The polarity of the molecule. Polar and non-polar molecules

A molecule is polar if the center of the negative charge does not coincide with the center of the positive one. Such a molecule is a dipole: two charges of equal magnitude and opposite in sign are separated in space.

A dipole is usually denoted by the symbol where the arrow points from the positive end of the dipole to the negative. A molecule has a dipole moment, which is equal to the magnitude of the charge multiplied by the distance between the charge centers:

Dipole moments of molecules can be measured; some found values ​​are given in table. 1.2. The values ​​of dipole moments serve as a measure of the relative polarity of various molecules.

Table 1.2 (see scan) Dipole moments

There is no doubt that the molecules are polar, if only the bonds in it are polar. We will consider bond polarity because the polarity of a molecule can be thought of as the sum of the polarities of the individual bonds.

Molecules such as have a dipole moment equal to zero, that is, they are non-polar. Two identical atoms in any given molecule, of course, have the same electronegativity and equally own electrons; the charge is zero and therefore the dipole moment is also zero.

The type molecule has a large dipole moment Although the hydrogen fluoride molecule is small, the electronegative fluorine strongly attracts electrons; although the distance is small, the charge is large, and hence the dipole moment is also large.

Methane and carbon tetrachloride have zero dipole moments. Individual bonds, at least in carbon tetrachloride, are polar: however, due to the symmetry of the tetrahedral arrangement, they compensate each other (Fig. 1.9). In methyl chloride, the polarity of the carbon-chlorine bond is not compensated and the dipole moment of methyl chloride is. Thus, the polarity of the molecules depends not only on the polarity of the individual bonds, but also on their direction, i.e., on the shape of the molecule.

The dipole moment of ammonia is It can be considered as the total dipole moment (vector sum) of three moments of individual bonds having the direction shown in the figure.

Rice. 1.9. Dipole moments of some molecules. Polarity of bonds and molecules.

Similarly, we can consider the dipole moment of water equal to

What dipole moment should be expected for nitrogen trifluoride, which, like ammonia, has a pyramidal structure? Fluorine is the most electronegative element, and it certainly draws electrons strongly from nitrogen; therefore, the nitrogen-fluorine bonds must be strongly polar and their vector sum must be large - much more than for ammonia with its not very polar -bonds.

What gives the experiment? The dipole moment of nitrogen trifluoride is only He is much less than the dipole moment of ammonia.

How to explain this fact? In the above consideration, the lone pair of electrons was not taken into account. B (as well as in this pair occupies the -orbital and its contribution to the dipole moment should have the opposite direction compared to the total moment of the nitrogen-fluorine bonds (Fig. 1.10); these moments of the opposite sign, obviously, have approximately the same value, and as a result, there is a small dipole moment, the direction of which is unknown.In ammonia, the dipole moment is probably determined mainly by this free electron pair, and it is increased by the sum of the bond moments.Similarly, lone pairs of electrons should contribute to the dipole moments of water and, of course, any other molecules in which they are present.

Based on the values ​​of dipole moments, valuable information about the structure of molecules can be obtained. For example, any structure of carbon tetrachloride that results in a polar molecule can be ruled out only on the basis of the magnitude of the dipole moment.

Rice. 1.10. Dipole moments of some molecules. The contribution of the lone pair of electrons. The dipole moment due to the lone pair of electrons has a direction opposite to the direction of the total vector of bond moments.

Thus, the dipole moment confirms the tetrahedral structure of carbon tetrachloride (although it does not, since other structures are possible that would also give a non-polar molecule).

Task 1.4. Which of the two possible structures listed below would also have to have a zero dipole moment? a) Carbon is located in the center of the square, at the corners of which there are chlorine atoms, b) Carbon is located at the top of the tetrahedral pyramid, and chlorine atoms are at the corners of the base.

Task 1.5. Although the carbon-oxygen and boron-fluorine bonds must be polar, the dipole moment of the compounds is zero. Propose an arrangement of atoms for each compound, causing a zero dipole moment.

For most compounds, the dipole moment has never been measured. The polarity of these compounds can be predicted from their structure. The polarity of the bonds is determined by the electronegativity of the atoms; if the angles between the bonds are known, then the polarity of the molecule can be determined, also taking into account unpaired pairs of electrons.

Polarity.

Depending on the location of the common electron pair (electron density) between the nuclei of atoms, non-polar and polar bonds are distinguished.

A nonpolar bond is formed by atoms of elements with the same electronegativity. The electron density is distributed symmetrically with respect to the nuclei of atoms.

The bond between atoms with different electronegativity is called polar. The shared electron pair is biased towards the more electronegative element. The centers of gravity of positive (b +) and negative (b -) charges do not match. The greater the difference in the electronegativity of the elements forming the bond, the higher the polarity of the bond. If the electronegativity difference is less than 1.9, the bond is considered polar covalent.

For a diatomic molecule, the polarity of the molecule is the same as the polarity of the bond. In polyatomic molecules, the total dipole moment of a molecule is equal to the vector sum of the moments of all its bonds. The dipole vector is directed from + to –

Example 3 Using the method of valence bonds, determine the polarity of the molecules of tin (II) chloride and tin (IV) chloride.

50 Sn refers to p-elements.

Valence electrons 5s 2 5p 2 . The distribution of electrons over quantum cells in the normal state:

Chemical formulas of tin (IV) chloride -SnCl 4, tin (II) chloride - SnCl 2

To construct the geometric shape of molecules, we depict the orbitals of unpaired valence electrons, taking into account their maximum overlap

Rice. 4. Geometric shape of SnCl 2 and SnCl 4 molecules

The electronegativity Sn is 1.8. Cl - 3.0. Bond Sn - Cl, polar, covalent. Let us depict the vectors of dipole moments of polar bonds.

in SnCl 2 and SnCl 4 molecules

SnCl 2 - polar molecule

SnCl 4 is a non-polar molecule.

Substances, depending on temperature and pressure, can exist in a gaseous, liquid and solid state of aggregation.

In the gaseous state, substances are in the form of individual molecules.

In the liquid state in the form of aggregates, where the molecules are connected by intermolecular van der Waals forces or hydrogen bonds. Moreover, the more polar the molecules, the stronger the bond and, as a result, the higher the boiling point of the liquid.

In solids, structural particles are connected both by intramolecular and intermolecular bonds. Classify: ionic, metallic, atomic (covalent), molecular crystals and crystals with mixed bonds.

CONTROL TASKS

73. Why are the elements chlorine and potassium active, and the element argon, located between them, is inactive?

74. Using the method of valence bonds, explain why the water molecule (H 2 O) is polar, and the methane molecule (CH 4) is non-polar?

75. The substance carbon monoxide (II) is an active substance, and carbon monoxide (IV) is classified as a low-active substance. Explain using the method of valence bonds.

76. How the strength of nitrogen and oxygen molecules changes. Explain using the method of valence bonds.

77. Why are the properties of a sodium chloride (NaCl) crystal different from those of a sodium (Na) crystal? What type of bonding occurs in these crystals?

78. Using the method of valence bonds, determine the polarity of the molecules of aluminum chloride and hydrogen sulfide.

79. What type of hydroxide is rubidium hydroxide? Explain using the method of valence bonds.

80. The boiling point of liquid hydrogen fluoride is 19.5 0 C, and liquid hydrogen chloride (- 84.0 0 C). Why such a big difference in boiling points?

81. Using the method of valence bonds, explain why carbon tetrachloride (CCl 4) is non-polar, and chloroform (CHCl 3) is a polar substance?

82. How does the bond strength change in CH 4 - SnH 4 molecules? Explain using the method of valence compounds.

83. What possible compounds form the elements: lead and bromine? Determine the polarity of these bonds.

84. Using the method of valence bonds, determine the polarity of nitrogen molecules and nitrogen (III) bromide.

85. The boiling point of water is 100 0 C, and hydrogen sulfide (60.7 0 C). Why such a big difference in boiling points?

86. Determine in which compound a stronger bond is tin bromide or carbon bromide? Determine the polarity of these compounds.

87. Using the method of valence bonds, determine the polarity of the molecules of gallium iodide and bismuth iodide.

88. Using the theory of chemical bonding, explain why xenon is a noble (low active) element.

89. Indicate the type of hybridization (sp, sp 2, sp 3) in the compounds: BeCl 2, SiCl 4. Depict the geometric shapes of the molecules.

90. Draw the spatial arrangement of bonds in molecules: boron hydride and phosphorus (III) hydride. Determine the polarity of the molecules.

Guidelines for control tasks in the discipline " Chemistry» for students of non-chemical specialties of distance learning. Part 1.

Compiled by: Associate Professor, Ph.D. Obukhov V.M.

assistant Kostareva E.V.

Signed for publication No. 1

Order no. ed. l.

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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 electric 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, the 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

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, NH 3 and NF 3 molecules have the same trigonal-pyramidal shape, and 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.

Electronegativity of atoms of elements. Relative electronegativity. Change in periods and groups of the Periodic system. The polarity of a chemical bond, the polarity of molecules and ions.

Electronegativity (e.o.) is the ability of an atom to displace electron pairs towards itself.
Meroy e.o. is the energy arithmetically equal to ½ the sum of the ionization energy I and the electron similarity energy E
E.O. = ½ (I+E)

Relative electronegativity. (OEO)

Fluorine, as the strongest e.o element, is assigned a value of 4.00 relative to which the other elements are considered.

Changes in periods and groups of the Periodic system.

Within periods, as the nuclear charge increases from left to right, electronegativity increases.

The fewest value is observed in alkali and alkaline earth metals.

Greatest- for halogens.

The higher the electronegativity, the stronger the non-metallic properties of the elements.

Electronegativity (χ) is a fundamental chemical property of an atom, a quantitative characteristic of the ability of an atom in a molecule to displace common electron pairs towards itself.

The modern concept of the electronegativity of atoms was introduced by the American chemist L. Pauling. L. Pauling used the concept of electronegativity to explain the fact that the energy of the A-B heteroatomic bond (A, B are symbols of any chemical elements) is generally greater than the geometric mean of the A-A and B-B homoatomic bonds.

The highest value of e.o. fluorine, and the lowest is cesium.

The theoretical definition of electronegativity was proposed by the American physicist R. Mulliken. Based on the obvious position that the ability of an atom in a molecule to attract an electronic charge to itself depends on the ionization energy of the atom and its electron affinity, R. Mulliken introduced the concept of the electronegativity of the atom A as the average value of the binding energy of the outer electrons during the ionization of valence states ( for example, from A− to A+) and on this basis proposed a very simple relation for the electronegativity of an atom:

where J1A and εA are the ionization energy of an atom and its electron affinity, respectively.
Strictly speaking, an element cannot be ascribed a permanent electronegativity. The electronegativity of an atom depends on many factors, in particular, on the valence state of the atom, the formal oxidation state, the coordination number, the nature of the ligands that make up the environment of the atom in the molecular system, and some others. Recently, more and more often, to characterize electronegativity, the so-called orbital electronegativity is used, which depends on the type of atomic orbital involved in the formation of a bond, and on its electron population, i.e. on whether the atomic orbital is occupied by an unshared electron pair, singly populated by an unpaired electron, or is vacant. But, despite the known difficulties in interpreting and determining electronegativity, it always remains necessary for a qualitative description and prediction of the nature of bonds in a molecular system, including the bond energy, electronic charge distribution and degree of ionicity, force constant, etc. One of the most developed in the current approach is the Sanderson approach. This approach was based on the idea of ​​equalizing the electronegativity of atoms during the formation of a chemical bond between them. Numerous studies have found relationships between the Sanderson electronegativity and the most important physicochemical properties of inorganic compounds of the vast majority of the elements of the periodic table. A modification of Sanderson's method, based on the redistribution of electronegativity between the atoms of a molecule for organic compounds, also turned out to be very fruitful.

2) The polarity of the chemical bond, the polarity of molecules and ions.

What is in the abstract and in the textbook - Polarity is associated with a dipole moment. It appears as a result of the displacement of a common electron pair to one of the atoms. Polarity also depends on the difference in the electronegativity of the atoms being bonded. two atoms, the more polar is the chemical bond between them. Depending on how the electron density is redistributed during the formation of a chemical bond, several types of it are distinguished. The limiting case of chemical bond polarization is a complete transition from one atom to another.

In this case, two ions are formed, between which an ionic bond arises. In order for two atoms to be able to create an ionic bond, it is necessary that their e.o. differed greatly. If e.o. are equal, then a non-polar covalent bond is formed. The most common polar covalent bond is formed between any atoms that have different e.o.

The effective charges of atoms can serve as a quantitative estimate of the polarity of a bond. The effective charge of an atom characterizes the difference between the number of electrons belonging to a given atom in a chemical compound and the number of electrons of a free atom. An atom of a more electronegative element attracts electrons more strongly, so the electrons are closer to it, and it receives some negative charge, which is called effective, and its partner has the same positive effective charge. If the electrons that form a bond between atoms belong to them equally, the effective charges are zero.

For diatomic molecules, it is possible to characterize the polarity of the bond and determine the effective charges of atoms based on measuring the dipole moment M = q * r where q is the charge of the dipole pole, which is equal to the effective charge for a diatomic molecule, r is the internuclear distance. The dipole moment of the bond is a vector quantity. It is directed from the positively charged part of the molecule to its negative part. The effective charge on the atom of an element does not coincide with the oxidation state.

The polarity of molecules largely determines the properties of substances. Polar molecules turn towards each other with oppositely charged poles, and mutual attraction arises between them. Therefore, substances formed by polar molecules have higher melting and boiling points than substances whose molecules are non-polar.

Liquids whose molecules are polar have a higher dissolving power. Moreover, the greater the polarity of the solvent molecules, the higher the solubility of polar or ionic compounds in it. This dependence is explained by the fact that the polar molecules of the solvent, due to the dipole-dipole or ion-dipole interaction with the solute, contribute to the decomposition of the solute into ions. For example, a solution of hydrogen chloride in water, whose molecules are polar, conducts electricity well. A solution of hydrogen chloride in benzene does not have an appreciable electrical conductivity. This indicates the absence of hydrogen chloride ionization in the benzene solution, since the benzene molecules are nonpolar.

Ions, like an electric field, have a polarizing effect on each other. When two ions meet, their mutual polarization occurs, i.e. displacement of the electrons of the outer layers relative to the nuclei. The mutual polarization of ions depends on the charges of the nucleus and ion, the radius of the ion, and other factors.

Within the groups of e.o. decreases.

The metallic properties of the elements increase.

Metallic elements at the external energy level contain 1,2,3 electrons and are characterized by a low value of ionization potentials and e.o. because metals show a pronounced tendency to donate electrons.
Non-metallic elements have a higher ionization energy.
As the outer shell of nonmetals is filled, the atomic radius decreases within the periods. On the outer shell, the number of electrons is 4,5,6,7,8.

The polarity of a chemical bond. Polarity of molecules and ions.

The polarity of a chemical bond is determined by the displacement of the bonds of an electron pair to one of the atoms.

A chemical bond arises due to the redistribution of electrons in valence orbitals, resulting in a stable electronic configuration of a noble gas, due to the formation of ions or the formation of common electron pairs.
A chemical bond is characterized by energy and length.
The measure of bond strength is the energy expended to break the bond.
For example. H - H = 435 kJmol-1

Electronegativity of atomic elements
Electronegativity is a chemical property of an atom, a quantitative characteristic of the ability of an atom in a molecule to attract electrons to itself from atoms of other elements.
Relative electronegativity

The first and most famous scale of relative electronegativity is the L. Pauling scale, obtained from thermochemical data and proposed in 1932. The electronegativity value of the most electronegative element fluorine, (F) = 4.0, is arbitrarily taken as the reference point in this scale.

Elements of group VIII of the periodic system (noble gases) have zero electronegativity;
The conditional boundary between metals and non-metals is considered to be the value of relative electronegativity equal to 2.

The electronegativity of the elements of the periodic system, as a rule, increases sequentially from left to right in each period. Within each group, with a few exceptions, electronegativity consistently decreases from top to bottom. Electronegativity is used to characterize a chemical bond.
Bonds with a smaller difference in the electronegativity of atoms are referred to as polar covalent bonds. The smaller the difference in the electronegativity of the atoms forming a chemical bond, the lower the degree of ionicity of this bond. The zero difference in the electronegativity of atoms indicates the absence of an ionic character in the bond formed by them, i.e., its pure covalence.

Polarity of a chemical bond, polarity of molecules and ions
The polarity of chemical bonds, a characteristic of a chemical bond, showing the redistribution of electron density in space near the nuclei compared to the initial distribution of this density in the neutral atoms that form this bond.

Almost all chemical bonds, with the exception of bonds in diatomic homonuclear molecules, are polar to one degree or another. Usually covalent bonds are weakly polar, ionic bonds are strongly polar.

For example:
covalent non-polar: Cl2, O2, N2, H2,Br2

covalent polar: H2O, SO2, HCl, NH3, etc.

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 °. A similar arrangement of 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.