7.11. The structure of substances with a covalent bond
Substances in which of all types of chemical bonds only covalent is present are divided into two unequal groups: molecular (very much) and non-molecular (much less).
Crystals solid molecular substances consist of weakly interconnected forces of intermolecular interaction of molecules. Such crystals do not have high strength and hardness (remember ice or sugar). They also have low melting and boiling points (see table 22).
Table 22. Melting and boiling points of some molecular substances
Substance |
Substance |
||||
| H 2 | – 259 | – 253 | Br 2 | – 7 | 58 |
| N 2 | – 210 | – 196 | H 2 O | 0 | 100 |
| HCl | – 112 | – 85 | P 4 | 44 | 257 |
| NH 3 | – 78 | – 33 | C 10 H 8 (naphthalene) | 80 | 218 |
| SO 2 | – 75 | – 10 | S 8 | 119 |
Unlike their molecular counterparts, non-molecular substances with a covalent bond form very hard crystals. Diamond crystals (the hardest substance) are of this type.
In a diamond crystal (Figure 7.5), each carbon atom is bonded to four other carbon atoms by simple covalent bonds (sp 3 -hybridization). Carbon atoms form a three-dimensional framework. Essentially, the entire diamond crystal is one huge and very strong molecule.
Silicon crystals, which are widely used in radio electronics and electronic engineering, have the same structure.
If you replace half of the carbon atoms in a diamond with silicon atoms without disturbing the skeleton structure of the crystal, you get a silicon carbide crystal SiC - also a very hard substance used as an abrasive material. Common quartz sand (silicon dioxide) is also of this type. crystalline substances... Quartz is a very solid substance; under the name "emery" it is also used as an abrasive. The structure of quartz is easy to obtain if oxygen atoms are inserted between every two silicon atoms in a silicon crystal. In this case, each silicon atom will be associated with four oxygen atoms, and each oxygen atom - with two silicon atoms.
Crystals of diamond, silicon, quartz and similar in structure are called atomic crystals.
Atomic crystal - a crystal consisting of atoms of one or more elements linked by chemical bonds.
A chemical bond in an atomic crystal can be covalent or metallic.
As you already know, any atomic crystal, like an ionic one, is a huge "supermolecule". Structural formula such a "supermolecule" cannot be written down - you can only show a fragment of it, for example:
Unlike molecular substances, substances that form atomic crystals are among the most refractory (see table 23.).
Table 23. Melting and boiling points of some non-molecular substancesfrom covalent bonds
Such high melting points are quite understandable, if we remember that when these substances melt, not weak intermolecular, but strong chemical bonds are broken. For the same reason, many substances that form atomic crystals do not melt when heated, but decompose or immediately pass into a vapor state (sublime), for example, graphite sublimes at 3700 o C.
| Silicon - Si. Very hard, brittle silicon crystals are similar in appearance to metallic ones, however, they are non-metallic. By the type of electrical conductivity, this substance belongs to semiconductors, which determines its enormous importance in the modern world. Silicon is the most important semiconductor material. Radios, televisions, computers, modern telephones, electronic clocks, solar batteries and many other household and industrial devices contain transistors, microcircuits and photocells made of high-purity silicon monocrystals as the most important structural elements. Technical silicon is used in steel production and non-ferrous metallurgy. According to its chemical properties, silicon is a rather inert substance, it reacts only at high temperatures Silicon dioxide - SiO 2. Another name for this substance is silica. Silicon dioxide occurs naturally in two forms: crystalline and amorphous. Many semi-precious and semi-precious stones are varieties of crystalline silicon dioxide (quartz): rock crystal, jasper, chalcedony, agate. and opal is an amorphous form of silica. Quartz is very widespread in nature, because dunes in deserts, and sandbanks of rivers and seas are all quartz sand. Quartz is a colorless crystalline very hard and refractory substance. In terms of hardness, it is inferior to diamond and corundum, but, nevertheless, it is widely used as an abrasive material. Quartz sand is widely used in the construction and building materials industry. Quartz glass is used for the manufacture of laboratory glassware and scientific instruments, as it does not crack when the temperature changes suddenly. According to their chemical properties silicon dioxide is an acidic oxide, but reacts with alkalis only when fusion is made. At high temperatures, silicon carbide - carborundum - is obtained from silicon dioxide and graphite. Carborundum is the second hardest substance after diamond; it is also used for the manufacture of grinding wheels and "sandpaper". |
7.12. The polarity of the covalent bond. Electronegativity
Recall that isolated atoms of different elements have different tendencies to both give and receive electrons. These differences persist after the formation of a covalent bond. That is, the atoms of some elements tend to attract the electron pair of the covalent bond stronger than the atoms of other elements.
Consider a molecule HCl.
Using this example, let's see how you can estimate the displacement of the electron bond cloud using molar ionization energies and means to the electron. 1312 kJ / mol, and 1251 kJ / mol - the difference is insignificant, about 5%. 73 kJ / mol, and 349 kJ / mol - here the difference is much greater: the electron affinity energy of the chlorine atom is almost five times that of the hydrogen atom. Hence, we can conclude that the electron pair of the covalent bond in the hydrogen chloride molecule is largely displaced towards the chlorine atom. In other words, bond electrons spend more time near the chlorine atom than near the hydrogen atom. This uneven distribution of electron density leads to a redistribution of electric charges inside the molecule. Partial (excess) charges appear on the atoms; on the hydrogen atom - positive, and on the chlorine atom - negative.
In this case, the bond is said to be polarized, and the bond itself is called a polar covalent bond.
If the electron pair of the covalent bond is not displaced to any of the bonded atoms, that is, the bond electrons equally belong to the bonded atoms, then such a bond is called a non-polar covalent bond.
The concept of "formal charge" in the case of a covalent bond is also applicable. Only in the definition it should not be about ions, but about atoms. In general, the following definition can be given.
In molecules, in which covalent bonds were formed only by the exchange mechanism, the formal charges of the atoms are equal to zero. So, in the HCl molecule, the formal charges on the atoms of both chlorine and hydrogen are equal to zero. Consequently, in this molecule, the real (effective) charges on the chlorine and hydrogen atoms are equal to the partial (excess) charges.
It is not always easy to determine the sign of the partial charge on the atom of one or another element in the molecule from the molar ionization energies and affinity to the electrode, that is, to estimate in which direction the electron bond pairs are displaced. Usually one more energy characteristic of the atom is used for these purposes - electronegativity.
Currently, there is no single, generally accepted designation for electronegativity. You can designate it with the letters E / O. Also, there is not yet a single, generally accepted method for calculating electronegativity. Simplified, it can be represented as the half-sum of the molar ionization energies and electron affinity - this was one of the first methods of its calculation.
The absolute values \u200b\u200bof the electronegativities of atoms of various elements are used very rarely. Relative electronegativity is more commonly used, denoted by the letter c. Initially, this value was defined as the ratio of the electronegativity of an atom of a given element to the electronegativity of a lithium atom. Subsequently, the methods of its calculation changed somewhat.
Relative electronegativity is a dimensionless quantity. Its values \u200b\u200bare given in Appendix 10.
Since the relative electronegativity depends primarily on the ionization energy of the atom (the electron affinity energy is always much less), then in the system chemical elements it changes in about the same way as the ionization energy, that is, it increases diagonally from cesium (0.86) to fluorine (4.10). The values \u200b\u200bof the relative electronegativity of helium and neon given in the table are of no practical importance, since these elements do not form compounds.
Using the table of electronegativity, one can easily determine in the direction of which of the two atoms the electrons connecting these atoms are displaced, and, therefore, the signs of the partial charges arising on these atoms.
| H 2 O | The connection is polar | |||
| H 2 | Atoms are the same | H - H | Non-polar communication | |
| CO 2 | The connection is polar | |||
| Cl 2 | Atoms are the same | Cl - Cl | Non-polar communication | |
| H 2 S | The connection is polar |
Thus, in the case of the formation of a covalent bond between atoms of different elements, such a bond will always be polar, and in the case of the formation of a covalent bond between the atoms of one element (in simple substancesah) the connection is in most cases non-polar.
The greater the difference between the electronegativities of the bonded atoms, the more polar the covalent bond between these atoms is.
| Hydrogen sulfide H 2 S - a colorless gas with a characteristic rotten egg odor; poisonous. It is thermally unstable and decomposes when heated. Hydrogen sulfide is slightly soluble in water, its water solution called hydrogen sulfide acid. Hydrogen sulfide provokes (catalyzes) the corrosion of metals, it is this gas that is "to blame" for the darkening of silver. In nature, it is found in some mineral waters. In the process of life, some bacteria form it. Hydrogen sulfide is fatal to all living things. The hydrogen sulphide layer was found in the depths of the Black Sea and gives rise to concern for scientists: the life of marine life there is under constant threat. |
POLAR COVALENT BOND, NON-POLAR COVALENT BOND, ABSOLUTE ELECTRIC NEGATIVITY, RELATIVE ELECTRIC NEGATIVITY.
1. Experiments and subsequent calculations showed that the effective charge of silicon in silicon tetrafluoride is equal to +1.64 e, and of xenon in xenon hexafluoride +2.3 e. Determine the values \u200b\u200bof partial charges on fluorine atoms in these compounds. 2. Make the structural formulas of the following substances and, using the designations "" and "", characterize the polarity of covalent bonds in the molecules of these compounds: a) CH 4, CCl 4, SiCl 4; b) H 2 O, H 2 S, H 2 Se, H 2 Te; c) NH 3, NF 3, NCl 3; d) SO 2, Cl 2 O, OF 2.
3. Using the table of electronegativities, indicate in which of the compounds the bond is more polar: a) CCl 4 or SiCl 4; b) H 2 S or H 2 O; c) NF 3 or NCl 3; d) Cl 2 O or OF 2.
7.13. Donor-acceptor mechanism of bond formation
In the previous paragraphs, you learned in detail about two types of bonds: ionic and covalent. Recall that an ionic bond is formed when an electron is completely transferred from one atom to another. Covalent - when the unpaired electrons of the bonded atoms are socialized.
In addition, there is another mechanism for forming a connection. Let us consider it using the example of the interaction of an ammonia molecule with a boron trifluoride molecule:
As a result, both covalent and ionic bonds arise between nitrogen and boron atoms. In this case, the nitrogen atom is donorelectron pair ("gives" it for the formation of a bond), and the boron atom - acceptor("accepts" it when forming a bond). Hence the name of the mechanism for the formation of such a connection - " donor-acceptor ".
When a bond is formed by the donor-acceptor mechanism, both a covalent bond and an ionic bond are formed.
Of course, after the formation of a bond, due to the difference in the electronegativity of the bonded atoms, the polarization of the bond occurs, and partial charges appear that reduce the effective (real) charges of the atoms.
Let's look at other examples.
If next to the ammonia molecule there is a strongly polar hydrogen chloride molecule, in which there is a significant partial charge on the hydrogen atom, then the hydrogen atom will play the role of an electron pair acceptor. Its 1 s-AO, although not entirely empty, as in the boron atom in the previous example, but the electron density in the cloud of this orbital is significantly reduced.
The spatial structure of the resulting cation, ammonium ion NH 4 is similar to the structure of the methane molecule, that is, all four N-H bonds are exactly the same.
The formation of ionic crystals of ammonium chloride NH 4 Cl can be observed by mixing gaseous ammonia with gaseous hydrogen chloride:
NH 3 (g) + HCl (g) \u003d NH 4 Cl (cr)
The donor of an electron pair can be not only a nitrogen atom. It can be, for example, the oxygen atom of a water molecule. The water molecule will interact with the same hydrogen chloride as follows:
The resulting H 3 O cation is called oxonium ionand, as you will soon find out, is of great importance in chemistry.
In conclusion, consider the electronic structure of the carbon monoxide (carbon monoxide) CO molecule:
In it, in addition to three covalent bonds (triple bonds), there is also an ionic bond.
Conditions for bond formation by the donor-acceptor mechanism:
1) the presence of a lone pair of valence electrons in one of the atoms;
2) the presence of a free orbital on the valence sublevel of another atom.
The donor-acceptor mechanism of bond formation is widespread. It is especially common in the formation of compounds d-elements. Almost everyone's atoms d-elements have many free valence orbitals. Therefore, they are active acceptors of electron pairs.
DONOR-ACCEPTOR BOND FORMATION MECHANISM, AMMONIUM ION, OXONIUM ION, CONDITIONS OF BOND FORMATION BY DONOR-ACCEPTOR MECHANISM.
1. Make reaction equations and educational schemes
a) ammonium bromide NH 4 Br from ammonia and hydrogen bromide;
b) ammonium sulfate (NH 4) 2 SO 4 from ammonia and sulfuric acid.
2. Make up the reaction equations and interaction schemes of a) water with hydrogen bromide; b) water with sulfuric acid.
3. Which atoms in the four previous reactions are donors of an electron pair, and which are acceptors? Why? Explain the answer with diagrams of valence sublevels.
4. Structural formula of nitric acid The angles between the O – N – O bonds are close to 120 o. Define:
a) the type of hybridization of the nitrogen atom;
b) which AO of the nitrogen atom takes part in the formation of the -bond;
c) which AO of the nitrogen atom takes part in the formation of the β-bond by the donor-acceptor mechanism.
What do you think equal angle between H – O – N bonds in this molecule? 5. Make the structural formula of the cyanide ion CN (negative charge - on the carbon atom). It is known that cyanides (compounds containing such an ion) and carbon monoxide CO are strong poisons, and their biological effect is very close. Offer your explanation for the closeness of their biological action.
7.14. Metallic bond. Metals
A covalent bond is formed between atoms that are close in propensity to give and attach electrons, only when the sizes of the bonded atoms are small. In this case, the electron density in the overlapping region of electron clouds is significant, and the atoms are tightly bound, as, for example, in the HF molecule. If at least one of the bonded atoms has a large radius, the formation of a covalent bond becomes less favorable, since the electron density in the overlapping region of electron clouds is much lower in large atoms than in small ones. An example of such a molecule with a less strong bond is the HI molecule (using Table 21, compare the atomization energies of HF and HI molecules).
And yet, between the large atoms ( r o\u003e 1.1) a chemical bond arises, but in this case it is formed due to the socialization of all (or part) of the valence electrons of all bonded atoms. For example, in the case of sodium atoms, all 3 s-electrons of these atoms, while a single electron cloud is formed:
Atoms form a crystal with metal communication.
So both atoms of one element and atoms can communicate with each other. different elements... In the first case, simple substances are formed, called metals, and in the second - complex substances called intermetallic compounds.
Of all the substances with a metallic bond between atoms in school, you will publish only metals. What is the spatial structure of metals? Metal crystal consists of atomic coresleft after the socialization of valence electrons, and the electron cloud of the socialized electrons. Atomic cores usually form the closest packing, and the electron cloud occupies the entire remaining free volume of the crystal.
The main types of the densest packages are cubic closest packing (KPU) and hexagonal tight packing(GPU). The names of these packages are associated with the symmetry of the crystals in which they are realized. Some metals form crystals with a loose packing - body-centered cubic(BCC). Volumetric and ball-and-stick models of these packages are shown in Figure 7.6.
The cubic closest packing is formed by the atoms of Cu, Al, Pb, Au and some other elements. Hexagonal closest packing - atoms Be, Zn, Cd, Sc and a number of others. Body-centered cubic packing of atoms is present in crystals alkali metals, elements of VB and VIB groups. Some metals may have different structures at different temperatures. The reasons for such differences and structural features of metals are still not fully understood.
When melted, metal crystals turn into metallic liquids... In this case, the type of chemical bond between atoms does not change.
The metallic bond does not possess directionality and saturation. In this respect, it is similar to ionic bond.
In the case of intermetallic compounds, we can talk about the polarizability of the metal bond.
Characteristic physical properties metals:
1) high electrical conductivity;
2) high thermal conductivity;
3) high plasticity.
The melting points of different metals are very different from each other: the lowest melting point is for mercury (-39 o C), and the highest for tungsten (3410 o C).
| Beryllium Be - light gray light, fairly hard, but usually brittle metal. Melting point 1287 o C. In air, it is covered with an oxide film. Beryllium is a rather rare metal, living organisms in the process of their evolution practically did not contact with it, therefore it is not surprising that it is poisonous to the animal world. It is used in nuclear technology. Zinc Zn is a soft metal, white with a bluish tinge. Melting point 420 o C. In air and in water, it is covered with a thin dense film of zinc oxide, which prevents further oxidation. In production, it is used for galvanizing sheets, pipes, wire, protecting iron from corrosion. Tungsten W. It is the most refractory of all metals: the melting point of tungsten is 3387 o C. Usually tungsten is quite brittle, but after careful cleaning it becomes ductile, which allows you to draw out of it the thin wire from which the filaments of electric bulbs are made. However, most of the tungsten produced goes to the production of hard and wear-resistant alloys capable of retaining these properties when heated even up to 1000 o C. |
METAL, INTERMETAL JOINT, METAL BOND, TIGHT PACKING.
1. To characterize various packings, the concept of "space filling factor" is used, that is, the ratio of the volume of atoms to the volume of the crystal
where V a -volume of an atom
Z is the number of atoms in a unit cell,
V i- the volume of the unit cell.
Atoms in this case are represented by rigid balls of radius Rtouching each other. Ball volume V w \u003d (4/3) R 3 .
Determine the fill factor of the space for KPU and BCC packaging.
2. Using the values \u200b\u200bof the metal radii (Appendix 9), calculate the size of the unit cell of a) copper (KPU), b) aluminum (KPU) and c) cesium (BCC).
The covalent bond is carried out due to the sharing of electrons belonging to both atoms participating in the interaction. The electronegativities of non-metals are large enough, so there is no electron transfer.
Electrons in overlapping electron orbitals go into general use. In this case, a situation is created in which the outer electronic levels of atoms are filled, that is, an 8 or 2 electron outer shell is formed.
In contact with
Classmates
The state in which the electron shell is completely filled is characterized by the lowest energy and, accordingly, the maximum stability.
There are two mechanisms of formation:
- donor-acceptor;
- exchange.
In the first case, one of the atoms provides its own pair of electrons, and the second - a free electron orbital.
In the second, one electron comes to the common pair from each participant in the interaction.
Depending on which type are - atomic or molecular, compounds with a similar type of bond can vary significantly in physical and chemical characteristics.
Molecular substances most often gases, liquids or solids with low melting and boiling points, non-conductive, with low strength. These include: hydrogen (H 2), oxygen (O 2), nitrogen (N 2), chlorine (Cl 2), bromine (Br 2), rhombic sulfur (S 8), white phosphorus (P 4) and others simple substances; carbon dioxide (CO 2), sulfur dioxide (SO 2), nitrogen oxide V (N 2 O 5), water (H 2 O), hydrogen chloride (HCl), hydrogen fluoride (HF), ammonia (NH 3), methane (CH 4), ethyl alcohol (C 2 H 5 OH), organic polymers and others.
Atomic substances exist in the form of strong crystals with high boiling and melting points, insoluble in water and other solvents, many do not conduct electric current. An example is diamond, which has exceptional strength. This is because a diamond is a crystal made up of carbon atoms linked by covalent bonds. There are no individual molecules in a diamond. Also atomic structure possess substances such as graphite, silicon (Si), silicon dioxide (SiO 2), silicon carbide (SiC) and others.
Covalent bonds can be not only single (as in the Cl2 chlorine molecule), but also double, as in the O2 oxygen molecule, or triple, as, for example, in the N2 nitrogen molecule. At the same time, triples have more energy and are more durable than double and single.
The covalent bond can be formed both between two atoms of one element (non-polar), and between atoms of different chemical elements (polar).
It is not difficult to indicate the formula of a compound with a covalent polar bond if we compare the values \u200b\u200bof electronegativities that make up the molecules of atoms. No difference in electronegativity will determine non-polarity. If there is a difference, then the molecule will be polar.
Don't Miss: Education Mechanism, Specific Examples.
Covalent non-polar chemical bond
Characteristic for simple substances of non-metals... Electrons belong to atoms equally, and there is no shift in electron density.
An example is the following molecules:
H2, O2, O3, N2, F2, Cl2.
Exceptions are inert gases... Their external energy level is completely filled, and the formation of molecules is not energetically favorable for them, and therefore they exist in the form of separate atoms.
Also, an example of substances with a non-polar covalent bond would be, for example, PH3. Despite the fact that the substance consists of different elements, the values \u200b\u200bof the electronegativities of the elements do not actually differ, which means that the electron pair will not shift.
Covalent polar chemical bond
Considering the covalent polar bond, there are many examples: HCl, H2O, H2S, NH3, CH4, CO2, SO3, CCl4, SiO2, CO.
formed between atoms of non-metals with different electronegativity. In this case, the nucleus of the element with greater electronegativity attracts common electrons closer to itself.Diagram of the formation of a covalent polar bond
Depending on the mechanism of formation, common electrons of one of the atoms or both.
The picture clearly shows the interaction in the hydrochloric acid molecule.
A pair of electrons belongs to both one atom and the second, both, so the outer levels are filled. But the more electronegative chlorine attracts a pair of electrons a little closer to itself (while it remains common). The difference in electronegativity is not large enough for a pair of electrons to pass to one of the atoms completely. The result is a partial negative charge for chlorine and a partial positive charge for hydrogen. The HCl molecule is a polar molecule.
Physical and chemical properties of the bond
The connection can be characterized by the following properties: directivity, polarity, polarizability and saturation.
A chemical bond is the interaction of particles (ions or atoms), which occurs during the exchange of electrons located at the last electronic level. There are several types of such a bond: covalent (it is divided into non-polar and polar) and ionic. In this article we will dwell in more detail on the first type of chemical bonds - covalent. To be more precise, in its polar form.
A polar covalent bond is a chemical bond between the valence electron clouds of neighboring atoms. The prefix "co-" means in this case "jointly", and the stem "valent" is translated as strength or ability. Those two electrons that bind to each other are called an electron pair.
History
For the first time this term was used in a scientific context by the laureate Nobel Prize chemist Irving Lenngrum. It happened in 1919. In his work, the scientist explained that a bond in which electrons common to two atoms are observed is different from a metallic or ionic one. This means that it requires a separate name.Later, already in 1927, F. London and W. Heitler, taking as an example the hydrogen molecule as the chemically and physically simplest model, described the covalent bond. They got down to business from the other end, and their observations were substantiated using quantum mechanics.
The essence of the reaction
The process of converting atomic hydrogen into molecular hydrogen is a typical chemical reaction, a qualitative feature of which is a large release of heat when two electrons combine. It looks something like this: two helium atoms are approaching each other, having one electron in their orbit. Then these two clouds come together and form a new one, similar to a helium shell, in which two electrons are already rotating.
Completed electron shells are more stable than incomplete ones, so their energy is significantly lower than that of two separate atoms. When a molecule is formed, excess heat is dissipated in the environment.
Classification
In chemistry, two types of covalent bonds are distinguished:
- A covalent non-polar bond formed between two atoms of one non-metallic element, for example oxygen, hydrogen, nitrogen, carbon.
- A covalent polar bond arises between the atoms of different non-metals. The hydrogen chloride molecule is a good example. When the atoms of two elements combine with each other, then the unpaired electron from hydrogen is partially transferred to the last electronic level of the chlorine atom. Thus, a positive charge is formed on the hydrogen atom, and a negative charge on the chlorine atom.
Donor-acceptor bond is also a type of covalent bond. It consists in the fact that one atom of a pair provides both electrons, becoming a donor, and the atom receiving them, respectively, is considered an acceptor. When a bond is formed between atoms, the donor charge increases by one, and the acceptor charge decreases.
Semipolar connection - ee can be considered a donor-acceptor subspecies. Only in this case are atoms combined, one of which has a complete electron orbital (halogens, phosphorus, nitrogen), and the other has two unpaired electrons (oxygen). Communication formation takes place in two stages:
- first, one electron is torn off from the lone pair and joins the unpaired;
- unification of the remaining unpaired electrodes, that is, a covalent polar bond is formed.
Properties
The polar covalent bond has its own physicochemical propertiessuch as directivity, saturation, polarity, polarizability. They determine the characteristics of the molecules being formed.
The direction of the bond depends on the future molecular structure of the resulting substance, namely on geometric shape, which is formed by two atoms when attached.
Saturation shows how many covalent bonds one atom of a substance can form. This number is limited by the number of outer atomic orbitals.
The polarity of a molecule arises because an electron cloud formed from two different electrons is uneven along its entire circumference. This is due to the difference in negative charge in each of them. It is this property that determines whether the connection is polar or non-polar. When two atoms of one element combine, the electron cloud is symmetrical, which means that the bond is covalent non-polar. And if atoms of different elements combine, then an asymmetric electron cloud is formed, the so-called dipole moment of the molecule.
Polarizability reflects how actively electrons in a molecule are displaced by external physical or chemical agents, such as electrical or magnetic field, other particles.
The last two properties of the resulting molecule determine its ability to react with other polar reagents.
Sigma link and pi link
The formation of these bonds depends on the distribution density of electrons in the electron cloud during the formation of the molecule.
The sigma bond is characterized by the presence of a dense accumulation of electrons along the axis connecting the atomic nuclei, that is, in the horizontal plane.
Pi-bond is characterized by the compaction of electron clouds in the place of their intersection, that is, above and below the nucleus of the atom.
Visualizing a relationship in a formula record
For example, we can take the chlorine atom. Its external electronic level contains seven electrons. In the formula, they are arranged in three pairs and one unpaired electron around the designation of the element in the form of dots.
If we write down a chlorine molecule in the same way, it will be seen that two unpaired electrons have formed a pair common to two atoms, it is called divided. In addition, each of them received eight electrons.
Octet-doublet rule
The chemist Lewis, who proposed how a covalent polar bond is formed, was the first of his colleagues to formulate a rule explaining the stability of atoms when they combine into molecules. Its essence lies in the fact that chemical bonds between atoms are formed when a sufficient number of electrons are socialized to obtain an electronic configuration that repeats similar to the atoms of noble elements.
That is, during the formation of molecules for their stabilization, it is necessary that all atoms have a complete external electronic level. For example, hydrogen atoms, combining into a molecule, repeat the electron shell of helium, chlorine atoms, acquire similarities at the electronic level with an argon atom.
Link length
A covalent polar bond, among other things, is characterized by a certain distance between the nuclei of the atoms that form the molecule. They are located at such a distance from each other that the energy of the molecule is minimal. In order to achieve this, it is necessary that the electron clouds of atoms overlap each other as much as possible. There is a directly proportional relationship between atomic size and bond length. The larger the atom, the longer the bond between the nuclei.
A variant is possible when the atom forms not one but several covalent polar bonds. Then the so-called bond angles are formed between the nuclei. They can be from ninety to one hundred and eighty degrees. They determine geometric formula molecules.
Figure: 2.1. The formation of molecules from atoms is accompanied by redistribution of electrons of valence orbitals and leads to gain in energy, since the energy of the 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 atomic (Figure 2.1). The formation of molecules from atoms is accompanied by a redistribution of electrons of valence orbitals and leads to an energy gain, since the energy of molecules turns out to be less than the energy of noninteracting atoms(Appendix 3). The forces holding atoms in molecules are collectively called 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 bonds: covalent,ionic,metaland hydrogen.
1 Covalent bond
The 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, the overlap of the electron clouds of interacting atoms, accompanied by the release of energy, occurs (Fig. 2.1). In this case, a cloud with an increased density of negative charge appears between the positively charged atomic nuclei. Due to the action of the Coulomb forces of attraction between opposite charges, an increase in the density of the negative charge favors the convergence of nuclei.
A covalent bond is formed by unpaired electrons in the outer shells of atoms ... In this case, electrons with opposite spins form e-pair(Figure 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.
A measure of the strength of a chemical bond is energy E sv spent on breaking the bond (gain in energy when a compound is formed from individual atoms). Usually this energy is measured per 1 mol substancesand are expressed in kilojoules per mole (kJ ∙ mol –1). The energy of a single covalent bond is within the range of 200–2000 kJmol –1.
Figure: 2.2. A covalent bond is the most common type of chemical bond arising from the socialization of an electron pair through an exchange mechanism (and), when each of the interacting atoms supplies one electron, or through the donor-acceptor mechanism (b)when an electron pair is transferred for general use by one atom (donor) to another atom (acceptor).
A covalent bond has properties saturation and focus . Saturation of a covalent bond is understood as the ability of atoms to form a limited number of bonds with neighbors, determined by the number of their unpaired valence electrons. The directionality of the covalent bond reflects the fact that the forces holding the 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-polarcovalent bond, an electron cloud formed by a common pair of electrons is distributed in space symmetrically relative to the nuclei of both atoms. A non-polar covalent bond is formed between the atoms of simple substances, for example, between the same atoms of gases that form diatomic molecules (O 2, H 2, N 2, Cl 2, etc.).
When polarin a covalent bond, the bond electron cloud is displaced toward one of the atoms. The formation of a polar covalent bond between atoms is typical for complex substances. An example is the molecules of volatile inorganic compounds: HCl, H 2 O, NH 3, etc.
The degree of displacement of a common electron cloud to one of the atoms during the formation of a covalent bond (bond polarity ) is determined mainly by the charge of atomic nuclei and the radius of interacting atoms .
The greater the charge of an atomic nucleus, the more it attracts a cloud of electrons to itself. At the same time, the larger the radius of the atom, the weaker the outer electrons are kept near the atomic nucleus. The combined effect of these two factors is expressed in different abilities different atoms "Pull back" the cloud of covalent bonds.
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 the atom, the more the electron cloud of the covalent bond is displaced towards it .
A number of methods have been proposed to quantify electronegativity. In this case, the clearest physical meaning has the method proposed by the American chemist Robert S. Mulliken, who determined the electronegativity atom as half the sum of its energy E e electron and energy affinities E i atom ionization:
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(2.1)
Ionization energyatom is the energy that needs to be spent in order to "tear" an electron from it and remove it at an infinite distance. The ionization energy is determined by photoionization of atoms or by bombarding atoms with electrons accelerated in an electric field. The smallest value of the energy of photons or electrons, which becomes sufficient for the ionization of atoms, and is called their ionization energy E i ... Usually this energy is expressed in electron volts (eV): 1 eV \u003d 1.610 –19 J.
The atoms most willingly donate external electrons metalsthat contain a small number of unpaired electrons on the outer shell (1, 2, or 3). 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 an element: the lower the ionization energy, the more strongly metalpropertieselement.
In the same subgroup of the periodic table 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 a core. 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(also expressed in eV). The release (and not absorption) of energy when a charged electron is attached to some neutral atoms is explained by the fact that the most stable in nature are atoms with filled outer shells. Therefore, those atoms in which these shells are "slightly not filled" (ie, 1, 2 or 3 electrons are not enough before filling), it is energetically advantageous to attach electrons to themselves, turning into negatively charged ions 1. These 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 disadvantageous for them to attach additional electrons; additional energy is required to keep them inside the atoms. The electron affinity of nonmetal atoms is always positive, and the greater, the closer to the noble (inert) gas the nonmetal is located in periodic system... This indicates an increase non-metallic propertiesas we approach the end of the period.
From all that has been said, it is clear that the electronegativity (2.1) of atoms increases from left to right for the elements of each period and decreases from top to bottom for elements of the same group of Mendeleev's periodic system. It is easy to understand, however, that for characterizing the degree of polarity of the covalent bond between atoms, it is not the absolute value of electronegativity that is important, but the ratio of the electronegativities of the atoms forming the bond. therefore in practice, use the relative values \u200b\u200bof electronegativity(Table 2.1), taking the electronegativity of lithium as a unit.
To characterize the polarity of the covalent chemical bond, the difference in the relative electronegativities of the atoms is used... Usually the bond between atoms A and B is considered purely covalent if | A – B | 0.5.
