoms (exchange mechanism) or a lone pair in one atom and a free orbital in the other (donor-acceptor mechanism):

Based on the number of shared electron pairs, covalent bonds are divided into
• simple (single)- one pair of electrons,
• double- two pairs of electrons,
• triples- three pairs of electrons.

Double and triple bonds are called multiple bonds.

According to the distribution of electron density between the bonded atoms, a covalent bond is divided into non-polar And polar. A non-polar bond is formed between identical atoms, a polar one - between different ones.

Electronegativity- a measure of the ability of an atom in a substance to attract common electron pairs.
The electron pairs of polar bonds are shifted towards more electronegative elements. The displacement of electron pairs itself is called bond polarization. The partial (excess) charges formed during polarization are designated + and -, for example: .

Based on the nature of the overlap of electron clouds ("orbitals"), a covalent bond is divided into -bond and -bond.
-A bond is formed due to the direct overlap of electron clouds (along the straight line connecting the atomic nuclei), -a bond is formed due to lateral overlap (on both sides of the plane in which the atomic nuclei lie).

A covalent bond is directional and saturable, as well as polarizable.
The hybridization model is used to explain and predict the mutual direction of covalent bonds.

Hybridization of atomic orbitals and electron clouds- the supposed alignment of atomic orbitals in energy, and electron clouds in shape when an atom forms covalent bonds.
The three most common types of hybridization are: sp-, sp 2 and sp 3 -hybridization. For example:
sp-hybridization - in molecules C 2 H 2, BeH 2, CO 2 (linear structure);
sp 2-hybridization - in molecules C 2 H 4, C 6 H 6, BF 3 (flat triangular shape);
sp 3-hybridization - in molecules CCl 4, SiH 4, CH 4 (tetrahedral form); NH 3 (pyramidal shape); H 2 O (angular shape).

Metal connection- a chemical bond formed by sharing the valence electrons of all bonded atoms of a metal crystal. As a result, a single electron cloud of the crystal is formed, which easily moves under the influence of electrical voltage - hence the high electrical conductivity of metals.
A metallic bond is formed when the atoms being bonded are large and therefore tend to give up electrons. Simple substances with a metallic bond are metals (Na, Ba, Al, Cu, Au, etc.), complex substances are intermetallic compounds (AlCr 2, Ca 2 Cu, Cu 5 Zn 8, etc.).
The metal bond does not have directionality or saturation. It is also preserved in metal melts.

Hydrogen bond- an intermolecular bond formed due to the partial acceptance of a pair of electrons from a highly electronegative atom by a hydrogen atom with a large positive partial charge. It is formed in cases where one molecule contains an atom with a lone pair of electrons and high electronegativity (F, O, N), and the other contains a hydrogen atom bound by a highly polar bond to one of such atoms. Examples of intermolecular hydrogen bonds:

H—O—H OH 2 , H—O—H NH 3 , H—O—H F—H, H—F H—F.

Intramolecular hydrogen bonds exist in the molecules of polypeptides, nucleic acids, proteins, etc.

A measure of the strength of any bond is the bond energy.
Communication energy- the energy required to break a given chemical bond in 1 mole of a substance. The unit of measurement is 1 kJ/mol.

The energies of ionic and covalent bonds are of the same order of magnitude, the energy of hydrogen bonds is an order of magnitude less.

The energy of a covalent bond depends on the size of the bonded atoms (bond length) and on the multiplicity of the bond. The smaller the atoms and the greater the bond multiplicity, the greater its energy.

The ionic bond energy depends on the size of the ions and their charges. The smaller the ions and the greater their charge, the greater the binding energy.

Structure of matter

According to the type of structure, all substances are divided into molecular And non-molecular. Among organic substances, molecular substances predominate, among inorganic substances, non-molecular substances predominate.

Based on the type of chemical bond, substances are divided into substances with covalent bonds, substances with ionic bonds (ionic substances) and substances with metallic bonds (metals).

Substances with covalent bonds can be molecular or non-molecular. This significantly affects their physical properties.

Molecular substances consist of molecules connected to each other by weak intermolecular bonds, these include: H 2, O 2, N 2, Cl 2, Br 2, S 8, P 4 and other simple substances; CO 2, SO 2, N 2 O 5, H 2 O, HCl, HF, NH 3, CH 4, C 2 H 5 OH, organic polymers and many other substances. These substances do not have high strength, have low melting and boiling points, do not conduct electricity, and some of them are soluble in water or other solvents.

Non-molecular substances with covalent bonds or atomic substances (diamond, graphite, Si, SiO 2, SiC and others) form very strong crystals (with the exception of layered graphite), they are insoluble in water and other solvents, have high melting and boiling points, most of them they do not conduct electric current (except for graphite, which is electrically conductive, and semiconductors - silicon, germanium, etc.)

All ionic substances are naturally non-molecular. These are solid, refractory substances, solutions and melts of which conduct electric current. Many of them are soluble in water. It should be noted that in ionic substances, the crystals of which consist of complex ions, there are also covalent bonds, for example: (Na +) 2 (SO 4 2-), (K +) 3 (PO 4 3-), (NH 4 + )(NO 3-), etc. The atoms that make up complex ions are connected by covalent bonds.

Metals (substances with metallic bonds) very diverse in their physical properties. Among them there are liquid (Hg), very soft (Na, K) and very hard metals (W, Nb).

The characteristic physical properties of metals are their high electrical conductivity (unlike semiconductors, it decreases with increasing temperature), high heat capacity and ductility (for pure metals).

In the solid state, almost all substances are composed of crystals. Based on the type of structure and type of chemical bond, crystals (“crystal lattices”) are divided into atomic(crystals of non-molecular substances with covalent bonds), ionic(crystals of ionic substances), molecular(crystals of molecular substances with covalent bonds) and metal(crystals of substances with a metallic bond).

Tasks and tests on the topic "Topic 10. "Chemical bonding. Structure of matter."

• Types of chemical bond - Structure of matter grade 8–9

  Lessons: 2 Assignments: 9 Tests: 1

• Assignments: 9 Tests: 1

After working through this topic, you should understand the following concepts: chemical bond, intermolecular bond, ionic bond, covalent bond, metallic bond, hydrogen bond, simple bond, double bond, triple bond, multiple bonds, non-polar bond, polar bond, electronegativity, bond polarization , - and -bond, hybridization of atomic orbitals, binding energy.

You must know the classification of substances by type of structure, by type of chemical bond, the dependence of the properties of simple and complex substances on the type of chemical bond and the type of “crystal lattice”.

You must be able to: determine the type of chemical bond in a substance, the type of hybridization, draw up diagrams of bond formation, use the concept of electronegativity, a number of electronegativity; know how electronegativity changes in chemical elements of the same period and one group to determine the polarity of a covalent bond.

After making sure that everything you need has been learned, proceed to completing the tasks. We wish you success.

• O. S. Gabrielyan, G. G. Lysova. Chemistry 11th grade. M., Bustard, 2002.
• G. E. Rudzitis, F. G. Feldman. Chemistry 11th grade. M., Education, 2001.

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

This is why some electrons try to accept, while others try to give away. By replenishing each other in this way, the elements form various chemical molecules. But what keeps them together? Why do there exist substances of such strength that even the most serious instruments cannot be destroyed? Others, on the contrary, are destroyed by the slightest impact. All this is explained by the formation of various types of chemical bonds between atoms in molecules, the formation of a crystal lattice of a certain structure.

Types of chemical bonds in compounds

In total, there are 4 main types of chemical bonds.

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

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

Metals and their physical properties

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

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

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

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

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

Metal type chemical bond

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

All metals have the following characteristics:

• a small number of electrons per (except for some exceptions, which may have 6,7 and 8);
• large atomic radius;
• low ionization energy.

All this contributes to the easy separation of outer unpaired electrons from the nucleus. At the same time, the atom has a lot of free orbitals. The diagram of the formation of a metallic bond will precisely show the overlap of numerous orbital cells of different atoms with each other, which as a result form a common intracrystalline space. Electrons are fed into it from each atom, which begin to wander freely through different parts of the lattice. Periodically, each of them attaches to an ion at a site in the crystal and turns it into an atom, then detaches again to form an ion.

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

How exactly does a metal chemical bond realize itself? Various examples can be given. Let's try to look at it on a piece of lithium. Even if you take it the size of a pea, there will be thousands of atoms. So let’s imagine that each of these thousands of atoms gives up its single valence electron to the common crystalline space. At the same time, knowing the electronic structure of a given element, you can see the number of empty orbitals. Lithium will have 3 of them (p-orbitals of the second energy level). Three for each atom out of tens of thousands - this is the common space inside the crystal in which the “electron gas” moves freely.

A substance with a metal bond is always strong. After all, electron gas does not allow the crystal to collapse, but only displaces the layers and immediately restores them. It shines, has a certain density (usually high), fusibility, malleability and plasticity.

Where else is metal bonding sold? Examples of substances:

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

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

Metal bond: mechanism of formation

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

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

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

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

Types of metal crystals

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

1. Body-centric cubic lattice - alkali metals.
2. Hexagonal compact structure - all alkaline earths except barium.
3. Facet-centric cubic - aluminum, copper, zinc, many transition metals.
4. Mercury has a rhombohedral structure.
5. Tetragonal - indium.

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

Communication characteristics: non-directional

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

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

Saturability

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

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

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

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

Examples of bond formation in metals

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

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

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

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

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

One of the most active metals that reacts even with ordinary water with fire is potassium. What explains these properties? Again, in many ways - by a metal type of connection. This element has only 19 electrons, but they are located at 4 energy levels. That is, in 30 orbitals of different sublevels. Electronic structure: 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1 3d 0 4p 0 4d 0 4f 0 . Only two with very low ionization energy. They break away freely and go into the common electronic space. There are 22 orbitals for movement per atom, that is, a very large free space for “electron gas”.

Similarities and differences with other types of connections

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

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

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

Types of bonds in metals of different states of aggregation

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

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