exchange covalent bond. The method of valence bonds. Donor-acceptor and dative mechanisms
Chemical bond.
Different substances have different structures. Of all currently known substances, only inert gases exist in the form of free (isolated) atoms, which is due to the high stability of their electronic structures. All other substances (and there are currently more than 10 million of them) are composed of bonded atoms.
Note: those parts of the text that can not be learned and disassembled are in italics.
The formation of molecules from atoms leads to a gain in energy, since under normal conditions the molecular state is more stable than the atomic state.
An atom can have from one to eight electrons in its outer energy level. If the number of electrons in the outer level of an atom is the maximum that it can accommodate, then this level is called completed. Completed levels are characterized by great durability. These are the outer levels of noble gas atoms: helium has two electrons at the outer level (s 2), the rest have eight electrons each (ns 2 np 6). The outer levels of atoms of other elements are incomplete and in the process of chemical interaction they are completed.
A chemical bond is formed by valence electrons, but it is carried out in different ways. There are three main types of chemical bonds: covalent, ionic and metallic.
covalent bond
Let us consider the mechanism of the emergence of a covalent bond using the example of the formation of a hydrogen molecule:
H + H \u003d H 2; Q = 436 kJ
The nucleus of a free hydrogen atom is surrounded by a spherically symmetric electron cloud formed by a 1 s electron. When atoms approach up to a certain distance, their electron clouds (orbitals) partially overlap
As a result, a molecular two-electron cloud arises between the centers of both nuclei, which has a maximum electron density in the space between the nuclei; an increase in the density of the negative charge favors a strong increase in the forces of attraction between the nuclei and the molecular cloud.
So, a covalent bond is formed as a result of the overlapping of electron clouds of atoms, accompanied by the release of energy. If for hydrogen atoms approaching before touching, the distance between the nuclei is 0.106 nm, then after the overlap of the electron clouds (formation of the H 2 molecule), this distance is 0.074 nm. The greatest overlap of electron clouds occurs along the line connecting the nuclei of two atoms (this occurs when a σ-bond is formed). The stronger the chemical bond, the greater the overlap of electron orbitals. As a result of the emergence chemical bond between two hydrogen atoms, each reaches the electron configuration of the noble gas atom helium.
Depicting chemical bonds is customary in different ways:
1) with the help of electrons in the form of dots placed at the chemical sign of the element. Then the formation of a hydrogen molecule can be shown by the scheme
H∙ + H∙ →H:H
2) often, especially in organic chemistry, a covalent bond is represented by a dash (dash) (for example, H-H), which symbolizes a common pair of electrons.
A covalent bond in a chlorine molecule is also carried out using two common electrons, or an electron pair:
Lone pair of electrons, there are 3 of them in an atom
← Lone pair of electrons,
There are 6 of them in a molecule.
unpaired electron shared or shared pair of electrons
As you can see, each chlorine atom has three lone pairs and one unpaired electron. The formation of a chemical bond occurs due to the unpaired electrons of each atom. The unpaired electrons bond into a common pair of electrons, also called a shared pair.
If one covalent bond has arisen between atoms (one common electron pair), then it is called single; if more, then a multiple of double (two common electron pairs), triple (three common electron pairs).
A single bond is represented by one dash (stroke), a double bond by two, and a triple bond by three. A dash between two atoms shows that they have a pair of electrons generalized, as a result of which a chemical bond was formed. With the help of such dashes, structural formulas molecules.
So, in the chlorine molecule, each of its atoms has a completed external level of eight electrons (s 2 p 6), and two of them (an electron pair) equally belong to both atoms. The overlap of electron orbitals during the formation of a molecule is shown in Fig: 
In the nitrogen molecule N 2, atoms have three common electron pairs:
:N+ + N: → :N:::N:
Obviously, a nitrogen molecule is stronger than a hydrogen or chlorine molecule, which is the reason for the significant inertness of nitrogen in chemical reactions.
A chemical bond carried out by electron pairs is called a covalent bond.
Mechanisms for the formation of a covalent bond.
A covalent bond is formed not only by overlapping one-electron clouds, is an exchange mechanism for the formation of a covalent bond.
In the exchange mechanism, atoms share the same number of electrons.
Another mechanism of its formation is also possible - the donor-acceptor mechanism. In this case, the chemical bond is formed by undivided electron pair of one atom and free orbitals of another atom.
Consider, as an example, the mechanism of formation of the ammonium ion NH 4 +
When ammonia reacts with HCl, chemical reaction:
NH 3 + HCl \u003d NH 4 Cl or in abbreviated ionic form: NH 3 + H + \u003d NH 4 +
At the same time, in the ammonia molecule, the nitrogen atom has undivided a couple of electrons (two-electron cloud):
USE OF NEW INFORMATION
TECHNOLOGIES IN CHEMISTRY LESSONS
Time is quickly running forward, and if earlier the school needed to create a theoretical base and educational and methodological support, now there is everything necessary to increase the efficiency of its work. And this is the great merit of the national project "Education". Of course, we, teachers, experience great difficulties in terms of mastering modern technologies. Our inability to work with a computer affects, and it takes a lot of time to master it. But still very interesting and exciting! Moreover, the result is obvious. Children are interested in the lessons, a variety of classes are very fast and informative.
People often think that chemistry is harmful and dangerous. We often hear: “Environmentally friendly products!”, “I heard that you are being poisoned with chemicals!”... But this is not true! We, teachers of chemistry, are faced with the task of convincing schoolchildren that chemistry is a creative science, that it is the productive force of society, and its products are used in all industries, Agriculture and without chemicalization, the further development of civilization is impossible.
The widespread introduction of chemicals, substances, methods and technological methods requires highly educated specialists with a solid base of chemical knowledge. To do this, our school has a specialized chemical and biological class, which provides quality training schoolchildren to continue chemical education. In order for students in high school to choose this particular profile, in the 9th grade there is an elective course "Chemistry in everyday life", the purpose of which is to help children get acquainted with professions directly related to the subjects of chemistry and biology. Even if students do not choose a chemical and biological profile in high school, the knowledge about the substances that they constantly encounter in everyday life will be useful in life.
In the classroom elective course Lectures come first. In preparing for them, I use information Internet resources. Many illustrations, diagrams, video collections, laboratory work materials, slides are displayed on the screen, and based on them, I lead my story. My explanation technology has changed significantly. The children are very interested, they listen to the story with great attention and desire.
Chemistry is an experimental science. A large number of time for laboratory work. But it happens that some reagents are not in the laboratory, and a virtual laboratory comes to the rescue. With the help of a special program, students can conduct a virtual experiment. The guys study the effect of synthetic detergents on various types of fabrics, solubility in water mineral fertilizers, the environment of their solution, the qualitative composition of food (carbohydrates, proteins, fats). Using a computer, they keep their own experimental diary, where they record the topic of laboratory work, their observations, and conclusions on the correct use of these substances in everyday life. The advantages of a virtual laboratory are safety, no need for laboratory equipment, and time costs are minimal.
At the end of the course, students must pass a test on any topic studied. They are faced with the task of choosing in what form to sum up. The most traditional is a test in the form of an abstract, message or report. For their preparation, children use materials from Internet resources. In this, of course, I help them: I clearly set the task, formulating the questions that the students must answer, and indicate the address of the site with information on the relevant topic.
But this form is already a little outdated, and some guys began to choose project activities. They work individually, in groups, in teams. The search for information is not complete without using the capabilities of the Internet. Before releasing them for free search, I give them an orientation: search method, keywords, phrases, titles search engines, work with which may be useful, addresses of sites on the Internet.
Children also choose a test in the form of a game, tasks and exercises for which they develop themselves. It can be a test-turntable, “Smarts and smarts”, “How to become a millionaire?”, “What? Where? When?", various puzzles.
I also arrange a presentation of the resulting product with the involvement of remote technologies. By posting the results of activities on the Internet on the website of the school or class, students get the opportunity to evaluate their work not only with the help of their classmates, but also with the help of children and teachers from other schools, discuss these results, look at them with different eyes.
From the point of view of the new media pedagogy, we live in an extremely interesting time. The rapid introduction of modern technologies forces us to approach old positions in a new way. Pre-profile training at our school exists for four years, and each time I review the course of the lessons, because. new perspectives are opening up, fruitful links are emerging between traditional teaching methods and the new tasks of society, information and knowledge. Indeed, media education has become part of general education. At the same time, the children develop communication skills, interest in new technologies, enthusiasm, individual activity, creativity, they actively cooperate, exchange their own opinions.
I am convinced that the use of information technology can provide a developed learning culture. This is success in teaching and learning. Apply information technology! Move from old forms of classes that have lost their effectiveness to newer, more advanced and modern ones!
The use of new information technologies in educational process can be illustrated by the example of one of the lessons in general chemistry in the 11th grade.
The mechanism of formation and properties of a covalent bond
The purpose of the lesson. Recall from the course of the 8th grade the mechanism of formation of a covalent bond, to study the donor-acceptor mechanism and the properties of a covalent bond.
Equipment. Electronegativity table of chemical elements, codograms of st- and n-bonds, training disk "General Chemistry" from a series of training programs of Cyril and Methodius with diagrams and models of molecules, ball-and-stick models of molecules, work card with tasks and tests, interactive whiteboard, computer, tasks for consolidation and control of knowledge with remote control.
During the classes
The lecture is conducted with the help of the training disk "General Chemistry".
Repetition of the material covered
Recall with students, due to which a bond is formed between the atoms of non-metals. Complete tasks 1, 2 on the work card (see appendix).
Learning new material
Covalent bond formation mechanism:
a) exchange (for example, H 2, Cl 2, HC1);
b) donor-acceptor (for example, NH 4 C1).
Immediately, students write down their homework in the margins: Depict the formation of the hydronium ion H 3 O + from H ion + and water molecules.
Types of covalent bonds: polar and non-polar (according to the composition of the molecule).
Properties of a covalent bond.
multiplicity(single, one and a half, double, triple).
Bond energy is the amount of energy released during the formation of a chemical bond or expended on breaking it.
Link length is the distance between the nuclei of atoms in a molecule.
The energy and length of the bond are interconnected. Show by example how these properties are interconnected, how they affect the strength of the molecule (project onto the board):
With an increase in the number of bonds between atoms in a molecule, the bond length decreases, and its energy increases, for example (project onto a board):Saturability- this is the ability of atoms to form a certain and limited number of bonds. Show with ball-and-stick examples
molecules Cl 2, H 2 O, CH 4, HNO 3.
Orientation. Consider patterns of overlapping electron clouds during the formation of σ- and π-bonds, project onto the board (Fig.).
Fix tasks 6, 7 on the work card (see appendix).
Small break!
1. Let's start the list in order,
Because the first element.
(It forms, by the way, water -
very important point).
Let's imagine a molecule
Convenient formula H 2 .
Let's add -
There is no lighter substance in the world!
2. N 2 is a nitrogen molecule.
Known to be colorless
gas. A lot of knowledge, but let's
Let's replenish them anyway.
3. He is everywhere and everywhere:
And in stone, in air, in water,
He is in the morning dew
And blue in the sky.
(Oxygen.)
4. Mushroom pickers found a small swamp in the forest, from which gas bubbles escaped in places. The match ignited the gas, and a faint flame began to wander through the swamp. What is this gas? (Methane)
Continuation of the lesson.
Polarizability is the ability of a covalent bond to change its polarity under the influence of an external electric field(pay attention to such different concepts as bond polarity and molecular polarizability).
Consolidation of the studied material
Control on the studied topic is carried out using remote controls.
The survey is conducted within 3 minutes, 10 questions at the price of one point, 30 seconds are given for the answer, the questions are projected onto an interactive whiteboard. When scoring 9-10 points - score "5", 7-8 points - score "4", 5-6 points - score "3".
Questions for consolidation
1. The bond that is formed due to common electron pairs is called:
a) ionic; b) covalent; c) metal.
2. A covalent bond is formed between atoms:
a) metals; b) non-metals; c) metal and non-metal.
3. The mechanism for the formation of a covalent bond due to the lone electron pair of one atom and the free orbital of another is called:
a) donor-acceptor; b) inert; c) catalytic.
4. Which of the molecules has a covalent bond?
a) Zn; b) Cu O; c) NH3.
5. The multiplicity of bonds in a nitrogen molecule is equal to:
a) three; b) two; c) unit.
6. The smallest bond length in a molecule:
a) H2S; b) SF6; c) SO 2 ; d) SOr
7. When electron clouds overlap along the axis connecting the nuclei of interacting atoms, the following is formed:
a) σ-bond; b) π bond; c) ρ-bond.
8. A nitrogen atom has a possible number of unpaired electrons:
a) 1; b) 2; at 3.
9. Bond strength increases in the series:
a) H 2 O - H 2 S; 6) NH 3 - PH 3; c) CS 2 - C O 2; d) N 2 - O 2
10. The hybrid s-orbital has the form:
a) a ball b) wrong eight; c) the correct eight.
The results are immediately displayed on the screen, we make a report on each question.
Analysis of homework (see appendix - work card), § 6 of the textbook by O.S. Gabrielyan, G. Glysov “Chemistry. Grade 11 ”(M .: Drofa, 2006), abstract in a notebook.
Application
work card
1. Match the names of the substance and the type of connection.
1) Potassium chloride;
2) oxygen;
3) magnesium;
4) carbon tetrachloride.
a) Covalent non-polar;
b) ionic;
c) metal;
d) covalent polar.
2. Between the atoms of which elements will the chemical bond have an ionic character?
a) NnO; b) Si and C1; c) Na and O; d) P and Br.
3. The bond length is expressed in:
a) nm; b) kg; c) j; d) m 3.
4. Where is the chemical bond the strongest: in the Cl 2 or O 2 molecule?
5. In which molecule is the strength of the hydrogen bond greater: H 2 O or H 2 S?
6. Continue the sentence: “The connection formed by the overlapping of electron clouds along the line connecting the nuclei of atoms is called ............................................ ......",
7. Sketch the patterns of overlapping electron orbitals during the formation of a π bond.
8. Homework. "General chemistry in tests, tasks, exercises" O.S. Gabrielyan (Moscow: Drofa, 2003), work 8A, option 1, 2.
Ticket number 11
Ticket number 12
Ticket number 13
Ticket number 14
Ticket number 15.
EXAMINATION TICKET No. 11
Redox reactions. The oxidation state of an element. Examples of oxidizing and reducing agents.
The method of valence bonds (MVS). Exchange and donor-acceptor mechanisms of covalent bond formation.
Answer:
Redox reactions(OVR) - reactions that go with a change in s.d. atoms. Redox reactions are chemical reactions that occur with a change in the oxidation states of the atoms that make up the reactants, realized by the redistribution of electrons between the oxidizing atom and the reducing atom.
Oxidation state(s.d.) - the charge that is attributed to the atom, considering it an ion
Oxidizer (Ox) accepts electrons.
Restorer (Red) - donates electrons
Ox 1 + Red 2 Red 1 + Ox 2
Ox1 + ne– → Red1
Cu2+ + 2e– → Cu0
CuSO 4 + Zn → ZnSO 4 + Cu
Red2–ne– → Ox2
Zn0 – 2e– → Zn2+
Valence bond method
1927 - Heitler and London Quantum-mechanical calculation of the hydrogen molecule
Communication formation mechanisms
|
Exchange A + BA : AT |
Donor-acceptor BUT : +VA : AT BF 3 + F – – : NH 3 + H + + |
The mechanism of formation of a covalent bond.
MVS makes it possible to distinguish three mechanisms for the formation of a covalent bond: exchange, donor-acceptor, and dative.
exchange mechanism. It includes those cases of the formation of a chemical bond, when each of the two bonded atoms allocates one electron for socialization, as if exchanging them. To bind the nuclei of two atoms, the electrons must be in the space between the nuclei. This area in the molecule is called the binding area (the area where the electron pair is most likely to stay in the molecule). In order for the exchange of unpaired electrons between atoms, the overlap of atomic orbitals is necessary. This is the action of the exchange mechanism for the formation of a covalent chemical bond. Atomic orbitals can only overlap if they have the same symmetry properties about the internuclear axis.
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|
Donor-acceptor and dative mechanisms.
The donor-acceptor mechanism is associated with the transfer of a lone pair of electrons from one atom to a vacant atomic orbital of another atom. For example, the formation of an ion -:
The vacant p-AO in the boron atom in the BF 3 molecule accepts a pair of electrons from the fluoride ion (donor). In the resulting anion, four B-F covalent bonds are equivalent in length and energy. In the original molecule, all three B-F bonds were formed by the exchange mechanism.
Atoms whose outer shell consists only of s- or p-electrons can be either donors or acceptors of a lone pair of electrons. Atoms that have valence electrons on the d-AO can simultaneously act as both donors and acceptors. To distinguish between these two mechanisms, the concepts of the dative mechanism of bond formation were introduced.
Examination ticket number 12
The second law of thermodynamics. Entropy, her physical meaning and calculation methods. The change in the entropy of the system as a probabilistic criterion for the direction of the process.
Osmosis. osmotic pressure. Van't Hoff's law for solutions of non-electrolytes.
Answer:
Second law of thermodynamics
AT isolated system, a spontaneous process is possible only with an increase in entropy.
KS- a bond carried out by an electron pair belonging to both atoms.
Conditions for the formation of the CS: it is formed between atoms with high electronegativity. (electronicity - the ability of atoms to attract electrons to themselves).
∆Χ is the electronegativity difference of 2 atoms, if ∆Χ≤1.4, the bond is polar
KS can. formed:
1 - between any atoms of non-metals (because all non-metals have high values of electro-rejection), ex: HCl, electro-resistance values - according to the tables, for H=2.1, for Cl=3.1, - ∆Χ=3.1-2.1= 1≤1.4, this bond is covalent and polar.
2 - between the atoms of a non-metal and a metal, if the metal is in a high degree of oxidation, ex: CrCl6 for Cr=2.4, ΔΧ=3.1-2.4=0.7≤1.4 - this is a covalent polar bond.
Mechanisms for the formation of CS:
1- exchange mechanism- 2 atoms exchange electrons, forming a common electron pair belonging to both and called "shared". Molecules of volatile inorganic compounds can serve as an example: HCl, H 2 O, H 2 S, NH 3, etc. The formation of the HCl molecule can be represented by the H. + scheme. Cl: \u003d H: Cl: The electron pair is shifted to the chlorine atom, since the relative electronegativity of the chlorine atom (2.83) is greater than that of the hydrogen atom (2.1).
2 - donor-acceptor mechanism: - lies in the fact that a pair of electrons of one atom (donor) occupies a free orbital of another atom (acceptor) Let us consider as an example the mechanism of formation of the ammonium ion. In the ammonia molecule, the nitrogen atom has an unshared pair of electrons (a two-electron cloud): .
The hydrogen ion has a free (not filled) 1s orbital, which can be denoted as □H+. When an ammonium ion is formed, a two-electron cloud of nitrogen becomes common for nitrogen and hydrogen atoms, i.e. it turns into a molecular electron cloud. So, there is a fourth covalent bond. The process of formation of the ammonium ion can be represented by the scheme
| + □H+ → |
The charge of the hydrogen ion becomes common (it is delocalized, i.e. dispersed between all atoms), and the two-electron cloud (lone electron pair) belonging to nitrogen becomes common with hydrogen.
A covalent bond can be polar (complex molecules) and non-polar (simple molecules).
Properties of a covalent bond
A covalent bond has a number of important properties. These include: saturation and directionality.
Saturability - characteristic property covalent bond. It manifests itself in the ability of atoms to form a limited number of covalent bonds. This is due to the fact that one orbital of an atom can take part in the formation of only one covalent chemical bond. This property determines the composition of molecular chemical compounds. So, when hydrogen atoms interact, an H 2 molecule is formed, and not H 3. The third hydrogen atom cannot join, since the spin of its electron will be parallel to the spin of one of the paired electrons in the molecule. The ability to form one or another number of covalent bonds in atoms various elements limited to obtaining the maximum number of unpaired valence electrons.
Orientation- property of a covalent bond that determines the geometric structure of the molecule. The reason for the directionality of the bond is that the overlap of electron orbitals is possible only if they have a certain mutual orientation, which provides the highest electron density in the region of their overlap. In this case, the strongest chemical bond is formed.
Covalent (non-polar, polar) bond. Mechanisms for the formation of a covalent bond
With the help of chemical bonds, the atoms of elements in the composition of substances are held near each other. The type of chemical bond depends on the distribution of electron density in the molecule.
chemical bond- mutual adhesion of atoms in a molecule and a crystal lattice under the influence of electric forces of attraction between atoms. An atom at its outer energy level can contain from one to eight electrons. Valence electrons- electrons of the outer, outer electron layers involved in chemical bonding. Valence- the property of the atoms of an element to form a chemical bond.
covalent bond is formed due to common electron pairs arising at the outer and pre-outer sublevels of the bonded atoms.
The shared electron pair is carried out through exchange or donor-acceptor mechanism. Exchange mechanism for the formation of a covalent bond- pairing of two unpaired electrons belonging to different atoms. Donor-acceptor mechanism of covalent bond formation- the formation of a bond due to a pair of electrons of one atom (donor) and a vacant orbital of another atom (acceptor).
There is There are two main types of covalent bonds: non-polar and polar.
Covalent non-polar bond occurs between non-metal atoms of the same chemical element(O2, N2, Cl2) - the electronic bond cloud formed by a common pair of electrons is distributed in space symmetrically with respect to the nuclei of both atoms.
covalent polar bond occurs between atoms of various non-metals (HCl, CO2, N2O) - the electron cloud of the bond is shifted to an atom with a higher electronegativity.
The more the electron clouds overlap, the stronger the covalent bond.
Electronegativity- the ability of atoms of a chemical element to pull towards themselves common electron pairs involved in the formation of a chemical bond.
Properties of a covalent bond: 1) energy; 2) length; 3) saturation; 4) orientation.
Link length- the distance between the nuclei of atoms that form a bond.
Bond energy is the amount of energy required to break the bond.
Saturability- the ability of atoms to form a certain number of covalent bonds.
Orientation of the covalent bond- a parameter that determines the spatial structure of molecules, their geometry, shape.
Hybridization- Alignment of orbitals in shape and energy. There are several forms of overlapping electron clouds with the formation of?-bonds and?-bonds (?-bond is much stronger than?-bond,?-bond can only be with?-bond). A covalent bond is a bond that occurs between atoms due to the formation of common electron pairs. It is also based on the idea that atoms acquire an energetically favorable and stable electronic configuration of 8 electrons (for a hydrogen atom of 2). Atoms receive such a configuration not by donating or gaining electrons, as in an ionic bond, but by forming common electron pairs. The mechanism of formation of such a bond can be exchange or donor-acceptor.
The exchange mechanism includes cases when one electron is involved in the formation of an electron pair from each atom. For example, hydrogen: H2 H. + H. >N:N or N-N. The bond arises due to the formation of a common electron pair due to the union of unpaired electrons. Each atom has one s-electron. The H atoms are equivalent and the pairs equally belong to both atoms. According to the same principle, the formation of common electron pairs (overlapping p-electron clouds) occurs during the formation of the Cl2 molecule. When an N2 molecule is formed, 3 common electron pairs are formed. The p-orbitals overlap. The bond is called non-polar.
When a hydrogen chloride molecule is formed, the orbital of the s-electron of hydrogen and the orbital of the p-electron of chlorine H-Cl overlap. The bonding electron pair is displaced towards the chlorine atom, resulting in the formation of a dipole, which is measured by the dipole moment. The connection is called polar.
According to the donor-acceptor mechanism, the ammonium ion is formed. The donor (nitrogen) has an electron pair, the acceptor has an (H +) free orbital, which the electron nitrogen pair can occupy. In the ammonium ion, three bonds of nitrogen with hydrogen are formed by the exchange mechanism, and one by the donor-acceptor mechanism. All 4 connections are equal.
Covalent bonds are classified not only by the mechanism of formation of common electron pairs connecting atoms, but also by the way the electron orbitals overlap, by the number of common pairs, and also by their displacement. According to the method of overlapping - y (sigma s-s, s-p, p-p) p ( rr dumbbells overlap in two places). In the nitrogen molecule, there is one y-bond and two p-bonds between the atoms, which are located in two mutually perpendicular planes.
According to the number of common electron pairs, they distinguish: single H2, HCl; double C2H4, CO2; triple N2.
According to the degree of bias: polar and non-polar. The bond between atoms with the same electronegativity is non-polar, while those with different electronegativity are polar.
Research by scientists led to the conclusion that the chemical bond in the hydrogen molecule is carried out by the formation of a pair of electrons with oppositely directed spins. Each electron occupies a place in the quantum cells of both atoms, i.e. moves in a force field formed by two force centers - the nuclei of hydrogen atoms. This concept of the mechanism of formation of a chemical bond was developed by the scientists Heitler and London on the example of hydrogen. This was extended to more complex molecules. The theory of chemical bond formation developed on this basis was called the method of valence bonds. The VS method gave a theoretical explanation of the most important properties of a covalent bond, made it possible to understand the structure a large number molecules. Although this method did not turn out to be universal and in some cases is not able to correctly describe the structure and properties of molecules, it nevertheless played an important role in the development of the quantum mechanical theory of chemical bonding and has not lost its significance to this day. The VS method is based on the following provisions:
A covalent bond is formed by two electrons with opposite spins, and this electron pair belongs to two atoms.
The stronger the covalent bond, the more the interacting electron clouds overlap.
The geometric shape of the s-orbital is spherical, smeared from the center to the edges (more dense at the core, and less at the edges). The p-electron orbitals are dumbbells directed along the coordinate axes. Clouds of d-electrons have a more complex shape. The orbital hybridization method proceeds from the assumption that during the formation of molecules, instead of the initial s-, p-, d-, f-orbitals (clouds), such equivalent “mixed” or hybrid electron clouds are formed that are elongated towards neighboring atoms, due to which their more complete overlap with electron clouds of other atoms. Energy is expended on hybridization, for which it pays off with more complete overlap. This results in a stronger molecule. The energy spent on hybridization is compensated by the energy released during bond formation. An example is a methane molecule. As a result of the overlap of four hybrid sp3 orbitals of a carbon atom and 4 s orbitals of 4 hydrogen atoms, a tetrahedral model of the methane molecule is formed with four y bonds, at an angle of 1090. If 3-p orbitals hybridize in a molecule, then sp2 hybridization - ethylene molecule, if 2 sp orbitals - hybridization (acetylene). For elements of the 3rd and subsequent periods, d-electrons also participate in the formation of hybrid clouds. In this case, 6 equivalent hybrid clouds are formed, elongated to the vertices of the octahedron sp3 d2-hybridization. The central atom of the complex ion has such hybridization. This explains their octahedral structure.
A covalent bond is directional. The overlap region is located in a certain direction with respect to the interacting atoms.
The nature of the distribution of electrons in molecular orbitals makes it possible to explain magnetic properties particles. Molecules whose total spin is equal to zero exhibit diamagnetic properties, i.e. in an external magnetic field, their own magnetic moments are oriented against the direction of the field. Molecules whose total spin is different from zero exhibit paramagnetic properties, i.e. in an external magnetic field, their own magnetic moments are oriented in the direction of the field. Thus, the H2 molecule is diamagnetic.
The geometric shape of the molecules depends on the direction of the chemical bond. The nuclei of atoms of molecules with sp-hybridization of atomic orbitals are located in the same plane, sp2 - directed to the vertices of the triangle, sp3 - to the vertices of the tetrahedron
