A covalent bond, depending on how the shared electron pair occurs, can be formed by exchange or donor-acceptor mechanism.
Exchange mechanism The formation of a covalent bond is realized in cases where both an atomic orbital and an unpaired electron located in this orbital participate in the formation of a common electron pair from each atom.
For example, in a hydrogen molecule. Interacting hydrogen atoms containing single electrons with opposite spins in atomic s-orbitals form a common electron pair, the movement of which in the H2 molecule occurs within the boundaries of the σ-molecular orbital, which arises when two s-atomic orbitals merge:
In the ammonia molecule, the nitrogen atom, having three single electrons and one electron pair in the four atomic orbitals of the outer energy level, forms three common electron pairs with the s-electrons of three hydrogen atoms. These electron pairs in the NH 3 molecule are located in three σ-molecular orbitals, each of which arises when the atomic orbital of a nitrogen atom merges with the s-orbital of a hydrogen atom:
Thus, in an ammonia molecule, the nitrogen atom forms three σ-bonds with hydrogen atoms and has unshared electron pair.
Donor-acceptor mechanism the formation of a covalent bond occurs in cases where one neutral atom or ion (donor) has an electron pair in the atomic orbital of the outer energy level, and the other ion or neutral atom (acceptor)- free (vacant) orbital. When atomic orbitals merge, a molecular orbital appears in which there is a common electron pair that previously belonged to the donor atom:
According to the donor-acceptor mechanism, for example, the formation of a covalent bond between an ammonia molecule and a hydrogen ion occurs with the appearance of ammonium + ion. In the ammonia molecule, the nitrogen atom in the outer layer has a free electron pair, which allows this molecule to act as a donor. The hydrogen ion (acceptor) has a free s-orbital. Due to the fusion of the atomic orbitals of the nitrogen atom and the hydrogen ion, a σ-molecular orbital arises, and the free pair of electrons of the nitrogen atom becomes common to the connecting atoms:
Or H + + NH 3 [ H NH 3 ] +
In the ammonium ion +, the covalent N-H bond formed by the donor-acceptor mechanism is equal in energy and length to the other three covalent N-H bonds formed by the exchange mechanism.
The boron atom forms the boron fluoride molecule BF 3 due to the overlap of the electron orbitals occupied in the excited state by unpaired electrons with the electron orbitals of fluorine. In this case, the boron atom retains one vacant orbital, due to which a fourth chemical bond can be formed through the donor-acceptor mechanism.
A bond formed by a donor-acceptor mechanism is often called donor-acceptor, coordination or coordinated. However, this is not a special type of bond, but only a different mechanism for the formation of a covalent bond.
The donor-acceptor mechanism for the formation of covalent bonds is characteristic of complex compounds: the role of the acceptor is usually played by d-metal ions, which can usually provide two, four or six free atomic orbitals of the s-, p-, d-type, which significantly expands their ability to form covalent communications.
For example, Ag + and Cu 2+ ions, respectively, provide two and four free atomic orbitals, and the donor of electron pairs can be, for example, two or four molecules of ammonia or cyanide ion:
Acceptor Donor 
In these cases, covalent bonds arise between the donors and the acceptor with the formation of complex cations (silver and copper ammonia) or an anion (copper cyanide).
A covalent bond is a bond that most often binds non-metal atoms in molecules and crystals. We talk about what kind of chemical bond is called covalent in this article.
What is a covalent chemical bond?
A covalent chemical bond is a bond achieved through the formation of shared (bonding) electron pairs.
If there is one common pair of electrons between two atoms, then such a bond is called single; if there are two, it is double; if there are three, it is triple.
A bond is usually denoted by a horizontal line between atoms. For example, in a hydrogen molecule there is a single bond: H-H; in an oxygen molecule there is a double bond: O=O; There is a triple bond in a nitrogen molecule:
Rice. 1. Triple bond in a nitrogen molecule.
The higher the bond multiplicity, the stronger the molecule: the presence of a triple bond explains the high chemical stability of nitrogen molecules.
Formation and types of covalent bonds
There are two mechanisms for the formation of a covalent bond: the exchange mechanism and the donor-acceptor mechanism:
- exchange mechanism. In the exchange mechanism, to form a shared electron pair, the two bonding atoms each provide one unpaired electron. This is exactly what happens, for example, when a hydrogen molecule is formed.
Rice. 2. Formation of a hydrogen molecule.
A common electron pair belongs to each of the bonded atoms, that is, their electron shell is complete.
- donor-acceptor mechanism. In the donor-acceptor mechanism, the shared electron pair is represented by one of the bonding atoms, the one that is more electronegative. The second atom represents an empty orbital for a shared electron pair.
Rice. 3. Formation of ammonium ion.
This is how ammonium ion NH 4 + is formed. This positively charged ion (cation) is formed when ammonia gas reacts with any acid. In an acid solution, there are hydrogen cations (protons), which in a hydrogen environment form the hydronium cation H 3 O+. The formula of ammonia is NH 3: the molecule consists of one nitrogen atom and three hydrogen atoms connected by single covalent bonds via an exchange mechanism. The nitrogen atom remains with one lone pair of electrons. He provides it as a common one, as a donor, to the hydrogen ion H+, which has a free orbital.
Covalent chemical bonds in chemical substances can be polar or non-polar. A bond does not have a dipole moment, that is, polarity, if two atoms of the same element that have the same electronegativity value are bonded. Thus, in a hydrogen molecule the bond is non-polar.
In the hydrogen chloride HCl molecule, atoms with different electronegativity are connected by a covalent single bond. The shared electron pair is shifted towards chlorine, which has a higher electron affinity and electronegativity. A dipole moment arises and the bond becomes polar. In this case, partial charge separation occurs: the hydrogen atom becomes the positive end of the dipole, and the chlorine atom becomes the negative end.
Any covalent bond has the following characteristics: energy, length, multiplicity, polarity, polarizability, saturation, directionality in space
What have we learned?
A covalent chemical bond is formed by the overlap of a pair of valence electron clouds. This type of bond can be formed by a donor-acceptor mechanism, as well as by an exchange mechanism. A covalent bond can be polar or nonpolar and is characterized by the presence of length, multiplicity, polarity, and direction in space.
Test on the topic
Evaluation of the report
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As already mentioned, a common electron pair that carries out a covalent bond can be formed due to unpaired electrons present in unexcited interacting atoms. This occurs, for example, during the formation of molecules such as H2, HC1, Cl2. Here, each atom has one unpaired electron; When two such atoms interact, a common electron pair is created - a covalent bond occurs.
An unexcited nitrogen atom has three unpaired electrons:
Consequently, due to unpaired electrons, the nitrogen atom can participate in the formation of three covalent bonds. This is what happens, for example, in N2 or NH3 molecules, in which the covalency of nitrogen is 3.
However, the number of covalent bonds may be greater than the number of unpaired electrons available to an unexcited atom. Thus, in the normal state, the outer electronic layer of the carbon atom has a structure that is depicted by the diagram:
Due to the available unpaired electrons, a carbon atom can form two covalent bonds. Meanwhile, carbon is characterized by compounds in which each of its atoms is connected to neighboring atoms by four covalent bonds (for example, CO 2, CH 4, etc.). This turns out to be possible due to the fact that with the expenditure of some energy one of the 2x electrons present in the atom can be transferred to sublevel 2 R as a result, the atom goes into an excited state, and the number of unpaired electrons increases. Such an excitation process, accompanied by the “pairing” of electrons, can be represented by the following diagram, in which the excited state is marked with an asterisk next to the element symbol:
There are now four unpaired electrons in the outer electron layer of the carbon atom; therefore, the excited carbon atom can participate in the formation of four covalent bonds. In this case, an increase in the number of covalent bonds created is accompanied by the release of more energy than is expended on transferring the atom to an excited state.
If the excitation of an atom, leading to an increase in the number of unpaired electrons, is associated with very large energy costs, then these costs are not compensated by the energy of formation of new bonds; then such a process as a whole turns out to be energetically unfavorable. Thus, oxygen and fluorine atoms do not have free orbitals in the outer electron layer:
Here, an increase in the number of unpaired electrons is possible only by transferring one of the electrons to the next energy level, i.e. in a state 3s. However, such a transition is associated with a very large expenditure of energy, which is not covered by the energy released when new bonds arise. Therefore, due to unpaired electrons, an oxygen atom can form no more than two covalent bonds, and a fluorine atom can form only one. Indeed, these elements are characterized by a constant covalency equal to two for oxygen and one for fluorine.
Atoms of elements of the third and subsequent periods have an “i-sublevel” in the outer electronic layer, to which they can transition upon excitation s- and p-electrons of the outer layer. Therefore, here additional opportunities arise to increase the number of unpaired electrons. Thus, a chlorine atom, which in an unexcited state has one unpaired electron
can be transferred, with the expenditure of some energy, into excited states (ES), characterized by three, five or seven unpaired electrons: 
Therefore, unlike the fluorine atom, the chlorine atom can participate in the formation of not only one, but also three, five or seven covalent bonds. Thus, in chlorous acid HClO 2 the covalency of chlorine is three, in perchloric acid HClO 3 it is five, and in perchloric acid HClO 4 it is seven. Similarly, a sulfur atom, which also has an unoccupied 36SiO level, can go into excited states with four or six unpaired electrons and, therefore, participate in the formation of not only two, like oxygen, but also four or six covalent bonds. This can explain the existence of compounds in which sulfur exhibits a covalency of four (SO 2, SCl 4) or six (SF 6).
In many cases, covalent bonds also arise due to paired electrons present in the outer electron layer of the atom. Consider, for example, the electronic structure of the ammonia molecule:
Here, the dots indicate electrons that originally belonged to the nitrogen atom, and the crosses indicate those that originally belonged to the hydrogen atoms. Of the eight outer electrons of the nitrogen atom, six form three covalent bonds and are common to the nitrogen atom and hydrogen atoms. But two electrons belong only to nitrogen and form lone electron pair. Such a pair of electrons can also participate in the formation of a covalent bond with another atom if there is a free orbital in the outer electron layer of this atom. An unfilled ls orbital exists, for example, in the hydrogen ion H +, which is generally devoid of electrons:
Therefore, when an NH 3 molecule interacts with a hydrogen ion, a covalent bond occurs between them; the lone pair of electrons on the nitrogen atom becomes shared between the two atoms, resulting in the formation of an ion ammonium NH 4:
Here the covalent bond arose due to a pair of electrons that originally belonged to one atom (donor electron pair), and a free orbital of another atom (acceptor electron pair). This method of forming a covalent bond is called donor-acceptor. In the example considered, the electron pair donor is a nitrogen atom, and the acceptor is a hydrogen atom.
Experience has established that the four N-H bonds in the ammonium ion are equivalent in all respects. It follows from this that a bond formed by the donor-acceptor method does not differ in its properties from a covalent bond created by unpaired electrons of interacting atoms.
Another example of a molecule in which there are bonds formed in a donor-acceptor manner is the molecule of nitrogen oxide (I) N 2 O.
Previously, the structural formula of this compound was depicted as follows:
According to this formula, the central nitrogen atom is connected to neighboring atoms by five covalent bonds, so that its outer electron layer contains ten electrons (five electron pairs). But such a conclusion contradicts the electronic structure of the nitrogen atom, since its outer L-layer contains only four orbitals (one 5- and three p-orbitals) and cannot accommodate more than eight electrons. Therefore, the given structural formula cannot be considered correct.
Let us consider the electronic structure of nitric oxide (I), and the electrons of individual atoms will be alternately designated by dots or crosses. The oxygen atom, which has two unpaired electrons, forms two covalent bonds with the central nitrogen atom:
Due to the unpaired electron remaining on the central nitrogen atom, the latter forms a covalent bond with the second nitrogen atom:
Thus, the outer electronic layers of the oxygen atom and the central nitrogen atom are filled: stable eight-electron configurations are formed here. But the outermost electron layer of the outermost nitrogen atom contains only six electrons; this atom can therefore be an acceptor of another electron pair. The central nitrogen atom adjacent to it has a lone electron pair and can act as a donor. This leads to the formation of another covalent bond between nitrogen atoms by the donor-acceptor method:
Now each of the three atoms that make up the N 2 O molecule has a stable eight-electron structure of the outer layer. If a covalent bond formed by a donor-acceptor method is designated, as is customary, by an arrow directed from the donor atom to the acceptor atom, then the structural formula of nitric oxide (I) can be represented as follows:
Thus, in nitric oxide (I) the covalency of the central nitrogen atom is four, and the outer one is two.
The examples considered show that atoms have a variety of possibilities for the formation of covalent bonds. The latter can be created due to unpaired electrons of an unexcited atom, and due to unpaired electrons appearing as a result of excitation of the atom (“pairing” of electron pairs), and, finally, by the donor-acceptor method. However, the total number of covalent bonds that a given atom can form is limited. It is determined by the total number of valence orbitals, i.e. those orbitals whose use for the formation of covalent bonds turns out to be energetically favorable. Quantum mechanical calculations show that similar orbitals include S- and p-orbitals of the outer electron layer and d-orbitals of the previous layer; in some cases, as we saw with the examples of chlorine and sulfur atoms, the b-orbitals of the outer layer can also be used as valence orbitals.
Atoms of all elements of the second period have four orbitals in the outer electron layer in the absence of ^-orbitals in the previous layer. Consequently, the valence orbitals of these atoms can accommodate no more than eight electrons. This means that the maximum covalency of elements in the second period is four.
Atoms of elements of the third and subsequent periods can be used to form covalent bonds not only s- And R-, but also ^-orbitals. There are known compounds of ^-elements in which the formation of covalent bonds involves s- And R-orbitals of the outer electron layer and all five
The ability of atoms to participate in the formation of a limited number of covalent bonds is called saturation covalent bond.
- A covalent bond formed in a donor-acceptor manner is sometimes called a donor-acceptor bond for short. By this term, however, one should understand not a special type of bond, but only a certain method of formation of a covalent bond.
USING NEW INFORMATION
TECHNOLOGY IN CHEMISTRY LESSONS
Time is quickly moving forward, and if earlier the school needed to create a theoretical base and educational and methodological support, now it has everything necessary to increase the efficiency of its work. And this is a 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 us, and mastering it takes a lot of time. But still very interesting and exciting! Moreover, the result is obvious. Children are interested in lessons; a variety of activities are held very quickly and informatively.
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 so! We, chemistry teachers, 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 branches of industry, agriculture, and without chemicalization the further development of civilization is impossible.
The widespread introduction of chemicals, substances, methods and technological techniques requires highly educated specialists with a solid base of chemical knowledge. For this purpose, our school has a specialized chemical and biological class, which provides high-quality preparation for schoolchildren to continue their 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 become familiar with professions related directly to the subjects of chemistry and biology. Even if students do not choose a chemical and biological major in high school, knowledge about substances that they constantly encounter in everyday life will be useful in life.
In elective course classes, the first place is given to lectures. When preparing for them, I use online information resources. Many illustrations, diagrams, video collections, laboratory materials, slides are displayed on the screen, and based on them I tell my story. My technology of explanation 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 amount of time is allocated for laboratory classes. But it happens that some reagents are not available in the laboratory, and a virtual laboratory comes to the rescue. Using a special program, students can conduct a virtual experiment. The children study the effect of synthetic detergents on various types of fabrics, the solubility of mineral fertilizers in water, the medium of their solution, and 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 take a test on any topic studied. They are faced with the task of choosing in what form to summarize. The most traditional one is a test in the form of an abstract, message or report. To prepare them, children use materials from Internet resources. In this, of course, I help them: I clearly set the task, while formulating the questions that 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. Searching for information is not complete without using the power of the Internet. Before releasing them into a free search, I give them orientation: search techniques, keywords, phrases, names of search engines that may be useful to work with, addresses of Internet sites.
Children also choose a test in the form of a game, tasks and exercises for which they develop themselves. This could be a spin test, “Smart Men and Women”, “How to Become a Millionaire?”, “What? Where? When?”, various puzzles.
I also arrange a presentation of the resulting product using remote technologies. By posting the results of their activities on the Internet on the school or class website, students have the opportunity to evaluate their work not only with the help of their classmates, but also with children and teachers from other schools, discuss these results, and look at them with different eyes.
From the point of view of new media pedagogy, we live in extremely interesting times. The rapid introduction of modern technologies forces us to approach old positions in a new way. Pre-professional training at our school has been going on for four years, and every time I review the course of the lessons, because... New perspectives are opening up, fruitful connections are emerging between traditional teaching methods and new challenges 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, passion, individual activity, creativity, they actively collaborate and exchange their own opinions.
I am convinced that the use of information technology can provide a developed educational culture. This is success in teaching and learning. Use information technology! Move from old forms of exercise that have lost their effectiveness to newer, more advanced and modern ones!
The use of new information technologies in the educational process can be illustrated by the example of one of the lessons in general chemistry in the 11th grade.
Mechanism of formation and properties of covalent bonds
The purpose of the lesson. Recall from the 8th grade course the mechanism of formation of a covalent bond, study the donor-acceptor mechanism and the properties of a covalent bond.
Equipment. Table of electronegativity of chemical elements, codograms of st- and l-bonds, educational disk “General Chemistry” from the series of educational 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 using the educational disk “General Chemistry”.
Repetition of covered material
Recall with students how bonds are formed between non-metal atoms. Complete tasks 1, 2 on the work card (see appendix).
Learning new material
Mechanism of covalent bond formation:
a) exchange (for example, H 2, Cl 2, HC1);
b) donor-acceptor (using the example of NH 4 C1).
Immediately, students write down their homework in the margins: Describe the formation of the hydronium ion H 3 ABOUT + from H ion + and water molecules.
Types of covalent bonds: polar and nonpolar (according to the composition of the molecule).
Properties of covalent bonds.
Multiplicity(single, one and a half, double, triple).
Communication energy- this is the amount of energy released during the formation of a chemical bond or spent on its breaking.
Link length is the distance between the nuclei of atoms in a molecule.
Energy and bond length are interrelated. Show with an example how these properties are interconnected, how they affect the strength of the molecule (project onto the board):
As the number of bonds between atoms in a molecule increases, the bond length decreases and its energy increases, for example (project onto the board):Saturability is the ability of atoms to form a certain and limited number of bonds. Show with examples of ball-and-rod
molecules Cl 2, H 2 O, CH 4, HNO 3.
Directionality. Consider drawings of the overlap of electron clouds during the formation of σ- and π-bonds, project them 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.
(By the way, it forms water -
A very significant point).
Let's imagine its molecule
Convenient formula H 2.
Let us significantly add -
There is no lighter substance in the world!
2. N 2 - nitrogen molecule.
It is known to be colorless
gas. A lot of knowledge, but let's
Let's replenish their stock.
3. He is everywhere and everywhere:
And in stone, in air, in water,
He is in the morning dew,
And in the blue sky.
(Oxygen.)
4. Mushroom pickers found a small swamp in the forest, from which gas bubbles were bursting out in places. The gas flared up from the match, and a faintly glowing flame began to wander through the swamp. What kind of gas is this? (Methane.)
Continuation of the lesson.
Polarizability- this 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 polarizability of the molecule).
Reinforcing the material learned
Control on the studied topic is carried out using remote controls.
The survey is conducted for 3 minutes, 10 questions worth one point, 30 seconds are allotted for answers, questions are projected onto the interactive board. If you score 9-10 points - score “5”, 7-8 points – score “4”, 5-6 points – score “3”.
Questions for consolidation
1. A bond that is formed due to shared 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 of formation of a covalent bond due to a lone electron pair of one atom and a free orbital of another is called:
a) donor-acceptor; b) inert; c) catalytic.
4. Which molecule has a covalent bond?
a) Zn; b) Cu O; c) NH 3.
5. The bond multiplicity in a nitrogen molecule is equal to:
a) three; b) two; c) one.
6. The bond length is the shortest in a molecule:
a) H 2 S; b) SF 6; 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. The 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) ball; b) irregular eight; c) regular 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. 11th grade" (M.: Bustard, 2006), notes in a notebook.
Application
Work card
1. Match the names of the substance and the type of bond.
1) Potassium chloride;
2) oxygen;
3) magnesium;
4) carbon tetrachloride.
a) Covalent nonpolar;
b) ionic;
c) metal;
d) covalent polar.
2. Between the atoms of which elements the chemical bond will be ionic in nature?
a) NnO; b) Si and C1; c) Na and O; d) P and Br.
3. The length of the connection 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. Which molecule has a stronger hydrogen bond: H 2 O or H 2 S?
6. Continue the sentence: “The bond formed by the overlap of electron clouds along the line connecting the nuclei of atoms is called............................. ......",
7. Draw diagrams of the overlap of electron orbitals during the formation of a π bond.
8. Homework. “General chemistry in tests, problems, exercises” by O.S. Gabrielyan (M.: Drofa, 2003), work 8A, option 1, 2.
The idea of forming a chemical bond using a pair of electrons belonging to both connecting atoms was expressed in 1916 by the American physical chemist J. Lewis.
Covalent bonds exist between atoms in both molecules and crystals. It occurs both between identical atoms (for example, in H2, Cl2, O2 molecules, in a diamond crystal) and between different atoms (for example, in H2O and NH3 molecules, in SiC crystals). Almost all bonds in molecules of organic compounds are covalent (C-C, C-H, C-N, etc.).
There are two mechanisms for the formation of covalent bonds:
1) exchange;
2) donor-acceptor.
Exchange mechanism of covalent bond formationlies in the fact that each of the connecting atoms provides one unpaired electron for the formation of a common electron pair (bond). The electrons of interacting atoms must have opposite spins.
Let us consider, for example, the formation of a covalent bond in a hydrogen molecule. When hydrogen atoms come closer, their electron clouds penetrate into each other, which is called overlapping of electron clouds (Fig. 3.2), the electron density between the nuclei increases. The nuclei attract each other. As a result, the energy of the system decreases. When atoms come very close together, the repulsion of nuclei increases. Therefore, there is an optimal distance between the nuclei (bond length l), at which the system has minimum energy. In this state, energy is released, called the binding energy E St.
Rice. 3.2. Diagram of electron cloud overlap during the formation of a hydrogen molecule
Schematically, the formation of a hydrogen molecule from atoms can be represented as follows (a dot means an electron, a line means a pair of electrons):
N + N→N: N or N + N→N - N.
In general terms for AB molecules of other substances:
A + B = A: B.
Donor-acceptor mechanism of covalent bond formationlies in the fact that one particle - the donor - represents an electron pair to form a bond, and the second - the acceptor - represents a free orbital:
A: + B = A: B.
donor acceptor
Let's consider the mechanisms of formation of chemical bonds in the ammonia molecule and ammonium ion.
1. Education
The nitrogen atom has two paired and three unpaired electrons at its outer energy level:
The hydrogen atom in the s sublevel has one unpaired electron.
In the ammonia molecule, the unpaired 2p electrons of the nitrogen atom form three electron pairs with the electrons of 3 hydrogen atoms:
In the NH 3 molecule, 3 covalent bonds are formed according to the exchange mechanism.
2. Formation of a complex ion - ammonium ion.
NH 3 + HCl = NH 4 Cl or NH 3 + H + = NH 4 +
The nitrogen atom remains with a lone pair of electrons, i.e. two electrons with antiparallel spins in one atomic orbital. The atomic orbital of the hydrogen ion contains no electrons (vacant orbital). When an ammonia molecule and a hydrogen ion approach each other, an interaction occurs between the lone pair of electrons of the nitrogen atom and the vacant orbital of the hydrogen ion. The lone pair of electrons becomes common to the nitrogen and hydrogen atoms, and a chemical bond occurs according to the donor-acceptor mechanism. The nitrogen atom of the ammonia molecule is the donor, and the hydrogen ion is the acceptor:
It should be noted that in the NH 4 + ion all four bonds are equivalent and indistinguishable; therefore, in the ion the charge is delocalized (dispersed) throughout the complex.
The considered examples show that the ability of an atom to form covalent bonds is determined not only by one-electron, but also by 2-electron clouds or the presence of free orbitals.
According to the donor-acceptor mechanism, bonds are formed in complex compounds: - ;
2+ ;
2- etc.
A covalent bond has the following properties:
- saturation;
