It is extremely rare for chemical substances to consist of individual, unrelated atoms of chemical elements. Such a structure in normal conditions possesses only a small number of gases called noble: 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.

Thus, in particular, if an XY molecule is formed during the interaction of X and Y atoms, this means that internal energy 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 atoms of non-metals of the same chemical element. Since the atoms are identical, so is their ability to draw shared electrons. 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 formation of the fourth covalent N-H bonds 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 any substance is bonded to an element with a high electronegativity (nitrogen, oxygen, or fluorine), such a substance is characterized by such a phenomenon as a hydrogen bond.

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 anomalous heat melting water. In addition to water, strong hydrogen bonds are also formed in substances such as hydrogen fluoride, ammonia, oxygen-containing acids, phenols, alcohols, amines.

And a two-electron three-center bond.

Taking into account the statistical interpretation of the wave function by M. Born, the probability density of finding binding electrons is concentrated in the space between the nuclei of the molecule (Fig. 1). In the theory of repulsion of electron pairs, the geometric dimensions of these pairs are considered. So, for the elements of each period, there is a certain average radius of the electron pair (Å):

0.6 for elements up to neon; 0.75 for elements up to argon; 0.75 for elements up to krypton and 0.8 for elements up to xenon.

Characteristic properties of a covalent bond

The characteristic properties of a covalent bond - directionality, saturation, polarity, polarizability - determine the chemical and physical properties connections.

• The direction of the bond is due to the molecular structure of the substance and the geometric shape of their molecule.

The angles between two bonds are called bond angles.

• Saturation - the ability of atoms to form a limited number of covalent bonds. The number of bonds formed by an atom is limited by the number of its outer atomic orbitals.

• The polarity of the bond is due to the uneven distribution of the electron density due to differences in the electronegativity of the atoms.

On this basis, covalent bonds are divided into non-polar and polar (non-polar - a diatomic molecule consists of identical atoms (H 2, Cl 2, N 2) and the electron clouds of each atom are distributed symmetrically with respect to these atoms; polar - a diatomic molecule consists of atoms of different chemical elements, and the common electron cloud shifts towards one of the atoms, thereby forming an asymmetry in the distribution of electric charge in the molecule, generating a dipole moment of the molecules s).

• The polarizability of a bond is expressed in the displacement of bond electrons under the influence of an external electric field, including that of another reacting particle. Polarizability is determined by electron mobility. The polarity and polarizability of covalent bonds determine the reactivity of molecules with respect to polar reagents.

However, twice winner Nobel Prize L. Pauling pointed out that "in some molecules there are covalent bonds due to one or three electrons instead of a common pair." A one-electron chemical bond is realized in the molecular hydrogen ion H 2 + .

The molecular hydrogen ion H 2 + contains two protons and one electron. The single electron of the molecular system compensates for the electrostatic repulsion of two protons and keeps them at a distance of 1.06 Å (the length of the chemical bond H 2 +). The center of the electron density of the electron cloud of the molecular system is equidistant from both protons by the Bohr radius α 0 =0.53 A and is the center of symmetry of the molecular hydrogen ion H 2 + .

History of the term

The term "covalent bond" was first introduced by Nobel Prize winner Irving Langmuir in 1919. The term referred to a chemical bond, due to the shared possession of electrons, as opposed to a metallic bond, in which the electrons were free, or from an ionic bond, in which one of the atoms donated an electron and became a cation, and the other atom accepted an electron and became an anion.

Communication education

A covalent bond is formed by a pair of electrons shared between two atoms, and these electrons must occupy two stable orbitals, one from each atom.

A + B → A: B

As a result of socialization, electrons form a filled energy level. A bond is formed if their total energy at this level is less than in the initial state (and the difference in energy will be nothing more than the bond energy).

According to the theory of molecular orbitals, the overlap of two atomic orbitals leads in the simplest case to the formation of two molecular orbitals (MOs): binding MO And antibonding (loosening) MO. Shared electrons are located on a lower energy binding MO.

Formation of a bond during the recombination of atoms

However, the mechanism of interatomic interaction for a long time remained unknown. Only in 1930, F. London introduced the concept of dispersion attraction - the interaction between instantaneous and induced (induced) dipoles. At present, the attractive forces due to the interaction between fluctuating electric dipoles of atoms and molecules are called "London forces".

The energy of such an interaction is directly proportional to the square of the electronic polarizability α and inversely proportional to the distance between two atoms or molecules to the sixth power.

Bond formation by the donor-acceptor mechanism

In addition to the homogeneous mechanism for the formation of a covalent bond described in the previous section, there is a heterogeneous mechanism - the interaction of oppositely charged ions - the proton H + and the negative hydrogen ion H -, called the hydride ion:

H + + H - → H 2

When the ions approach, the two-electron cloud (electron pair) of the hydride ion is attracted to the proton and eventually becomes common to both hydrogen nuclei, that is, it turns into a binding electron pair. The particle that supplies an electron pair is called a donor, and the particle that accepts this electron pair is called an acceptor. Such a mechanism for the formation of a covalent bond is called donor-acceptor.

H + + H 2 O → H 3 O +

The proton attacks the lone electron pair of the water molecule and forms a stable cation that exists in aqueous solutions acids.

Similarly, a proton is attached to an ammonia molecule with the formation of a complex ammonium cation:

NH 3 + H + → NH 4 +

In this way (according to the donor-acceptor mechanism for the formation of a covalent bond) one obtains big class onium compounds, which includes ammonium, oxonium, phosphonium, sulfonium and other compounds.

A hydrogen molecule can act as an electron pair donor, which, upon contact with a proton, leads to the formation of a molecular hydrogen ion H 3 +:

H 2 + H + → H 3 +

The binding electron pair of the molecular hydrogen ion H 3 + belongs simultaneously to three protons.

Types of covalent bond

There are three types of covalent chemical bonds that differ in the mechanism of formation:

1. Simple covalent bond. For its formation, each of the atoms provides one unpaired electron. When a simple covalent bond is formed, the formal charges of the atoms remain unchanged.

• If the atoms that form a simple covalent bond are the same, then the true charges of the atoms in the molecule are also the same, since the atoms that form the bond equally own a shared electron pair. Such a connection is called non-polar covalent bond. Simple substances have such a bond, for example: 2, 2, 2. But not only non-metals of the same type can form a covalent non-polar bond. A covalent non-polar bond can also be formed by non-metal elements, the electronegativity of which has equal value, for example, in a PH 3 molecule, the bond is covalent non-polar, since the EO of hydrogen is equal to the EO of phosphorus.

• If the atoms are different, then the degree of ownership of a socialized pair of electrons is determined by the difference in the electronegativity of the atoms. An atom with greater electronegativity attracts a pair of bond electrons to itself more strongly, and its true charge becomes negative. An atom with less electronegativity acquires, respectively, the same positive charge. If a compound is formed between two different non-metals, then such a compound is called polar covalent bond.

In the ethylene molecule C 2 H 4 there is a double bond CH 2 \u003d CH 2, its electronic formula is: H: C:: C: H. The nuclei of all ethylene atoms are located in the same plane. Three electron clouds of each carbon atom form three covalent bonds with other atoms in the same plane (with angles between them of about 120°). The cloud of the fourth valence electron of the carbon atom is located above and below the plane of the molecule. Such electron clouds of both carbon atoms, partially overlapping above and below the plane of the molecule, form a second bond between carbon atoms. The first, stronger covalent bond between carbon atoms is called a σ-bond; the second, weaker covalent bond is called π (\displaystyle \pi )-communication.

covalent bond(atomic bond, homeopolar bond) - a chemical bond formed by the overlap (socialization) of paravalent electron clouds. The electronic clouds (electrons) that provide communication are called common electron pair.

The characteristic properties of a covalent bond - directionality, saturation, polarity, polarizability - determine the chemical and physical properties of compounds.

The direction of the bond is due to the molecular structure of the substance and the geometric shape of their molecule. The angles between two bonds are called bond angles.

Saturation - the ability of atoms to form a limited number of covalent bonds. The number of bonds formed by an atom is limited by the number of its outer atomic orbitals.

The polarity of the bond is due to the uneven distribution of the electron density due to differences in the electronegativity of the atoms. On this basis, covalent bonds are divided into non-polar and polar (non-polar - a diatomic molecule consists of identical atoms (H 2, Cl 2, N 2) and the electron clouds of each atom are distributed symmetrically with respect to these atoms; polar - a diatomic molecule consists of atoms of different chemical elements, and the common electron cloud shifts towards one of the atoms, thereby forming an asymmetry in the distribution of electric charge in the molecule, generating a dipole moment of the molecules s).

The polarizability of a bond is expressed in the displacement of bond electrons under the influence of an external electric field, including that of another reacting particle. Polarizability is determined by the electron mobility. The polarity and polarizability of covalent bonds determine the reactivity of molecules with respect to polar reagents.

Communication education

A covalent bond is formed by a pair of electrons shared between two atoms, and these electrons must occupy two stable orbitals, one from each atom.

A + B → A: B

As a result of socialization, electrons form a filled energy level. A bond is formed if their total energy at this level is less than in the initial state (and the difference in energy will be nothing more than the bond energy).

Electron filling of atomic (at the edges) and molecular (in the center) orbitals in the H 2 molecule. The vertical axis corresponds to the energy level, the electrons are indicated by arrows reflecting their spins.

According to the theory of molecular orbitals, the overlap of two atomic orbitals leads in the simplest case to the formation of two molecular orbitals (MOs): binding MO And antibonding (loosening) MO. Shared electrons are located on a lower energy binding MO.

Types of covalent bond

There are three types of covalent chemical bonds that differ in the mechanism of formation:

1. Simple covalent bond. For its formation, each of the atoms provides one unpaired electron. When a simple covalent bond is formed, the formal charges of the atoms remain unchanged.

If the atoms that form a simple covalent bond are the same, then the true charges of the atoms in the molecule are also the same, since the atoms that form the bond equally own a socialized electron pair. Such a connection is called non-polar covalent bond. Simple substances have such a bond, for example: O 2, N 2, Cl 2. But not only non-metals of the same type can form a covalent non-polar bond. Non-metal elements whose electronegativity is of equal value can also form a covalent non-polar bond, for example, in the PH 3 molecule, the bond is covalent non-polar, since the EO of hydrogen is equal to the EO of phosphorus.

· If the atoms are different, then the degree of possession of a socialized pair of electrons is determined by the difference in the electronegativity of the atoms. An atom with greater electronegativity attracts a pair of bond electrons to itself more strongly, and its true charge becomes negative. An atom with less electronegativity acquires, respectively, the same positive charge. If a compound is formed between two different non-metals, then such a compound is called polar covalent bond.

2. Donor-acceptor bond. To form this type of covalent bond, both electrons provide one of the atoms - donor. The second of the atoms involved in the formation of a bond is called acceptor. In the resulting molecule, the formal charge of the donor increases by one, while the formal charge of the acceptor decreases by one.

3. Semipolar connection. It can be considered as a polar donor-acceptor bond. This type of covalent bond is formed between an atom that has an unshared pair of electrons (nitrogen, phosphorus, sulfur, halogens, etc.) and an atom with two unpaired electrons (oxygen, sulfur). The formation of a semipolar bond proceeds in two stages:

1. Transfer of one electron from an atom with an unshared pair of electrons to an atom with two unpaired electrons. As a result, an atom with an unshared pair of electrons turns into a radical cation (a positively charged particle with an unpaired electron), and an atom with two unpaired electrons into a radical anion (a negatively charged particle with an unpaired electron).

2. Socialization of unpaired electrons (as in the case of a simple covalent bond).

When a semipolar bond is formed, an atom with an unshared pair of electrons increases its formal charge by one, and an atom with two unpaired electrons decreases its formal charge by one.

σ bond and π bond

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

In the ethylene molecule C 2 H 4 there is a double bond CH 2 \u003d CH 2, its electronic formula is: H: C:: C: H. The nuclei of all ethylene atoms are located in the same plane. Three electron clouds of each carbon atom form three covalent bonds with other atoms in the same plane (with angles between them of about 120°). The cloud of the fourth valence electron of the carbon atom is located above and below the plane of the molecule. Such electron clouds of both carbon atoms, partially overlapping above and below the plane of the molecule, form a second bond between carbon atoms. The first, stronger covalent bond between carbon atoms is called a σ-bond; the second, less strong covalent bond is called a bond.

In a linear acetylene molecule

H-S≡S-N (N: S::: S: N)

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

All six carbon atoms of the C 6 H 6 cyclic benzene molecule lie in the same plane. σ-bonds act between carbon atoms in the plane of the ring; the same bonds exist for each carbon atom with hydrogen atoms. Each carbon atom spends three electrons to make these bonds. Clouds of the fourth valence electrons of carbon atoms, having the shape of eights, are located perpendicular to the plane of the benzene molecule. Each such cloud overlaps equally with the electron clouds of neighboring carbon atoms. In the benzene molecule, not three separate -bonds are formed, but a single -electronic system of six electrons, common to all carbon atoms. The bonds between the carbon atoms in the benzene molecule are exactly the same.

Examples of substances with a covalent bond

A simple covalent bond connects atoms in the molecules of simple gases (H 2, Cl 2, etc.) and compounds (H 2 O, NH 3, CH 4, CO 2, HCl, etc.). Compounds with a donor-acceptor bond -ammonium NH 4 +, tetrafluoroborate anion BF 4 - and others. Compounds with a semipolar bond - nitrous oxide N 2 O, O - -PCl 3 +.

Crystals with a covalent bond are dielectrics or semiconductors. Typical examples of atomic crystals (the atoms in which are interconnected by covalent (atomic) bonds are diamond, germanium and silicon.

the only known to man A substance with an example of a covalent bond between a metal and carbon is cyanocobalamin, known as vitamin B12.

Ionic bond- a very strong chemical bond formed between atoms with a large difference (> 1.5 on the Pauling scale) of electronegativity, at which the common electron pair completely passes to an atom with a higher electronegativity. This is the attraction of ions as oppositely charged bodies. An example is the compound CsF, in which the "degree of ionicity" is 97%. Consider the method of formation using the example of sodium chloride NaCl. Electronic configuration sodium and chlorine atoms can be represented: 11 Na 1s2 2s2 2p 6 3s1; 17 Cl 1s2 2s2 2p6 3s2 3p5. These are atoms with incomplete energy levels. Obviously, to complete them, it is easier for a sodium atom to give up one electron than to add seven, and it is easier for a chlorine atom to add one electron than to give up seven. In a chemical interaction, the sodium atom completely gives up one electron, and the chlorine atom accepts it. Schematically, this can be written as: Na. - l e -> Na + sodium ion, stable eight-electron 1s2 2s2 2p6 shell due to the second energy level. :Cl + 1e --> .Cl - chlorine ion, stable eight electron shell. Electrostatic attraction forces arise between the Na+ and Cl- ions, as a result of which a compound is formed. An ionic bond is an extreme case of the polarization of a covalent polar bond. Formed between typical metal and non-metal. In this case, the electrons from the metal completely pass to the non-metal. Ions are formed.

If a chemical bond is formed between atoms that have a very large electronegativity difference (EO > 1.7 according to Pauling), then the shared electron pair is completely transferred to the atom with a larger EO. The result of this is the formation of a compound of oppositely charged ions:

Between the formed ions there is an electrostatic attraction, which is called ionic bonding. Rather, such a view is convenient. In fact, the ionic bond between atoms in its pure form is not realized anywhere or almost nowhere; usually, in fact, the bond is partly ionic and partly covalent. At the same time, the relationship of complex molecular ions can often be considered purely ionic. Key differences ionic bonds from other types of chemical bonds are non-directional and unsaturable. That is why crystals formed due to ionic bonding gravitate towards various close packings of the corresponding ions.

characteristic of such compounds is good solubility in polar solvents (water, acids, etc.). This is due to the charged parts of the molecule. In this case, the dipoles of the solvent are attracted to the charged ends of the molecule, and, as a result, brownian motion, "pull" the molecule of the substance into parts and surround them, preventing them from reconnecting. The result is ions surrounded by dipoles of the solvent.

When such compounds are dissolved, as a rule, energy is released, since the total energy of the formed solvent-ion bonds is greater than the anion-cation bond energy. Exceptions are many salts of nitric acid (nitrates), which, when dissolved, absorb heat (solutions cool). Last fact explained on the basis of the laws that are considered in physical chemistry.

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

It can be concluded that chemistry is a must, to understand it and to know how our world or some of its separate parts works is interesting, and, moreover, useful.

Now we have to deal with such a term as a covalent bond, which, by the way, can be both polar and non-polar. By the way, the very word "covalent" is formed from the Latin "co" - together and "vales" - having force.

Term occurrences

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

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

Examples, substances with a polar:

Types of covalent bond

Two types are also distinguished - these are polar, and, accordingly, non-polar bonds. We will analyze the features of each of them separately.

Covalent polar - education

What is the term "polar"?

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

As a result, at polarity, the electron density shifts to a more electronegative one, and a partial negative charge arises on it. In turn, the nucleus, whose electronegativity is lower, has, accordingly, a partial positive charge.

We conclude: polar arises between various non-metals, which differ in the value of electronegativity, and electrons are located closer to the nucleus with greater electronegativity.

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

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

The formula of a substance with a covalent polar bond

Covalent non-polar, difference between polar and non-polar

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

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

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

Eg, substances with a non-polar covalent bond:

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

Examples of a covalent non-polar bond and a scheme for the formation of a covalent non-polar bond

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

covalent non-polar are always atoms of the same substance. H2. CL2.

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

Rice. 2.1. The formation of molecules from atoms is accompanied by redistribution of electrons of valence orbitals and leads to gain in energy because the energy of molecules is less than the energy of non-interacting atoms. The figure shows a diagram of the formation of a non-polar covalent chemical bond between hydrogen atoms.

§2 chemical bond

Under normal conditions, the molecular state is more stable than the atomic state. (fig.2.1). The formation of molecules from atoms is accompanied by a redistribution of electrons in valence orbitals and leads to a gain in energy, since the energy of molecules is less than the energy of non-interacting atoms(Appendix 3). The forces that hold atoms in molecules have received a generalized name chemical bond.

The chemical bond between atoms is carried out by valence electrons and has an electrical nature . There are four main types of chemical bonding: covalent,ionic,metal And hydrogen.

1 Covalent bond

A chemical bond carried out by electron pairs is called atomic, or covalent. . Compounds with covalent bonds are called atomic, or covalent. .

When a covalent bond occurs, an overlap of electron clouds of interacting atoms occurs, accompanied by energy release (Fig. 2.1). In this case, a cloud with an increased negative charge density arises between positively charged atomic nuclei. Due to the action of the Coulomb forces of attraction between opposite charges, an increase in the negative charge density favors the approach of the nuclei.

A covalent bond is formed by unpaired electrons in the outer shells of atoms . In this case, electrons with opposite spins form electron pair(Fig. 2.2), common to interacting atoms. If one covalent bond has arisen between atoms (one common electron pair), then it is called single, two-double, etc.

Energy is a measure of the strength of a chemical bond. E sv spent on the destruction of the bond (gain in energy during the formation of a compound from individual atoms). Usually this energy is measured per 1 mol substances and are expressed in kilojoules per mol (kJ ∙ mol -1). The energy of a single covalent bond is in the range of 200–2000 kJmol–1.

Rice. 2.2. The covalent bond is the most general form chemical bond arising due to the socialization of an electron pair through the exchange mechanism (A), when each of the interacting atoms supplies one electron, or through the donor-acceptor mechanism (b) when an electron pair is shared by one atom (donor) to another atom (acceptor).

A covalent bond has properties satiety and focus . The saturation of a covalent bond is understood as the ability of atoms to form a limited number of bonds with their neighbors, determined by the number of their unpaired valence electrons. The directionality of a covalent bond reflects the fact that the forces that hold atoms near each other are directed along the straight line connecting the atomic nuclei. Besides, covalent bond can be polar or non-polar .

When non-polar covalent bond electron cloud formed common pair electrons, is distributed in space symmetrically with respect to the nuclei of both atoms. A nonpolar covalent bond is formed between atoms simple substances, for example, between identical atoms of gases that form diatomic molecules (O 2, H 2, N 2, Cl 2, etc.).

When polar covalent bond electron cloud bond is shifted to one of the atoms. The formation of a polar covalent bond between atoms is characteristic of complex substances. Molecules of volatile inorganic compounds can serve as an example: HCl, H 2 O, NH 3, etc.

The degree of displacement of the common electron cloud to one of the atoms during the formation of a covalent bond (degree of polarity of a bond ) determined mainly by the charge atomic nuclei and radius of interacting atoms .

The greater the charge of the atomic nucleus, the stronger it attracts a cloud of electrons. At the same time, the larger the atomic radius, the weaker the outer electrons are held near the atomic nucleus. The cumulative effect of these two factors is expressed in the different ability of different atoms to "pull" the cloud of covalent bonds towards themselves.

The ability of an atom in a molecule to attract electrons to itself is called electronegativity. . Thus, electronegativity characterizes the ability of an atom to polarize a covalent bond: the greater the electronegativity of an atom, the more the electron cloud of a covalent bond is shifted towards it .

A number of methods have been proposed to quantify electronegativity. At the same time, the method proposed by the American chemist Robert S. Mulliken, who determined the electronegativity  an atom as half the sum of its energy E e electron and energy affinities E i atom ionization:

. (2.1)

Ionization energy of an atom is called the energy that needs to be expended in order to “tear off” an electron from it and remove it to an infinite distance. The ionization energy is determined by photoionization of atoms or by bombarding atoms with electrons accelerated in an electric field. That smallest value of the energy of photons or electrons, which becomes sufficient for the ionization of atoms, is called their ionization energy E i. Usually this energy is expressed in electron volts (eV): 1 eV = 1.610 -19 J.

Atoms are the most willing to give away their outer electrons. metals, which contain a small number of unpaired electrons (1, 2 or 3) on the outer shell. These atoms have the lowest ionization energy. Thus, the value of the ionization energy can serve as a measure of the greater or lesser "metallicity" of the element: the lower the ionization energy, the stronger must be expressed metalproperties element.

In the same subgroup of the periodic system of elements of D.I. Mendeleev, with an increase in the ordinal number of an element, its ionization energy decreases (Table 2.1), which is associated with an increase in the atomic radius (Table 1.2), and, consequently, with a weakening of the bond of external electrons with the nucleus. For elements of the same period, the ionization energy increases with increasing serial number. This is due to a decrease in the atomic radius and an increase in the nuclear charge.

Energy E e, which is released when an electron is attached to a free atom, is called electron affinity(expressed also in eV). The release (rather than absorption) of energy when a charged electron is attached to some neutral atoms is explained by the fact that atoms with filled outer shells are the most stable in nature. Therefore, for those atoms in which these shells are “slightly unfilled” (i.e., 1, 2, or 3 electrons are missing before filling), it is energetically beneficial to attach electrons to themselves, turning into negatively charged ions 1 . Such atoms include, for example, halogen atoms (Table 2.1) - elements of the seventh group (main subgroup) of the periodic system of D.I. Mendeleev. The electron affinity of metal atoms is usually zero or negative, i.e. it is energetically unfavorable for them to attach additional electrons, additional energy is required to keep them inside atoms. The electron affinity of non-metal atoms is always positive and the greater, the closer to the noble (inert) gas the non-metal is located in periodic system. This indicates an increase non-metallic properties as we approach the end of the period.

From all that has been said, it is clear that the electronegativity (2.1) of atoms increases in the direction from left to right for elements of each period and decreases in the direction from top to bottom for elements of the same group of the Mendeleev periodic system. It is not difficult, however, to understand that to characterize the degree of polarity of a covalent bond between atoms, it is not the absolute value of the electronegativity that is important, but the ratio of the electronegativity of the atoms forming the bond. That's why in practice, they use the relative values ​​of electronegativity(Table 2.1), taking the electronegativity of lithium as a unit.

To characterize the polarity of a covalent chemical bond, the difference in the relative electronegativity of atoms is used. Usually the bond between atoms A and B is considered purely covalent, if |  A – B|0.5.