How to determine the polarity of a molecule in chemistry. How to determine the polarity of a bond? Forward and reverse polarity

28.08.2020

When a covalent bond is formed between dissimilar atoms, the bonding pair of electrons shifts towards the more electronegative atom. This leads to the polarization of molecules, so all diatomic molecules consisting of dissimilar elements turn out to be polar to some extent. In more complex molecules, the polarity also depends on the geometry of the molecule. For the appearance of polarity, it is necessary that the centers of distribution of positive and negative charges do not coincide.

In the CO 2 molecule, the carbon-oxygen bonds are polar, with a certain positive charge on the carbon atom, and the same negative charge on each of the oxygen atoms. Therefore, the center of positive charge is concentrated on the carbon atom. Since the oxygen atoms are located on the same straight line, but both sides of the carbon atom (linear molecule) are at equal distances, the positive charge is neutralized. Thus, despite the polarity of each bond in CO., the entire molecule as a whole is non-polar and the reason for this is

Rice. 434. Examples of the structure and polarity of a molecule is its linear structure. On the contrary, the S=C=0 molecule is polar, since the carbon-sulfur and carbon-oxygen bonds have different length and different polarity. On fig. 4.34 shows the structures and polarity of some molecules.

It follows from the above examples that if the atoms or groups of atoms attached to the central atom are the same or located symmetrically relative to it (linear, flat triangular, tetrahedral and other structures), then the molecule will be nonpolar. If unequal groups are attached to the central atom or if there is an asymmetric arrangement of groups, then the molecules are polar.

When considering polar bonds, the effective charge of atoms in a molecule is important. For example, in the HC1 molecule, the binding electron cloud is shifted towards the more electronegative chlorine atom, as a result of which the charge of the hydrogen nucleus is not compensated, and the electron density on the chlorine atom becomes excessive compared to the charge of its nucleus. Therefore, the hydrogen atom is positively polarized, and the chlorine atom is negatively polarized. The hydrogen atom has a positive charge, and the chlorine atom has a negative charge. This charge 8, called the effective charge, is usually established experimentally. So, for hydrogen 8 H \u003d +0.18, and for chlorine 5 C, \u003d -0.18 of the absolute charge of the electron, as a result, the bond in the HC1 molecule is 18% ionic (i.e., the degree of ionicity is 0.18 ).

Since the polarity of the bond depends on the degree of displacement of the bonding pair of electrons towards the more electronegative element, the following must be taken into account:

a) electronegativity (EO) - not strict physical quantity, which can be determined directly experimentally;
b) the value of electronegativity is not constant, but depends on the nature of the other atom with which this atom is bonded;
c) the same atom in a given chemical bond can sometimes function both as an electropositive and as an electronegative one.

Experimental evidence suggests that relative electronegativities (RERs) can be assigned to elements, the use of which makes it possible to judge the degree of polarity of the bond between atoms in a molecule (see also paragraphs 3.6 and 4.3).

In a molecule consisting of two atoms, the greater the polarity of the covalent bond, the higher the RER of one of them, therefore, with an increase in the RER of the second element, the degree of ionicity of the compound increases.

To characterize the reactivity of molecules, not only the nature of the electron density distribution is important, but also the possibility of its change under the influence of an external influence. The measure of this change is the polarizability of the bond, i.e. its ability to become polar or even more polar. The polarization of the bond occurs both under the influence of an external electric field, and under the influence of another molecule that is a partner in the reaction. The result of these influences may be the polarization of the bond, accompanied by its complete break. In this case, the binding pair of electrons remains at the more electronegative atom, which leads to the formation of opposite ions. This type of bond breaking is called teterolytic. For instance:

In the above example of an asymmetric bond cleavage, hydrogen is split off in the form of an H + -ion, and the binding pair of electrons remains with chlorine, so the latter is converted into an anion C1.

In addition to this type of bond rupture, a symmetrical bond rupture is also possible, when not ions are formed, but atoms and radicals. This type of bond breaking is called homolytic.

In molecules, the positive charges of the nuclei are compensated by the negative charges of the electrons. However, positive and negative charges can be spatially separated. Let us assume that the molecule consists of atoms of different elements (HC1, CO, etc.). In this case, the electrons are shifted to an atom with a higher electronegativity and the centers of gravity of positive and negative charges do not coincide, forming electric dipole- a system of two equal in magnitude and opposite in sign charges q, located at a distance l called dipole length. The length of the dipole is a vector quantity. Its direction is conditionally taken from the negative charge to the positive one. Such molecules are called polar molecules or dipoles.

The polarity of the molecule is greater, the greater the absolute value of the charge and the length of the dipole. The measure of polarity is the product q . l, called the electric moment of the dipole μ: μ = q. l.

Unit of measurement μ serves as Debye (D). 1 D \u003d 3.3. 10 -30 C. m.

In molecules consisting of two identical atoms, μ = 0. They are called nonpolar. If such a particle enters an electric field, then under the action of the field, polarization- displacement of the centers of gravity of positive and negative charges. An electric dipole moment arises in the particle, called induced dipole.

The dipole moment of a diatomic AB molecule can be identified with the dipole moment of the A-B bond in it. If the common electron pair is shifted to one of the atoms, then the electric moment of the bond dipole is not equal to zero. The relationship in this case is called polar covalent bond. If the electron pair is symmetrically located relative to the atoms, then the bond is called non-polar.

In a polyatomic molecule, a certain electric dipole moment can be assigned to each bond. Then the electric moment of the dipole of the molecule can be represented as the vector sum of the electric moments of the dipole of individual bonds. The existence or absence of a dipole moment in a molecule is related to its symmetry. Molecules that have a symmetrical structure are non-polar (μ = 0). These include diatomic molecules with identical atoms (H 2, C1 2, etc.), a benzene molecule, molecules with polar bonds BF 3, A1F 3, CO 2, BeC1 2, etc.

The electric moment of the dipole of a molecule is an important molecular parameter. Knowing the value of μ can indicate the geometric structure of the molecule. So, for example, the polarity of a water molecule indicates its angular structure, and the absence of a CO 2 dipole moment indicates its linearity.

Ionic bond

The limiting case of a covalent polar bond is an ionic bond. If the electronegativities of atoms differ very much (for example, atoms alkali metals and halogens), then when they approach each other, the valence electrons of one atom completely transfer to the second atom. As a result of this transition, both atoms become ions and take on the electronic structure of the nearest noble gas. For example, when sodium and chlorine atoms interact, they turn into Na + and Cl - ions, between which there is an electrostatic attraction. The ionic bond can be described in terms of the VS and MO methods, but it is usually considered using the classical laws of electrostatics.

Molecules in which there is a pure ionic bond are found in the vapor state of matter. Ionic crystals are made up of endless rows of alternating positive and negative ions bound by electrostatic forces. When ionic crystals are dissolved or melted, positive and negative ions pass into the solution or melt.

It should be noted that ionic bonds are very strong, therefore, to destroy ionic crystals, it is necessary to expend a lot of energy. This explains the fact that ionic compounds have high melting points.

Unlike a covalent bond, an ionic bond does not have the properties of saturation and directionality. The reason for this is that the electric field created by the ions has spherical symmetry and acts equally on all ions. Therefore, the number of ions surrounding a given ion and their spatial arrangement are determined only by the magnitudes of the charges of the ions and their sizes.

Considering ionic bond, it must be borne in mind that during the electrostatic interaction between ions, their deformation occurs, called polarization. On fig. 2.1, a two interacting electrostatically neutral ions are shown, which retain a perfectly spherical shape. On fig. 2.1, b the polarization of ions is shown, which leads to a decrease in the effective distance between the centers of positive and negative charges. The greater the polarization of the ions, the lower the degree of ionicity of the bond, i.e., the greater the covalent nature of the bond between them. In crystals, the polarization turns out to be low, since the ions are symmetrically surrounded by ions of the opposite sign and the ion is subjected to the same action in all directions.

In homonuclear molecules (H 2, F 2, etc.), the electron pair that forms the bond, in equally belongs to each atom, so the centers of positive and negative charges in the molecule coincide. Such molecules are non-polar.

However, in heteronuclear molecules, the contribution of the wave functions of different atoms to the coupling is not the same. Near one of the atoms, an excess electron density appears, therefore, an excess negative charge, and near the other, a positive one. In this case, one speaks of the displacement of an electron pair from one atom to another, but this should not be understood literally, but only as an increase in the probability of finding an electron pair near one of the nuclei of the molecule.

To determine the direction of such a shift and a semiquantitative estimate of its magnitude, the concept of electronegativity is introduced.

There are several scales of electronegativity. However, the elements are arranged in the electronegativity series in the same order, so the differences are insignificant, and the electronegativity scales are quite comparable.

According to R. Mulliken, electronegativity is half the sum of ionization energies and electron affinity (see Section 2.10.3):

The valence electron pair is shifted to a more electronegative atom.

It is more convenient to use not absolute values of electronegativity, but relative ones. The unit is the electronegativity of lithium 3 Li. The relative electronegativity of any element A is:

Heavy alkali metals have the lowest electronegativity. (X Fr = 0.7). The most electronegative element is fluorine (X F = 4.0). By periods, there is a general trend of increasing electronegativity, and by subgroups - its decrease (Table 3.4).

In the practical use of the data in this table (as well as data from other electronegativity scales), it should be borne in mind that in molecules consisting of three or more atoms, the value of electronegativity under the influence of neighboring atoms can change noticeably. Strictly speaking, a constant electronegativity cannot be attributed to an element at all. It depends on the valence state of the element, the type of compound, etc. Nevertheless, this concept is useful for a qualitative explanation of the properties of chemical bonds and compounds.

Table 3.4

Electronegativity of s- and p-elements according to Pauling

Period	Group
Period

The polarity of the bond is determined by the displacement of the valence electron pair in diatomic molecules and is quantitatively characterized dipole moment, or dipole electric moment, molecules. He is equal to the product distances between nuclei G in the molecule and the effective charge 5 corresponding to this distance:

Insofar as G considered to be a vector directed from positive to negative charge, the dipole moment is also a vector and has the same direction. The unit of dipole moment is the debye D (1D = 3.33 10 -30 C m).

The dipole moment of a complex molecule is defined as the vector sum of the dipole moments of all bonds. Therefore, if the AB I molecule is symmetrical with respect to the line of each bond, the total dipole moment of such a molecule, despite the polarity

ness connections A-B, is equal to zero: D = ^ D; = 0. Examples are

live the previously considered symmetrical molecules, the bonds in which are formed by hybrid orbitals: BeF 2, BF 3, CH 4, SF 6, etc.

Molecules in which bonds are formed by non-hybrid orbitals or hybrid orbitals involving lone pairs of electrons are asymmetric with respect to bond lines. The dipole moments of such molecules are not equal to zero. Examples of such polar molecules: H 2 S, NH 3 , H 2 0, etc. In fig. 3.18 shows a graphical interpretation of the summation of polar bond vectors in a symmetric BeF 2 (fl) molecule and an asymmetric H 2 S molecule (b).

Rice. 3.18. Dipole moments of BeF 2 (a) and H 2 S (b) molecules

As already noted, the greater the difference in the electronegativity of the atoms forming the bond, the more the valence electron pair shifts, the more polar the bond and, consequently, the greater the effective charge b, which is illustrated in Table. 3.5.

Table 3.5

Changing the nature of the bond in a series of compounds of elements of the II period with fluorine

In a polar bond, two components can be conditionally distinguished: an ionic one, due to electrostatic attraction, and a covalent one, due to the overlap of orbitals. As the difference in electronegativity increases OH the valence electron pair shifts more and more towards the fluorine atom, which acquires an increasingly negative effective charge. The contribution of the ionic component to the bond increases, while the proportion of the covalent component decreases. Quantitative changes turn into qualitative ones: in the UF molecule, the electron pair almost completely belongs to fluorine, and its effective charge approaches unity, i.e. to the charge of the electron. We can assume that two ions were formed: the Li + cation and the anion F~ and the bond is due only to their electrostatic attraction (the covalent component can be neglected). Such a connection is called ionic. It can be considered as extreme case of a covalent polar bond.

The electrostatic field does not have preferred directions. So ionic bond as opposed to covalent no directionality. An ion interacts with any number of ions of opposite charge. This is due to another distinctive property of the ionic bond - lack of saturation.

For ionic molecules, the binding energy can be calculated. If we consider ions as non-deformable balls with charges ±e, then the force of attraction between them, depending on the distance between the centers of the ions G can be expressed by the Coulomb equation:

The attraction energy is determined by the relation

When approaching, a repulsive force appears due to the interaction electron shells. It is inversely proportional to the distance to the power P:

where V is some constant. Exponent P much more than one and for various configurations of ions lies in the range from 5 to 12. Taking into account that the force is the derivative of energy with respect to distance, from equation (3.6) we obtain:

With change G change F np and F qtt . At some distance g 0 these forces are equalized, which corresponds to the minimum of the resulting interaction energy U Q . After transformation, you can get

This equation is known as the Born equation.

Minimum on the dependence curve U=f(r) correspond to the equilibrium distance r 0 and the energy U Q . This is the binding energy between ions. Even P is unknown, then we can estimate the value of the binding energy by taking 1 /P equal to zero:

The error will not exceed 20%.

For ions with charges z l and z 2 equations (3.7) and (3.8) take the form:

Since the existence of a bond approaching a purely ionic one in molecules of this type is problematic, the last equations should be considered a very rough approximation.

At the same time, the problems of polarity and ionicity of the bond can be approached from the opposite position - from the point of view of ion polarization. It is assumed that there is a complete transfer of electrons, and the molecule consists of isolated ions. Then the electron clouds are displaced under the action of the electric field created by the ions, - polarization ions.

Polarization is a two-pronged process that combines polarizing effect ions from their polarizability. Polarizability is the ability of an electron cloud of an ion, molecule or atom to deform under the action of the electrostatic field of another ion. The strength of this field determines the polarizing effect of the ion. It follows from equation (3.10) that the greater the polarizing effect of an ion, the greater its charge and the smaller its radius. The radii of cations, as a rule, are much smaller than the radii of anions; therefore, in practice, it is more often necessary to encounter polarization of anions under the action of cations, and not vice versa. The polarizability of ions also depends on their charge and radius. Ions of large size and charge are more easily polarized. The polarizing effect of an ion is reduced to pulling the electron cloud of an ion of opposite charge towards itself. As a result, the ionicity of the bond decreases; the bond becomes polar covalent. Thus, the polarization of ions reduces the degree of ionicity of the bond and is opposite to the polarization of the bond in its effect.

Polarization of ions in a molecule, i.e. an increase in the proportion of a covalent bond in it increases the strength of its decay into ions. In a series of compounds of a given cation with anions of the same type, the degree of dissociation in solutions decreases with an increase in the polarizability of the anions. For example, in the series of lead halides PbCl 2 - PbBr 2 - Pb 2, the radius of the halide anions increases, their polarizability increases, and the decay into ions weakens, which is expressed in a decrease in solubility.

When comparing the properties of salts with the same anion and sufficiently large cations, one should take into account the polarization of the cations. For example, the radius of the Hg 2+ ion is greater than the radius of the Ca 2+ ion, so Hg 2+ is more polarized than Ca 2+ . As a result, CaCl 2 is a strong electrolyte; dissociates completely in solution, and HgCl 2 - as a weak electrolyte, i.e. practically does not dissociate in solutions.

The polarization of ions in a molecule reduces its strength during decay into atoms or molecules. For example, in the series CaCl 2 - CaBr 2 - Ca1 2, the radius of halide ions increases, their polarization by the Ca 2+ ion increases, therefore, the temperature of thermal dissociation into calcium and halogen decreases: CaNa1 2 \u003d Ca + Na1 2.

If the ion is easily polarized, then its excitation requires little energy, which corresponds to the absorption of visible light quanta. This is the reason for the color of solutions of such compounds. An increase in polarizability leads to an increase in color, for example, in the series NiCl 2 - NiBr 2 - Nil 2 (increase in the polarizability of the anion) or in the series KC1 - CuCl 2 (increase in the polarizability of the cation).

The boundary between covalent polar and ionic bonds is very conditional. For molecules in the gaseous state, it is believed that with a difference in electronegativity AH > 2.5 bond is ionic. In solutions of polar solvents, as well as in the crystalline state, a strong influence is exerted, respectively, by solvent molecules and neighboring particles at sites crystal lattice. Therefore, the ionic nature of the bond manifests itself at a much smaller difference in electronegativity. In practice, we can assume that the bond between typical metals and nonmetals in solutions and crystals is ionic.

Rice. 32. Schemes of polar and non-polar molecules: a - polar molecule; b-non-polar molecule

In any molecule there are both positively charged particles - the nuclei of atoms, and negatively charged particles - electrons. For each kind of particles (or, rather, charges), one can find a point that will be, as it were, their "electric center of gravity." These points are called the poles of the molecule. If in a molecule the electrical centers of gravity of positive and negative charges coincide, the molecule will be non-polar. Such, for example, are H 2 and N 2 molecules formed by identical atoms, in which common pairs of electrons equally belong to both atoms, as well as many symmetrically constructed molecules with atomic bonds, for example, methane CH 4, CCl 4 tetrachloride.

But if the molecule is built asymmetrically, for example, it consists of two heterogeneous atoms, as we have already said, the common pair of electrons can be more or less shifted towardsone of the atoms. Obviously, in this case, due to the uneven distribution of positive and negative charges inside the molecule, their electrical centers of gravity will not coincide and a polar molecule will be obtained (Fig. 32).

Polar molecules are

Polar molecules are dipoles. This term denotes in general any electrically neutral system, i.e., a system consisting of positive and negative charges distributed in such a way that their electrical centers of gravity do not coincide.

The distance between the electric centers of gravity of those and other charges (between the poles of the dipole) is called the length of the dipole. The length of the dipole characterizes the degree of polarity of the molecule. It is clear that for different polar molecules the length of the dipole is different; the larger it is, the more pronounced the polarity of the molecule.

Rice. 33. Schemes of the structure of CO2 and CS2 molecules

In practice, the degree of polarity of certain molecules is determined by measuring the so-called dipole moment of the molecule m, which is defined as the product of the dipole length l on the charge of its pole e:

t =l e

The values of dipole moments are associated with certain properties of substances and can be determined experimentally. Order of magnitude T always 10 -18, since the electric charge

throne is 4.80 10 -10 electrostatic units, and the length of the dipole is a value of the same order as the diameter of the molecule, i.e. 10 -8 cm. Below are the dipole moments of the molecules of some inorganic substances.

Dipole moments of some substances

T 10 18

. . . .. …….. 0

Water……. 1.85

. . . ………..0

Hydrogen chloride……. 1.04

Carbon dioxide…….0

bromide. …… 0.79

Carbon disulfide…………0

Hydrogen iodide…….. 0.38

Hydrogen sulfide………..1.1

Carbon monoxide……. 0,11

Sulphur dioxide. . . ……1.6

Hydrocyanic acid……..2.1

Determining the values of dipole moments allows us to draw many interesting conclusions regarding the structure of various molecules. Let's look at some of these findings.

Rice. 34. Scheme of the structure of the water molecule

As expected, the dipole moments of hydrogen and nitrogen molecules are zero; molecules of these substancesare symmetrical and, therefore, the electric charges in them are distributed evenly. The absence of polarity in carbon dioxide and carbon disulfide shows that their molecules are also built symmetrically. The structure of the molecules of these substances is schematically shown in Fig. 33.

Somewhat unexpected is the presence of a rather large dipole moment near water. Since the formula for water is similar to the formulas for carbon dioxide

and carbon disulfide, one would expect that its molecules would be built in the same waysymmetrically, like the CS 2 and CO 2 molecules.

However, in view of the experimentally established polarity of water molecules (polarity of molecules), this assumption has to be discarded. At present, an asymmetric structure is attributed to the water molecule (Fig. 34): two hydrogen atoms are connected to an oxygen atom in such a way that their bonds form an angle of about 105 °. Similar arrangement atomic nuclei exists in other molecules of the same type (H 2 S, SO 2) that have dipole moments.

The polarity of water molecules explains many of its physical properties.

The polarity of a molecule must be distinguished from the polarity of a bond. For diatomic molecules of type AB, these concepts coincide, as has already been shown for the example of the HCl molecule. In such molecules the greater the difference in the electronegativity of the elements (∆EO), the greater the electric moment of the dipole. For example, in the series HF, HCl, HBr, HI, it decreases in the same sequence as the relative electronegativity.

Molecules can be polar and non-polar depending on the nature of the electron density distribution of the molecule. The polarity of a molecule is characterized by the value of the electric moment of the dipole μ they say , which is equal to the vector sum of the electric moments of the dipoles of all bonds and non-bonding electron pairs located on hybrid AOs: → →

 m-ly \u003d  ( connections) i +  ( unconnected electric pairs) j .

The result of addition depends on the polarity of the bonds, the geometric structure of the molecule, and the presence of unshared electron pairs. The polarity of a molecule is greatly influenced by its symmetry.

For example, a CO 2 molecule has a symmetrical linear structure:

Therefore, although the C=O bonds are highly polar, due to the mutual compensation of their electrical moments of the dipole, the CO 2 molecule is generally non-polar ( m-ly =  bonds = 0). For the same reason, the highly symmetric tetrahedral molecules CH 4, CF 4, the octahedral molecule SF 6, etc. are nonpolar.

In the corner H 2 O molecule, polar O–H bonds are located at an angle of 104.5º: → →

 H2O \u003d  O - H +  unconnected electric pair  0.

Therefore, their moments are not mutually compensated and the molecule turns out to be polar ().

The angular molecule SO 2, pyramidal molecules NH 3, NF 3, etc. also have an electric moment of the dipole. The absence of such a moment

indicates a highly symmetrical structure of the molecule, the presence of an electric moment of the dipole indicates the asymmetry of the structure of the molecule (Table 3.2).

Table 3.2

Structure and expected polarity of molecules

	Spatial Configuration	Expected polarity
	Linear	non-polar
	Linear	Polar
	Linear	non-polar
		Polar
	Linear	Polar
	plane triangular	non-polar
	Trigonal-pyramidal	Polar
	tetrahedral	non-polar

The value of the electric moment of the dipole of a molecule is strongly influenced by nonbonding electron pairs located in hybrid orbitals and having their own electric moment of the dipole (the direction of the vector is from the nucleus, along the axis of the hybrid AO). For example, the NH 3 and NF 3 molecules have the same trigonal-pyramidal shape, the polarity of the N–H and N–F bonds is also approximately the same. However, the electric moment of the NH 3 dipole is 0.49·10 -29 C·m, and NF 3 is only 0.07·10 -29 C·m. This is explained by the fact that in NH 3 the direction of the electric moment of the dipole of the bonding N–H and non-bonding electron pairs coincides and, upon vector addition, causes a large electric moment of the dipole. On the contrary, in NF 3, the moments of the N–F bonds and the electron pair are directed in opposite directions, therefore, when added, they are partially compensated (Fig. 3.15).

Figure 3.15. Addition of electric moments of the dipole of bonding and non-bonding electron pairs of NH 3 and NF 3 molecules

A non-polar molecule can be made polar. To do this, it must be placed in an electric field with a certain potential difference. Under the action of an electric field, the "centers of gravity" of positive and negative charges are displaced and an induced or induced electric moment of the dipole arises. When the field is removed, the molecule will again become non-polar.

Under the action of an external electric field, a polar molecule is polarized, i.e., a redistribution of charges occurs in it, and the molecule acquires a new value of the electric moment of the dipole, becomes even more polar. This can also occur under the influence of the field created by the approaching polar molecule. The ability of molecules to polarize under the action of an external electric field is called polarizability.

The polarity and polarizability of molecules determine the intermolecular interaction. The reactivity of a substance, its solubility, is associated with the electric moment of the dipole of a molecule. The polar molecules of liquids favor the electrolytic dissociation of the electrolytes dissolved in them.