There can be covalent bonds between atoms. Covalent bond (CC)
The idea of the formation of a chemical bond with the help of a pair of electrons belonging to both connecting atoms was put forward in 1916 by the American physicist and chemist J. Lewis.
A covalent bond exists between atoms in both molecules and crystals. It occurs both between the same atoms (for example, in the molecules of H2, Cl 2, O 2, in a diamond crystal), and between different atoms (for example, in the molecules of H 2 O and NH 3, in SiC crystals). Almost all bonds in molecules organic compounds are covalent (C-C, C-H, C-N, etc.).
There are two mechanisms of formation covalent bond:
1) exchange;
2) donor-acceptor.
Exchange mechanism for the formation of a covalent bondlies 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 the interacting atoms must then have opposite spins.
Consider, for example, the formation of a covalent bond in a hydrogen molecule. When hydrogen atoms approach each other, 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 are attracted to each other. As a result, the energy of the system is reduced. With a very strong approach of atoms, the repulsion of the nuclei increases. Therefore, there is an optimal distance between the nuclei (bond length l) at which the system has the minimum energy. In this state, energy is released, called the binding energy E St.
Rice. 3.2. Diagram of the overlapping of electron clouds 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):
H + H → H: H or H + H → H - N.
In general, 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 - presents an electron pair for the formation of a bond, and the second - an acceptor - a free orbital:
A: + B = A: B.
donor acceptor
Let us consider the mechanisms of the formation of chemical bonds in the ammonia molecule and the ammonium ion.
1. Education
The nitrogen atom has on the outside energy level two paired and three unpaired electrons:
The hydrogen atom at 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 by the exchange mechanism.
2. Formation of a complex ion - an ammonium ion.
NH 3 + HCl = NH 4 Cl or NH 3 + H + = NH 4 +
The nitrogen atom has an unshared pair of electrons, that is, two electrons with antiparallel spins in one atomic orbital. Atomic orbital hydrogen ion does not contain electrons (vacant orbital). When the ammonia molecule and the hydrogen ion approach each other, the lone pair of electrons of the nitrogen atom and the vacant orbital of the hydrogen ion interact. An unshared pair of electrons becomes common for nitrogen and hydrogen atoms, a chemical bond arises 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, the charge in the ion is delocalized (dispersed) throughout the complex.
The examples considered show that the ability of an atom to form covalent bonds is due not only to one-electron, but also to 2-electron clouds or the presence of free orbitals.
By the donor-acceptor mechanism, bonds are formed in complex compounds: -; 2+; 2- etc.
A covalent bond has the following properties:
- saturation;
- focus;
- polarity and polarizability.
KS- a bond carried out due to an electron pair belonging to both atoms.
Conditions for the formation of the COP: it forms between atoms with high electronegativity. (electrons - the ability of atoms to attract electrons to themselves).
∆Χ is the difference of electronegativity of 2 atoms, if ∆Χ≤1.4, the bond is polar
KS m. formed:
1 - between any atoms of non-metals (since all non-metals high values electricity), pr: HCl, the values of electricity are according to the tables, for H = 2.1, for Cl = 3.1, - ∆Χ = 3.1-2.1 = 1≤1.4, this is a covalent and polar bond.
2 - between the atoms of a non-metal and a metal, if the metal is in high degree oxidation, pr: CrCl6 for Cr = 2.4, ∆Χ = 3.1-2.4 = 0.7≤1.4 is a covalent polar bond.
Mechanisms of CS formation:
1- exchange mechanism- 2 atoms exchange electrons, forming a common electron pair belonging to both and called "shared". An example is the molecules of volatiles inorganic compounds: HCl, H 2 O, H 2 S, NH 3, etc. The formation of the HCl molecule can be represented by the scheme H. +. Сl: = Н: Cl: The electron pair is shifted towards 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). Consider as an example the mechanism of formation of an ammonium ion. In the ammonia molecule, the nitrogen atom has a lone 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. This means that the fourth covalent bond arises. The formation of an ammonium ion can be represented by the diagram
| + □ 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 is polar (complex molecules) and non-polar (simple molecules).
Covalent bond properties
The covalent bond has a number of important properties. These include saturation and focus.
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 the hydrogen atoms interact, the Н 2 molecule is formed, and not the Н 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 of various elements is limited by obtaining the maximum number of unpaired valence electrons.
Focus- a property of a covalent bond that determines the geometric structure of a molecule. The reason for the directionality of the coupling lies in the fact that the overlapping 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.
A covalent bond is a bond that most often binds atoms of non-metals in molecules and crystals. We are talking 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 carried out through the formation of common (bonding) electron pairs.
If there is one common electron pair between two atoms, then such a bond is called single (ordinary), if two - double, if three - triple.
The bond is usually denoted by a horizontal bar between atoms. For example, in a hydrogen molecule there is a single bond: H-H; in the oxygen molecule there is a double bond: O = O; in the nitrogen molecule there is a triple bond:
Rice. 1. Triple bond in the nitrogen molecule.
The higher the multiplicity of the bond, 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 for the formation of a common electron pair, two binding atoms provide one unpaired electron each. 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 connected atoms, that is, their electron shell is complete.
- donor-acceptor mechanism... In the donor-acceptor mechanism, a common electron pair is represented by one of the bonding atoms, the one that is more electronegative. The second atom represents a free orbital for a common electron pair.
Rice. 3. Formation of an ammonium ion.
This is how the ammonium ion NH 4 + is formed. This positively charged ion (cation) is formed when ammonia gas interacts with any acid. In an acid solution, there are hydrogen cations (protons), which in a hydrogen medium form the hydronium cation H 3 O +. The formula for ammonia NH 3: the molecule consists of one nitrogen atom and three hydrogen atoms linked by single covalent bonds by an exchange mechanism. The nitrogen atom is left with one lone electron pair. It provides it as a common one, as a donor, to the hydrogen ion H +, which has a free orbital.
Covalent chemical bond in chemicals can be polar and non-polar. A bond does not have a dipole moment, that is, polarity, if two atoms of the same element are bonded, having the same electronegativity value. So, 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 total electron pair turns out to be shifted towards chlorine, which has a higher electron affinity and electronegativity. A dipole moment arises, the bond becomes polar. In this case, a partial separation of charge occurs: the hydrogen atom becomes the positive end of the dipole, and the chlorine atom becomes negative.
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 overlapping a pair of valence electron clouds. This type of bond can be formed by the donor-acceptor mechanism, as well as by the exchange mechanism. The covalent bond is polar and non-polar and is characterized by the presence of length, multiplicity, polarity, directionality in space.
Test by topic
Assessment of the report
Average rating: 4.2. Total ratings received: 164.
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 happens, for example, during the formation of molecules such as H2, HC1, Cl2. Here each of the atoms has one unpaired electron; when two such atoms interact, a common electron pair is created - a covalent bond arises.
There are three unpaired electrons in an unexcited nitrogen atom:
Consequently, due to unpaired electrons, the nitrogen atom can participate in the formation of three covalent bonds. This happens, for example, in molecules N 2 or NH 3, in which the covalence of nitrogen is 3.
However, the number of covalent bonds can be more numbers unpaired electrons of an unexcited atom. So, in the normal state, the outer electronic layer of a 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 bonded 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 the sublevel 2 R as a result, the atom goes over into an excited state, and the number of unpaired electrons increases. Such an excitation process, accompanied by the "steaming" of electrons, can be represented by the following diagram, in which the excited state is marked with an asterisk at the element symbol:
There are now four unpaired electrons in the outer electron layer of the carbon atom; therefore, an excited carbon atom can participate in the formation of four covalent bonds. In this case, an increase in the number of created covalent bonds is accompanied by the release of more energy than is spent on transferring an 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 expenditures of energy, then these expenditures are not compensated by the energy of the 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 - only one. Indeed, these elements are characterized by a constant covalence equal to two for oxygen and one for fluorine.
The atoms of the elements of the third and subsequent periods have an "i-sublevel in the outer electron layer, to which, upon excitation, they can pass s- and p-electrons of the outer layer. Therefore, here appear additional features increasing the number of unpaired electrons. So, a chlorine atom, which has one unpaired electron in an unexcited state
can be converted at the expense of some energy into excited states (SR), 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. So, in chlorous acid HClO 2, the covalence of chlorine is three, in chloric acid HClO 3 - five, and in perchloric acid HClO 4 - seven. Similarly, a sulfur atom, which also has an unoccupied 3bCio level, can pass into excited states with four or six unpaired electrons and, therefore, participate in the formation of not only two, as in oxygen, but also four or six covalent bonds. This can explain the existence of compounds in which sulfur exhibits covalence equal to 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 an ammonia molecule:
Here, the dots represent the electrons that originally belonged to the nitrogen atom, and the crosses - those that 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 empty ls-orbital is present, for example, for the hydrogen ion H +, which is generally devoid of electrons:
Therefore, when the NH 3 molecule interacts with a hydrogen ion, a covalent bond arises between them; the lone pair of electrons of the nitrogen atom becomes common for two atoms, as a result of which an ion is formed ammonium NH 4:
Here, the covalent bond arose due to a pair of electrons that originally belonged to one atom (donor electron pair), and the free orbital of another atom (acceptor electronic pair). This method of forming a covalent bond is called donor-acceptor. In the considered example, the donor of the electron pair is the nitrogen atom, and the acceptor is the hydrogen atom.
Experience has established that four communication N-H in the ammonium ion are equivalent in all respects. It follows from this that the bond formed by the donor-acceptor method does not differ in its properties from the covalent bond created by the unpaired electrons of the interacting atoms.
Another example of a molecule in which there are bonds formed by the donor-acceptor method is the molecule of nitric 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 bonded to neighboring atoms by five covalent bonds, so that there are ten electrons (five electron pairs) in its outer electron layer. But this 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.
Consider the electronic structure of nitric oxide (I), and the electrons of individual atoms will be alternately denoted 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 at the central nitrogen atom, the latter forms a covalent bond with the second nitrogen atom:
Thus, the outer electron layers of the oxygen atom and the central nitrogen atom are filled: here, stable eight-electron configurations are formed. But in the outer electron layer of the outermost nitrogen atom there are 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 the covalent bond formed by the 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 covalence of the central nitrogen atom is four, and the extreme 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 both at the expense of unpaired electrons of an unexcited atom, and at the expense of unpaired electrons appearing as a result of the excitation of an atom ("unpairing" 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 total valence orbitals, i.e. those orbitals, the use of which for the formation of covalent bonds turns out to be energetically favorable. Quantum-mechanical calculation shows that such orbitals include S- and p-orbital of the outer electron layer and d-orbital of the previous layer; in some cases, as we have seen with the examples of chlorine and sulfur atoms, the bf-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 d-orbitals in the previous layer. Consequently, the valence orbitals of these atoms can accommodate no more than eight electrons. This means that the maximum covalence of the elements of 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. Known compounds of ^ -elements in which the formation of covalent bonds is involved 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 by a donor-acceptor method is sometimes briefly referred to as a donor-acceptor bond. However, this term should not be understood as a special type of bond, but only as a certain way of forming a covalent bond.
Chemical bond.
Different substances have different structure... Of all the substances known to date, only inert gases exist in the form of free (isolated) atoms, which is due to their high stability electronic structures... All other substances (and there are currently more than 10 million of them) are composed of bound atoms.
Note: those parts of the text that can not be learned or understood are highlighted in italics.
The formation of molecules from atoms leads to an energy gain, since under normal conditions the molecular state is more stable than the atomic one.
An atom at the outer energy level can contain from one to eight electrons. If the number of electrons at the outer level of the 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 on 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.
The 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 formation of a covalent bond using the example of the formation of a hydrogen molecule:
H + H = H 2; Q = 436 kJ
The nucleus of a free hydrogen atom is surrounded by a spherically symmetric electron cloud formed by 1 s-electron. When atoms approach to a certain distance, their electron clouds (orbitals) partially overlap
As a result, a molecular two-electron cloud appears between the centers of both nuclei, which has the maximum electron density in the space between the nuclei; an increase in the density of 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 the distance between the nuclei of the hydrogen atoms that have come close to touching is 0.106 nm, then after the overlap of the electron clouds (the formation of the H2 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 during the formation of a σ-bond). The greater the overlap of electron orbitals, the stronger the chemical bond. As a result of the formation of a chemical bond between two hydrogen atoms, each of them reaches the electronic configuration of the noble gas, helium.
It is customary to depict chemical bonds in different ways:
1) with the help of electrons in the form of dots, set 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 depicted by a dash (stroke) (for example, H-H), which symbolizes a common pair of electrons.
The covalent bond in the chlorine molecule is also carried out using two common electrons, or an electron pair:
Lone pair of electrons, there are 3 of them in the 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 the 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 depicted with one dash (stroke), a double bond with two, and a triple bond with three. A dash between two atoms shows that they have a generalized pair of electrons, as a result of which a chemical bond is formed. With the help of such dashes, the structural formulas of molecules are depicted.
So, in a chlorine molecule, each of its atoms has a complete outer level of eight electrons (s 2 p 6), and two of them (an electron pair) equally belong to both atoms. The overlapping 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, the nitrogen molecule is stronger than the hydrogen or chlorine molecule, which is the reason for the significant inertness of nitrogen in chemical reactions.
The chemical bond carried out by electron pairs is called covalent.
Mechanisms of covalent bond formation.
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 provide the same number of electrons for general use.
Another mechanism of its formation is also possible - donor-acceptor. In this case, the chemical bond arises due to unshared an electron pair of one atom and free orbitals of another atom.
Let us consider as an example the mechanism of formation of the ammonium ion NH 4 +
When ammonia interacts with HCl, chemical reaction:
NH 3 + HCl = NH 4 Cl or in abbreviated ionic form: NH 3 + H + = NH 4 +
In this case, the nitrogen atom in the ammonia molecule has unshared a couple of electrons (two-electron cloud):