Like the components of a constructor, atoms are connected to each other. And no matter how hard you try, you can connect only one block with a single block. Part for 4 cells, can hold no more than four. This principle holds true in chemistry. The valence of the atoms of the elements is responsible for the number of free cells.

The result of the interaction of atoms is the production of substances. The types of chemical bonding of atoms depend on the nature of the constituent elements.

Metals are distinguished by a small number of electrons at the external level compared to non-metals with a lower value of electronegativity. Now our task is to remember how the change in EO occurs in the periodic table or use the table "Relative electronegativity". The more active the non-metal, the higher it is, and this indicates that this element, when a bond is formed, will take electrons.

There are millions of things. These can be simple substances: metals iron Fe, gold Au, mercury Hg; non-metals sulfur S, phosphorus P, nitrogen N 2. So are complex substances: H 2 S, Ca 3 (PO 4) 2, (C 6 H 10 O 5) n, protein molecules, etc. The combination of elements that make up substances determines what types of bonds will exist between them .

covalent bond

Of all the elements, non-metals are in the minority. But having some features in the structure and the ability to have a variable valence, the number of compounds built by these elements is impressive.

To have an idea of ​​how atoms are connected, let's start with a hydrogen molecule H 2 .

Let's give free rein to fantasy, imagine what cannot be seen. Let's say that we picked up two identical parts that look like this:

There is only one combination of their connection, and between them there will be one common link. Let's move from our imagination to molecules. Imagine that we have two hydrogen atoms in front of us and our task is to combine them into a molecule. Twist the details mentally so that they come together, you need to put them on top of each other, linking them in a certain place. The dots next to it mean how many electrons are located on the outer layer.

Source

Hydrogen atoms, as parts, are connected by one bond, so the valency in this case of each of them will be equal to I. But the oxidation state will be equal to 0, since the substance is formed by an element with the same electronegativity value.

Consider how a molecule of the most common gas on our planet, nitrogen N 2, is formed.

Nitrogen has 3 unpaired electrons. It's like taking two pieces of a view and putting them together.

Thus, nitrogen is trivalent, and the degree

oxidation still remains equal to 0. Due to the common electron pair, nitrogen completes the outer layer 2s 2 2p 6 .

A covalent bond in a molecule consisting of one type of atoms, namely non-metals, is called non-polar.

During the construction of a molecule, the number of electrons tends to be completed. Consider how the O 2 molecule is formed. Each atom lacks 2 electrons and they compensate for this shortage with a common electron pair.

We also pay attention that the oxidation state is 0, because the atoms are equal partners, and their valency is II.

A covalent chemical bond formed by different non-metals is called polar.

Let's take two non-metallic elements Hydrogen and Chlorine. Let us indicate the electronic formulas of the outer layer.

After analyzing the values, E(N)< Э(Cl), приходим к выводу, чтобы принять конфигурацию благородного газа, хлор будет притягивать на себя единственный электрон водорода.

The scheme of a covalent bond formed by different elements is written in this form.

It is so important to note that in this situation Cl and H will not be equal partners, since the total electron density is concentrated at Cl. Hydrogen in an unequal battle, yields 1 electron to chlorine, which has as many as 7 of them. Hydrogen acquires a positive charge, chlorine - a negative one. The valences of H and Cl are equal to I. At that time, the oxidation states will be H + Cl −.

This type of formation of compounds occurs by the exchange mechanism. This means that in order to get a complete configuration, more electronegative ones accept electrons, less - they donate, but at the same time there is a common electron pair.

Non-metals form not only binary compounds, but possibly three or more elements will be included in the composition. For example, a molecule of carbonic acid H 2 CO 3 consists of 3 elements. How do they connect with each other. Electronegativity increases in the series EO (N)<ЭО (С) <ЭО(O). Определим степени окисления каждого элемента. Н + 2 С +4 О −2 3 . Это означает, что кислород будет притягивать на себя электроны углерода и водорода. Схематически это можно записать в следующем виде.

To build a structural formula, we write carbon in the center. It has 4 unpaired electrons. Since there are 3 oxygen atoms, each of them can accept 2 electrons. Then, by not tricky calculations, we see that 4 electrons will come from C and one from each N. We check our calculation, taking into account the neutrality of the molecule, we consider positive and negative charges.

H 2 + C +4 O 3 −2 (+1 ∙ 2) + (+4 ∙ 1) + (-2 ∙ 3) = 0

There is another mechanism of covalent bonding called donor-acceptor.

To understand this principle, let us describe the formation of a molecule that has a not entirely pleasant pungent, suffocating smell, ammonia NH 3 .

Of the 5 electrons at the disposal of the N atom, only 3 are bound. The valence of the N atom acquires the value III. At the same time, the oxidation state N −3 (having drawn 3 electrons from each H atom, becomes negative), hydrogen, on the contrary, having performed a “noble deed”, giving up an electron, acquires a positive charge H +. Two electrons are not involved in any way, they are highlighted in red. They are able to settle in a free cell of the H + ion. This place will be occupied by nitrogen electrons, which are indicated in red. The ammonium cation is formed by the donor-acceptor mechanism.

Previously unused "red" electrons N are "populated" in the empty s-orbital belonging to the hydrogen cation. The ammonium ion has 3 bonds that occur according to the exchange mechanism, as well as one, according to the donor-acceptor mechanism. That is why NH 3 easily interacts with acids and water.

Ionic bond

Ionic chemical bond is a boundary covalent polar. They differ in that for substances in which a covalent bond is localized, the existence of a joint electron pair is characteristic, while for an ionic bond, a complete return of electrons is characteristic. The consequence of recoil is the formation of charged particles - ions.

Calculations will help determine the type of relationship. If the difference between the electronegativity values ​​is greater than 1.7, then the substance is characterized by an ionic bond. If the value is less than 1.7, then the inherent polar bond. Consider two substances NaCl and CaC 2 . Both of them are formed by a metal (Na and Ca) and a non-metal (Cl and C). However, in one case the bond will be ionic, in the second - covalent polar.

The postulate of physics says that opposites attract. Those. positive ions attract negative ones and vice versa.

Let us suppose that it is necessary to obtain a substance from potassium and fluorine atoms. Each atom tends to take on the noble gas configuration. This can be achieved in two ways by donating or accepting electrons, thus forming ions with the desired configuration.

It is much easier for a potassium atom to give 1 electron than to take 7 from fluorine. Taking 1 electron, F has a completed level.

Like potassium, which easily gave up its electron, its cation took on the electronic formula of argon.

Calcium is a divalent metal, so two fluorine atoms are needed for interaction, since it is able to accept only one electron. The scheme for the formation of an ionic bond has the form.

This type of bond is localized in all salts, between the metal and the acid residue. In the above example for carbonic acid, the acid residue will be CO 3 2−, if instead of hydrogen we put sodium atoms, then the bond formation scheme looks like this.

It should be noted that an ionic bond will exist between Na and O, and between C and O a covalent polar one.

metal connection

Metals exist in different colors: black (iron), red (copper), yellow (gold), gray (silver), melt at different temperatures. However, all of them are united by the presence of brilliance, hardness, and electrical conductivity.

The metallic bond has similarities with the covalent non-polar. Metals are poor in electrons at the external level, therefore, when a bond is formed, they are not able to attract them to themselves, they are characterized by bestowal. Since the atomic radius in metals is large, this makes it easy for electrons to break off, forming cations.

Me 0 - ne = Me n+

Electrons are constantly moving from atom to ion and vice versa. The cations themselves can be compared to icebergs surrounded by negative particles.

Diagram of a metallic bond

hydrogen bond

Elements-non-metals of the II period (N, O, F) have a high value of electronegativity. This affects the ability to form a hydrogen bond between the polarized H + of one molecule and the anion N 3-, O -2, F -. A hydrogen bond can bring two different molecules together. For example, if we take two water molecules, they are connected to each other due to the H and O atoms.

The hydrogen chemical bond is depicted ... ... by a dotted line. Connecting with each other, molecules play and find an important role in living organisms. Hydrogen bonding builds the secondary structure of the DNA molecule.

Types of crystal lattices

In order to obtain a substance, and not just a set of molecules, it is necessary to “pack” the particles into a kind of framework - a crystal lattice.

Imagine a geometric figure in front of you - a cube, at the vertices there will be particles conditionally connected to each other.

There is a direct relationship between the structure of the atom and the type of crystal lattice.

Please note that compounds with a covalent non-polar bond are formed by molecular particles that are packed into a molecular crystal lattice. Most often these will be low-boiling and volatile compounds according to the temperature regime. These are substances known to you as oxygen O 2, chlorine Cl 2, bromine Br 2.

A covalent polar chemical bond is also characteristic of molecular compounds. This includes both organic: sucrose, alcohols, methane and inorganic compounds: acids, ammonia, non-metal oxides. Their existence happens both in liquid (H 2 O), solid (sulfur) and gaseous form (CO 2).

At the nodes of the atomic crystal lattice there are individual atoms, between which there is a covalent non-polar bond. The atomic crystal lattice is characteristic of diamond. At the moment it is the hardest substance. This type of bond is typical for a substance that covers a significant part of our planet, it is -SiO 2 (sand) and carborundum SiC, which has similar properties to diamond.

The ionic bond between atoms forms a crystal lattice, at the nodes of which there will be cations and anions. This structure combines a whole class of inorganic compounds of salts, consisting of metal cations and anions of the acid residue. Characteristic features of these substances will be high temperatures at which they melt and boil.

The metallic bond has a metallic crystal lattice. In its structure, one can draw a parallel with the ionic lattice. Atoms and ions will be placed at the nodes, and between them there will be an electron gas, consisting of electrons migrating from atom to electron.

Summarizing this information, we can draw a conclusion, knowing the composition and structure, we can predict properties and vice versa.

Topics of the USE codifier: Covalent chemical bond, its varieties and mechanisms of formation. Characteristics of a covalent bond (polarity and bond energy). Ionic bond. Metal connection. hydrogen bond

Intramolecular chemical bonds

Let us first consider the bonds that arise between particles within molecules. Such connections are called intramolecular.

chemical bond between atoms of chemical elements has an electrostatic nature and is formed due to interactions of external (valence) electrons, in more or less degree held by positively charged nuclei bonded atoms.

The key concept here is ELECTRONEGNATIVITY. It is she who determines the type of chemical bond between atoms and the properties of this bond.

is the ability of an atom to attract (hold) external(valence) electrons. Electronegativity is determined by the degree of attraction of external electrons to the nucleus and depends mainly on the radius of the atom and the charge of the nucleus.

Electronegativity is difficult to determine unambiguously. L. Pauling compiled a table of relative electronegativity (based on the bond energies of diatomic molecules). The most electronegative element is fluorine with meaning 4 .

It is important to note that in different sources you can find different scales and tables of electronegativity values. This should not be frightened, since the formation of a chemical bond plays a role atoms, and it is approximately the same in any system.

If one of the atoms in the chemical bond A:B attracts electrons more strongly, then the electron pair is shifted towards it. The more electronegativity difference atoms, the more the electron pair is displaced.

If the electronegativity values ​​of the interacting atoms are equal or approximately equal: EO(A)≈EO(V), then the shared electron pair is not displaced to any of the atoms: A: B. Such a connection is called covalent non-polar.

If the electronegativity of the interacting atoms differ, but not much (the difference in electronegativity is approximately from 0.4 to 2: 0,4<ΔЭО<2 ), then the electron pair is shifted to one of the atoms. Such a connection is called covalent polar .

If the electronegativity of the interacting atoms differ significantly (the difference in electronegativity is greater than 2: ΔEO>2), then one of the electrons almost completely passes to another atom, with the formation ions. Such a connection is called ionic.

The main types of chemical bonds are − covalent, ionic and metallic connections. Let's consider them in more detail.

covalent chemical bond

covalent bond – it's a chemical bond formed by formation of a common electron pair A:B . In this case, two atoms overlap atomic orbitals. A covalent bond is formed by the interaction of atoms with a small difference in electronegativity (as a rule, between two non-metals) or atoms of one element.

Basic properties of covalent bonds

• orientation,
• saturability,
• polarity,
• polarizability.

These bond properties affect the chemical and physical properties of substances.

Direction of communication characterizes the chemical structure and form of substances. The angles between two bonds are called bond angles. For example, in a water molecule, the H-O-H bond angle is 104.45 o, so the water molecule is polar, and in the methane molecule, the H-C-H bond angle is 108 o 28 ′.

Saturability is the ability of atoms to form a limited number of covalent chemical bonds. The number of bonds that an atom can form is called.

Polarity bonds arise due to the uneven distribution of electron density between two atoms with different electronegativity. Covalent bonds are divided into polar and non-polar.

Polarizability connections are the ability of bond electrons to be displaced by an external electric field(in particular, the electric field of another particle). The polarizability depends on the electron mobility. The farther the electron is from the nucleus, the more mobile it is, and, accordingly, the molecule is more polarizable.

Covalent non-polar chemical bond

There are 2 types of covalent bonding - POLAR and NON-POLAR .

Example . Consider the structure of the hydrogen molecule H 2 . Each hydrogen atom carries 1 unpaired electron in its outer energy level. To display an atom, we use the Lewis structure - this is a diagram of the structure of the external energy level of an atom, when electrons are denoted by dots. Lewis point structure models are a good help when working with elements of the second period.

H. + . H=H:H

Thus, the hydrogen molecule has one common electron pair and one H–H chemical bond. This electron pair is not displaced to any of the hydrogen atoms, because the electronegativity of hydrogen atoms is the same. Such a connection is called covalent non-polar .

Covalent non-polar (symmetrical) bond - this is a covalent bond formed by atoms with equal electronegativity (as a rule, the same non-metals) and, therefore, with a uniform distribution of electron density between the nuclei of atoms.

The dipole moment of nonpolar bonds is 0.

Examples: H 2 (H-H), O 2 (O=O), S 8 .

Covalent polar chemical bond

covalent polar bond is a covalent bond that occurs between atoms with different electronegativity (usually, different non-metals) and is characterized displacement common electron pair to a more electronegative atom (polarization).

The electron density is shifted to a more electronegative atom - therefore, a partial negative charge (δ-) arises on it, and a partial positive charge arises on a less electronegative atom (δ+, delta +).

The greater the difference in the electronegativity of atoms, the higher polarity connections and even more dipole moment . Between neighboring molecules and charges opposite in sign, additional attractive forces act, which increases strength connections.

Bond polarity affects the physical and chemical properties of compounds. The reaction mechanisms and even the reactivity of neighboring bonds depend on the polarity of the bond. The polarity of a bond often determines polarity of the molecule and thus directly affects such physical properties as boiling point and melting point, solubility in polar solvents.

Examples: HCl, CO 2 , NH 3 .

Mechanisms for the formation of a covalent bond

A covalent chemical bond can occur by 2 mechanisms:

1. exchange mechanism the formation of a covalent chemical bond is when each particle provides one unpaired electron for the formation of a common electron pair:

BUT . + . B= A:B

2. The formation of a covalent bond is such a mechanism in which one of the particles provides an unshared electron pair, and the other particle provides a vacant orbital for this electron pair:

BUT: + B= A:B

In this case, one of the atoms provides an unshared electron pair ( donor), and the other atom provides a vacant orbital for this pair ( acceptor). As a result of the formation of a bond, both electron energy decreases, i.e. this is beneficial for the atoms.

A covalent bond formed by the donor-acceptor mechanism, is not different by properties from other covalent bonds formed by the exchange mechanism. The formation of a covalent bond by the donor-acceptor mechanism is typical for atoms either with a large number of electrons in the external energy level (electron donors), or vice versa, with a very small number of electrons (electron acceptors). The valence possibilities of atoms are considered in more detail in the corresponding.

A covalent bond is formed by the donor-acceptor mechanism:

- in a molecule carbon monoxide CO(the bond in the molecule is triple, 2 bonds are formed by the exchange mechanism, one by the donor-acceptor mechanism): C≡O;

- in ammonium ion NH 4 +, in ions organic amines, for example, in the methylammonium ion CH 3 -NH 2 + ;

- in complex compounds, a chemical bond between the central atom and groups of ligands, for example, in sodium tetrahydroxoaluminate Na the bond between aluminum and hydroxide ions;

- in nitric acid and its salts- nitrates: HNO 3 , NaNO 3 , in some other nitrogen compounds;

- in a molecule ozone O 3 .

Main characteristics of a covalent bond

A covalent bond, as a rule, is formed between the atoms of non-metals. The main characteristics of a covalent bond are length, energy, multiplicity and directivity.

Chemical bond multiplicity

Chemical bond multiplicity - this is the number of shared electron pairs between two atoms in a compound. The multiplicity of the bond can be quite easily determined from the value of the atoms that form the molecule.

For example , in the hydrogen molecule H 2 the bond multiplicity is 1, because each hydrogen has only 1 unpaired electron in the outer energy level, therefore, one common electron pair is formed.

In the oxygen molecule O 2, the bond multiplicity is 2, because each atom has 2 unpaired electrons in its outer energy level: O=O.

In the nitrogen molecule N 2, the bond multiplicity is 3, because between each atom there are 3 unpaired electrons in the outer energy level, and the atoms form 3 common electron pairs N≡N.

Covalent bond length

Chemical bond length is the distance between the centers of the nuclei of atoms that form a bond. It is determined by experimental physical methods. The bond length can be estimated approximately, according to the additivity rule, according to which the bond length in the AB molecule is approximately equal to half the sum of the bond lengths in the A 2 and B 2 molecules:

The length of a chemical bond can be roughly estimated along the radii of atoms, forming a bond, or by the multiplicity of communication if the radii of the atoms are not very different.

With an increase in the radii of the atoms forming a bond, the bond length will increase.

For example

With an increase in the multiplicity of bonds between atoms (whose atomic radii do not differ, or differ slightly), the bond length will decrease.

For example . In the series: C–C, C=C, C≡C, the bond length decreases.

Bond energy

A measure of the strength of a chemical bond is the bond energy. Bond energy is determined by the energy required to break the bond and remove the atoms that form this bond to an infinite distance from each other.

The covalent bond is very durable. Its energy ranges from several tens to several hundreds of kJ/mol. The greater the bond energy, the greater the bond strength, and vice versa.

The strength of a chemical bond depends on the bond length, bond polarity, and bond multiplicity. The longer the chemical bond, the easier it is to break, and the lower the bond energy, the lower its strength. The shorter the chemical bond, the stronger it is, and the greater the bond energy.

For example, in the series of compounds HF, HCl, HBr from left to right the strength of the chemical bond decreases, because the length of the bond increases.

Ionic chemical bond

Ionic bond is a chemical bond based on electrostatic attraction of ions.

ions are formed in the process of accepting or giving away electrons by atoms. For example, the atoms of all metals weakly hold the electrons of the outer energy level. Therefore, metal atoms are characterized restorative properties the ability to donate electrons.

Example. The sodium atom contains 1 electron at the 3rd energy level. Easily giving it away, the sodium atom forms a much more stable Na + ion, with the electron configuration of the noble neon gas Ne. The sodium ion contains 11 protons and only 10 electrons, so the total charge of the ion is -10+11 = +1:

+11Na) 2 ) 8 ) 1 - 1e = +11 Na +) 2 ) 8

Example. The chlorine atom has 7 electrons in its outer energy level. To acquire the configuration of a stable inert argon atom Ar, chlorine needs to attach 1 electron. After the attachment of an electron, a stable chlorine ion is formed, consisting of electrons. The total charge of the ion is -1:

+17Cl) 2 ) 8 ) 7 + 1e = +17 Cl — ) 2 ) 8 ) 8

Note:

• The properties of ions are different from the properties of atoms!
• Stable ions can form not only atoms, but also groups of atoms. For example: ammonium ion NH 4 +, sulfate ion SO 4 2-, etc. Chemical bonds formed by such ions are also considered ionic;
• Ionic bonds are usually formed between metals and nonmetals(groups of non-metals);

The resulting ions are attracted due to electrical attraction: Na + Cl -, Na 2 + SO 4 2-.

Let us visually generalize difference between covalent and ionic bond types:

metal chemical bond

metal connection is the relationship that is formed relatively free electrons between metal ions forming a crystal lattice.

The atoms of metals on the outer energy level usually have one to three electrons. The radii of metal atoms, as a rule, are large - therefore, metal atoms, unlike non-metals, quite easily donate outer electrons, i.e. are strong reducing agents

Intermolecular interactions

Separately, it is worth considering the interactions that occur between individual molecules in a substance - intermolecular interactions . Intermolecular interactions are a type of interaction between neutral atoms in which new covalent bonds do not appear. The forces of interaction between molecules were discovered by van der Waals in 1869 and named after him. Van dar Waals forces. Van der Waals forces are divided into orientation, induction and dispersion . The energy of intermolecular interactions is much less than the energy of a chemical bond.

Orientation forces of attraction arise between polar molecules (dipole-dipole interaction). These forces arise between polar molecules. Inductive interactions is the interaction between a polar molecule and a non-polar one. A non-polar molecule is polarized due to the action of a polar one, which gives rise to an additional electrostatic attraction.

A special type of intermolecular interaction is hydrogen bonds. - these are intermolecular (or intramolecular) chemical bonds that arise between molecules in which there are strongly polar covalent bonds - H-F, H-O or H-N. If there are such bonds in the molecule, then between the molecules there will be additional forces of attraction .

Mechanism of Education The hydrogen bond is partly electrostatic and partly donor-acceptor. In this case, an atom of a strongly electronegative element (F, O, N) acts as an electron pair donor, and hydrogen atoms connected to these atoms act as an acceptor. Hydrogen bonds are characterized orientation in space and saturation .

The hydrogen bond can be denoted by dots: H ··· O. The greater the electronegativity of an atom connected to hydrogen, and the smaller its size, the stronger the hydrogen bond. It is primarily characteristic of compounds fluorine with hydrogen , as well as to oxygen with hydrogen , less nitrogen with hydrogen .

Hydrogen bonds occur between the following substances:

— hydrogen fluoride HF(gas, solution of hydrogen fluoride in water - hydrofluoric acid), water H 2 O (steam, ice, liquid water):

— solution of ammonia and organic amines- between ammonia and water molecules;

— organic compounds in which O-H or N-H bonds: alcohols, carboxylic acids, amines, amino acids, phenols, aniline and its derivatives, proteins, solutions of carbohydrates - monosaccharides and disaccharides.

The hydrogen bond affects the physical and chemical properties of substances. Thus, the additional attraction between molecules makes it difficult for substances to boil. Substances with hydrogen bonds exhibit an abnormal increase in the boiling point.

For example As a rule, with an increase in molecular weight, an increase in the boiling point of substances is observed. However, in a number of substances H 2 O-H 2 S-H 2 Se-H 2 Te we do not observe a linear change in boiling points.

Namely, at boiling point of water is abnormally high - not less than -61 o C, as the straight line shows us, but much more, +100 o C. This anomaly is explained by the presence of hydrogen bonds between water molecules. Therefore, under normal conditions (0-20 o C), water is liquid by phase state.

chemical bond

There are no single atoms in nature. All of them are in the composition of simple and complex compounds, where their combination into molecules is ensured by the formation of chemical bonds with each other.

The formation of chemical bonds between atoms is a natural, spontaneous process, since in this case the energy of the molecular system decreases, i.e. the energy of the molecular system is less than the total energy of the isolated atoms. This is the driving force behind the formation of a chemical bond.

The nature of chemical bonds is electrostatic, because Atoms are a collection of charged particles, between which the forces of attraction and repulsion act, which come into equilibrium.

Unpaired electrons located in outer atomic orbitals (or ready-made electron pairs) - valence electrons - participate in the formation of bonds. They say that when bonds are formed, electron clouds overlap, resulting in an area between the nuclei of atoms where the probability of finding electrons of both atoms is maximum.

chemical bond - this is the interaction of atoms, carried out by the exchange of electrons.

When a chemical bond is formed, atoms tend to acquire a stable eight-electron (or two-electron - H, He) outer shell, corresponding to the structure of the nearest inert gas atom, i.e. complete your outer level.

Classification of chemical bonds.

1. According to the mechanism of chemical bond formation.

a) exchange when both atoms that form a bond provide unpaired electrons for it.

For example, the formation of hydrogen molecules H 2 and chlorine Cl 2:

b) donor-acceptor , when one of the atoms provides a ready pair of electrons (donor) to form a bond, and the second atom provides an empty free orbital.

For example, the formation of an ammonium ion (NH 4) + (charged particle):

2. According to the way the electron orbitals overlap.

a) σ - connection (sigma), when the overlap maximum lies on the line connecting the centers of atoms.

For example,

H 2 σ (s-s)

Cl 2 σ(p-p)

HClσ(s-p)

b) π - connections (pi), if the overlap maximum does not lie on the line connecting the centers of atoms.

3. According to the method of achieving the completed electron shell.

Each atom tends to complete its outer electron shell, and there can be several ways to achieve such a state.

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

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

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

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

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

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

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

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

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

Depending on the degree of socialization of electrons, covalent, ionic and metallic bonds can be distinguished.

covalent bond

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

Examples of substances with a covalent polar bond:

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

A covalent non-polar bond is formed between non-metal atoms of the same chemical element. Since the atoms are identical, their ability to pull shared electrons is the same. In this regard, no displacement of the electron pair is observed:

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

There is also a donor-acceptor mechanism.

When a covalent bond is formed by the donor-acceptor mechanism, a common electron pair is formed due to the filled orbital of one atom (with two electrons) and the empty orbital of another atom. An atom that provides an unshared electron pair is called a donor, and an atom with a free orbital is called an acceptor. The donors of electron pairs are atoms that have paired electrons, for example, N, O, P, S.

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

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

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

HI< HBr < HCl < HF

Also, the bond energy depends on its multiplicity - the greater the bond multiplicity, the greater its energy. The bond multiplicity is the number of common electron pairs between two atoms.

Ionic bond

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

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

The formation of this type of bond is characteristic of the interaction of atoms of typical metals and typical nonmetals.

For example, potassium fluoride. A potassium cation is obtained as a result of the detachment of one electron from a neutral atom, and a fluorine ion is formed by attaching one electron to a fluorine atom:

Between the resulting ions, a force of electrostatic attraction arises, as a result of which an ionic compound is formed.

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

It has been established that electrons do not completely detach from the metal atom, but only shift towards the chlorine atom, as in a covalent bond.

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

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

metal connection

This type of bond is formed in metals.

The atoms of all metals have electrons on the outer electron layer that have a low binding energy with the atomic nucleus. For most metals, the loss of external electrons is energetically favorable.

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

M 0 - ne - \u003d M n +, where M 0 is a neutral metal atom, and M n + cation of the same metal. The figure below shows an illustration of the ongoing processes.

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

hydrogen bond

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

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

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