covalent bond is carried out due to the socialization of electrons belonging to both atoms participating in the interaction. The electronegativities of non-metals are large enough that electron transfer does not occur.
Electrons in overlapping electron orbitals are shared. 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.
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 education:
- donor-acceptor;
- exchange.
In the first case, one of the atoms provides its pair of electrons, and the second - a free electron orbital.
In the second, one electron from each participant in the interaction comes to the common pair.
Depending on what type they are- atomic or molecular, compounds with a similar type of bond can vary significantly in physicochemical 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), nitric 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.
Substances atomic exist in the form of strong crystals with high boiling and melting points, are insoluble in water and other solvents, many do not conduct electricity. An example is a diamond, which has exceptional strength. This is due to the fact that diamond is a crystal consisting of carbon atoms connected 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, triple ones have more energy and are more durable than double and single ones.
The covalent bond can be It is formed both between two atoms of the same 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 of the electronegativity that make up the molecules of atoms. The absence of a difference in electronegativity will determine non-polarity. If there is a difference, then the molecule will be polar.
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Covalent non-polar chemical bond
Typical for simple substances non-metals. The electrons belong to the atoms equally, and there is no displacement of the electron density.
The following molecules are examples:
H2, O2, O3, N2, F2, Cl2.
Exceptions are inert gases. Their external energy level is completely filled, and the formation of molecules is energetically unfavorable 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 of the electronegativity of the elements do not actually differ, which means that there will be no displacement of the electron pair.
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 an element with greater electronegativity attracts common electrons closer to itself.Scheme of the formation of a covalent polar bond
Depending on the mechanism of formation, common can become electrons of one or both atoms.
The picture clearly shows the interaction in the molecule of hydrochloric acid.
A pair of electrons belongs to both one atom and the second, both of them, so the outer levels are filled. But 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
Communication can be characterized by the following properties: directivity, polarity, polarizability and saturation.
Due to which molecules of inorganic and organic substances are formed. A chemical bond appears during the interaction of electric fields that are created by the nuclei and electrons of atoms. Therefore, the formation of a covalent chemical bond is associated with electrical nature.
What is a connection
This term refers to the result of the action of two or more atoms, which lead to the formation of a strong polyatomic system. The main types of chemical bonds are formed when the energy of the reacting atoms decreases. In the process of bond formation, atoms try to complete their electron shell.
Communication types
In chemistry, there are several types of bonds: ionic, covalent, metallic. There are two types of covalent bonds: polar and non-polar.
What is the mechanism of its creation? A covalent non-polar chemical bond is formed between atoms of identical non-metals that have the same electronegativity. In this case, common electron pairs are formed.
non-polar bond
Examples of molecules that have a non-polar covalent chemical bond include halogens, hydrogen, nitrogen, oxygen.
This connection was first discovered in 1916 by the American chemist Lewis. First, he put forward a hypothesis, and it was confirmed only after experimental confirmation.
A covalent chemical bond is associated with electronegativity. For non-metals, it has a high value. In the course of the chemical interaction of atoms, it is not always possible to transfer electrons from one atom to another; as a result, they are combined. A true covalent chemical bond appears between the atoms. Grade 8 of the regular school curriculum involves a detailed consideration of several types of communication.
Substances that have this type of bond normal conditions- liquids, gases, and solids with a low melting point.
Types of covalent bond
Let's dwell on this issue in more detail. What are the types of chemical bonds? The covalent bond exists in exchange, donor-acceptor variants.
The first type is characterized by the return of one unpaired electron by each atom to the formation of a common electronic bond.
Electrons united in a common bond must have opposite spins. Hydrogen can be considered as an example of this type of covalent bond. When its atoms approach each other, their electron clouds penetrate each other, which is called in science the overlapping of electron clouds. As a result, the electron density between the nuclei increases, and the energy of the system decreases.
At the minimum distance, the hydrogen nuclei repel each other, resulting in some optimal distance.
In the case of a donor-acceptor type of covalent bond, one particle has electrons, it is called a donor. The second particle has a free cell in which a pair of electrons will be placed.
polar molecules
How are polar covalent bonds formed? They arise in those situations when the bonded atoms of nonmetals have different electronegativity. In such cases, the socialized electrons are located closer to the atom, which has a higher electronegativity value. As an example of a covalent polar bond, bonds that arise in a hydrogen bromide molecule can be considered. Here, the public electrons that are responsible for the formation of a covalent bond are closer to bromine than to hydrogen. The reason for this phenomenon is that bromine has a higher electronegativity than hydrogen.
Methods for determining a covalent bond
How to identify covalent polar chemical bonds? To do this, you need to know the composition of the molecules. If it contains atoms of different elements, there is a covalent polar bond in the molecule. Nonpolar molecules contain atoms of one chemical element. Among the tasks offered under the school course chemistry, there are those that involve identifying the type of connection. Tasks of this type are included in the tasks of the final certification in chemistry in the 9th grade, as well as in the tests of the unified state exam in chemistry in 11th grade.
Ionic bond
What is the difference between covalent and ionic chemical bonds? If a covalent bond is characteristic of non-metals, then ionic bond is formed between atoms that have significant differences in electronegativity. For example, this is typical for compounds of elements of the first and second groups of the main PS subgroups (alkaline and alkaline earth metals) and elements of groups 6 and 7 of the main subgroups of the periodic table (chalcogens and halogens).
It is formed as a result of the electrostatic attraction of ions with opposite charges.
Ionic bond features
Because force fields oppositely charged ions are distributed evenly in all directions, each of them is able to attract particles opposite in sign. This characterizes the nondirectionality of the ionic bond.
The interaction of two ions with opposite signs does not imply complete mutual compensation of individual force fields. This contributes to the preservation of the ability to attract ions in other directions, therefore, unsaturation of the ionic bond is observed.
In an ionic compound, each ion has the ability to attract a certain number of others with opposite signs to itself in order to form an ionic crystal lattice. There are no molecules in such a crystal. Each ion is surrounded in a substance by a specific number of ions of a different sign.
metal connection
This type of chemical bond has certain individual features. Metals have an excess number of valence orbitals with a lack of electrons.
When individual atoms approach each other, their valence orbitals overlap, which contributes to the free movement of electrons from one orbital to another, making a connection between all metal atoms. These free electrons are the main feature metallic bond. It does not have saturation and directionality, since the valence electrons are distributed evenly throughout the crystal. The presence of free electrons in metals explains some of their physical properties: metallic luster, plasticity, malleability, thermal conductivity, opacity.
A type of covalent bond
It is formed between a hydrogen atom and an element that has a high electronegativity. There are intra- and intermolecular hydrogen bonds. This kind of covalent bond is the most fragile, it appears due to the action of electrostatic forces. The hydrogen atom has a small radius, and when this one electron is displaced or given away, hydrogen becomes a positive ion, acting on an atom with a large electronegativity.
Among characteristic properties covalent bonds are distinguished: saturation, directivity, polarizability, polarity. Each of these indicators has a certain value for the formed compound. For example, direction is geometric shape molecules.
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 physical chemist J. Lewis.
A covalent bond exists between atoms both in molecules and in crystals. It occurs both between identical atoms (for example, in H 2, Cl 2, O 2 molecules, in a diamond crystal), and between different atoms (for example, in H 2 O and NH 3 molecules, in SiC crystals). Almost all bonds in the molecules of organic compounds are covalent (C-C, C-H, C-N, etc.).
There are two mechanisms for the formation of a covalent bond:
1) exchange;
2) donor-acceptor.
Exchange mechanism for the formation of a covalent bondis that each of the connecting atoms provides for the formation of a common electron pair (bond) by one unpaired electron. The electrons of the interacting atoms must 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 each other, which is called the overlap 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 decreases. With a very strong approach of atoms, the repulsion of nuclei increases. Therefore, there is an optimal distance between the nuclei (bond length l) at which the system has a minimum energy. In this state, energy is released, called the binding energy E St.
Rice. 3.2. Scheme of overlapping 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 bar means a pair of electrons):
H + H→H: H or H + H→H - H.
IN general view for AB molecules of other substances:
A + B = A: B.
Donor-acceptor mechanism of covalent bond formationconsists in the fact that one particle - the donor - presents an electron pair for the formation of a bond, and the second - the acceptor - a free orbital:
A: + B = A: B.
donor acceptor
Consider the mechanisms of formation of chemical bonds in the ammonia molecule and the ammonium ion.
1. Education
The nitrogen atom has two paired and three unpaired electrons in its outer energy level:
The hydrogen atom on 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. The formation of a complex ion - an ammonium ion.
NH 3 + HCl = NH 4 Cl or NH 3 + H + = NH 4 +
The nitrogen atom has a lone pair of electrons, i.e. two electrons with antiparallel spins in the same atomic orbital. atomic orbital the hydrogen ion does not contain electrons (a vacant orbital). When an ammonia molecule and a hydrogen ion approach each other, the lone pair of electrons of the nitrogen atom and the vacant orbital of the hydrogen ion interact. The 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, in the ion the charge is delocalized (dispersed) over the entire 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 by 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:
- satiety;
- orientation;
- polarity and polarizability.
covalent bond(from the Latin "with" jointly and "vales" valid) is carried out by an electron pair belonging to both atoms. Formed between atoms of non-metals.
The electronegativity of non-metals is quite large, so that during the chemical interaction of two non-metal atoms, the complete transfer of electrons from one to the other (as in the case) is impossible. In this case, electron pooling is necessary to perform.
As an example, let's discuss the interaction of hydrogen and chlorine atoms:
H 1s 1 - one electron
Cl 1s 2 2s 2 2 p6 3 s2 3 p5 - seven electrons in the outer level
Each of the two atoms lacks one electron in order to have a complete outer electron shell. And each of the atoms allocates “for common use” one electron. Thus, the octet rule is satisfied. The best way to represent this is with the Lewis formulas:
Formation of a covalent bond
The shared electrons now belong to both atoms. The hydrogen atom has two electrons (its own and the shared electron of the chlorine atom), and the chlorine atom has eight electrons (its own plus the shared electron of the hydrogen atom). These two shared electrons form a covalent bond between the hydrogen and chlorine atoms. The particle formed when two atoms bond is called molecule.
Non-polar covalent bond
A covalent bond can form between two the same atoms. For example:
This diagram explains why hydrogen and chlorine exist as diatomic molecules. Thanks to the pairing and socialization of two electrons, it is possible to fulfill the octet rule for both atoms.
In addition to single bonds, a double or triple covalent bond can be formed, as, for example, in oxygen O 2 or nitrogen N 2 molecules. Nitrogen atoms each have five valence electrons, so three more electrons are required to complete the shell. This is achieved by sharing three pairs of electrons, as shown below:
Covalent compounds are usually gases, liquids, or relatively low-melting solids. One of the rare exceptions is diamond, which melts above 3,500°C. This is due to the structure of diamond, which is a continuous lattice of covalently bonded carbon atoms, and not a collection of individual molecules. In fact, any diamond crystal, regardless of its size, is one huge molecule.
A covalent bond occurs when the electrons of two nonmetal atoms join together. The resulting structure is called a molecule.
Polar covalent bond
In most cases, two covalently bonded atoms have different electronegativity and shared electrons do not belong equally to two atoms. Most of the time they are closer to one atom than to another. In a molecule of hydrogen chloride, for example, the electrons that form a covalent bond are located closer to the chlorine atom, since its electronegativity is higher than that of hydrogen. However, the difference in the ability to attract electrons is not so great that there is a complete transfer of an electron from a hydrogen atom to a chlorine atom. Therefore, the bond between hydrogen and chlorine atoms can be viewed as a cross between an ionic bond (full electron transfer) and a non-polar covalent bond (symmetrical arrangement of a pair of electrons between two atoms). The partial charge on atoms is denoted by the Greek letter δ. Such a connection is called polar covalent bond, and the hydrogen chloride molecule is said to be polar, that is, it has a positively charged end (hydrogen atom) and a negatively charged end (chlorine atom).
The table below lists the main types of bonds and examples of substances:
Exchange and donor-acceptor mechanism of covalent bond formation
1) Exchange mechanism. Each atom contributes one unpaired electron to a common electron pair.
2) Donor-acceptor mechanism. One atom (donor) provides an electron pair, and another atom (acceptor) provides an empty orbital for this pair.
Far from the last role at the chemical level of the organization of the world is played by the way the structural particles are connected, interconnected. The vast majority of simple substances, namely non-metals, have a covalent non-polar type of bond, with the exception of metals in their pure form, they have a special bonding method, which is realized through the socialization of free electrons in the crystal lattice.
The types and examples of which will be indicated below, or rather, the localization or partial displacement of these bonds to one of the binding participants, is explained precisely by the electronegative characteristic of one or another element. The shift occurs to the atom in which it is stronger.
Covalent non-polar bond
The "formula" of a covalent non-polar bond is simple - two atoms of the same nature unite the electrons of their valence shells into a joint pair. Such a pair is called shared because it equally belongs to both participants in the binding. It is thanks to the socialization of the electron density in the form of a pair of electrons that the atoms pass into a more stable state, since they complete their external electronic level, and the “octet” (or “doublet” in the case of a simple hydrogen substance H 2, it has a single s-orbital, for completion of which two electrons are needed) is the state of the outer level, to which all atoms aspire, since its filling corresponds to the state with the minimum energy.
An example of a non-polar covalent bond is in the inorganic and, no matter how strange it may sound, but also in organic chemistry Same. This type of bond is inherent in all simple substances - non-metals, except for noble gases, since the valence level of an inert gas atom is already completed and has an octet of electrons, which means that bonding with a similar one does not make sense for it and is even less energetically beneficial. In organics, non-polarity occurs in individual molecules of a certain structure and is conditional.
covalent polar bond
An example of a non-polar covalent bond is limited to a few molecules of a simple substance, while dipole compounds in which the electron density is partially shifted towards a more electronegative element are the vast majority. Any combination of atoms with different electronegativity values gives a polar bond. In particular, bonds in organics are covalent polar bonds. Sometimes ionic, inorganic oxides are also polar, and in salts and acids, the ionic type of binding predominates.
The ionic type of compounds is sometimes considered as an extreme case of polar bonding. If the electronegativity of one of the elements is much higher than that of the other, the electron pair is completely shifted from the bond center to it. This is how the separation into ions occurs. The one who takes the electron pair turns into an anion and gets a negative charge, and the one who loses an electron turns into a cation and becomes positive.
Examples of inorganic substances with a covalent non-polar bond type
Substances with a covalent non-polar bond are, for example, all binary gas molecules: hydrogen (H - H), oxygen (O \u003d O), nitrogen (in its molecule, 2 atoms are connected by a triple bond (N ≡ N)); liquids and solids: chlorine (Cl - Cl), fluorine (F - F), bromine (Br - Br), iodine (I - I). As well as complex substances consisting of atoms of different elements, but with the actual same value of electronegativity, for example, phosphorus hydride - PH 3.
Organics and non-polar binding
It is clear that everything is complex. The question arises, how can there be a non-polar bond in a complex substance? The answer is quite simple if you think a little logically. If the values of the electronegativity of the associated elements differ slightly and do not form in the compound, such a bond can be considered non-polar. This is exactly the situation with carbon and hydrogen: all C - H bonds in organics are considered non-polar.
An example of a non-polar covalent bond is the methane molecule, the simplest. It consists of one carbon atom, which, according to its valency, is connected by single bonds to four hydrogen atoms. In fact, the molecule is not a dipole, since there is no localization of charges in it, to some extent due to the tetrahedral structure. The electron density is evenly distributed.
An example of a non-polar covalent bond exists in more complex organic compounds. It is realized due to mesomeric effects, i.e. the successive withdrawal of the electron density, which quickly fades along the carbon chain. So, in a hexachloroethane molecule, the C - C bond is non-polar due to the uniform pulling of the electron density by six chlorine atoms.
Other types of connections
In addition to the covalent bond, which, by the way, can also be carried out according to the donor-acceptor mechanism, there are ionic, metallic and hydrogen bonds. Brief characteristics the penultimate two are presented above.
A hydrogen bond is an intermolecular electrostatic interaction that is observed if the molecule contains a hydrogen atom and any other that has unshared electron pairs. This type of bonding is much weaker than the others, but due to the fact that a lot of these bonds can form in the substance, it makes a significant contribution to the properties of the compound.
