What substances have a covalent bond? Covalent bond

Covalent bond(from the Latin “co” together and “vales” having force) is carried out due to the electron pair belonging to both atoms. Formed between non-metal atoms.

The electronegativity of nonmetals is quite high, so that during the chemical interaction of two nonmetal atoms, complete transfer of electrons from one to another (as in the case) is impossible. In this case, electron pooling is required to complete.

As an example, let's discuss the interaction of hydrogen and chlorine atoms:

H 1s 1 - one electron

Cl 1s 2 2s 2 2 p 6 3 s 2 3 p5 - seven electrons in the outer level

Each of the two atoms is missing one electron in order to have a complete outer shell of electrons. And each of the atoms allocates one electron “for common use.” Thus, the octet rule is satisfied. This is best represented using the Lewis formulas:

Formation of 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 by the bonding of two atoms is called molecule.

Non-polar covalent bond

A covalent bond can also form between two identical atoms. For example:

This diagram explains why hydrogen and chlorine exist as diatomic molecules. Thanks to the pairing and sharing 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 molecules of oxygen O 2 or nitrogen N 2. Nitrogen atoms 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 point solids. One of the rare exceptions is diamond, which melts above 3,500 °C. This is explained by 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 combine. 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 to two atoms equally. Most of the time they are closer to one atom than to another. In a hydrogen chloride molecule, for example, the electrons that form a covalent bond are located closer to the chlorine atom because its electronegativity is higher than that of hydrogen. However, the difference in the ability to attract electrons is not large enough for complete electron transfer from the hydrogen atom to the chlorine atom to occur. Therefore, the bond between hydrogen and chlorine atoms can be considered as a cross between an ionic bond (complete electron transfer) and a non-polar covalent bond (a symmetrical arrangement of a pair of electrons between two atoms). The partial charge on atoms is denoted by the Greek letter δ. This 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 the other atom (acceptor) provides an empty orbital for that pair.

Definition

A covalent bond is a chemical bond formed by atoms sharing their valence electrons. Required condition The formation of a covalent bond is the overlap of atomic orbitals (AO) on which the valence electrons are located. In the simplest case, the overlap of two AOs leads to the formation of two molecular orbitals (MO): a bonding MO and an antibonding (antibonding) MO. The shared electrons are located on the lower energy bonding MO:

Education Communications

Covalent bond (atomic bond, homeo polar connection) - a connection between two atoms due to electron sharing of two electrons - one from each atom:

A. + B. -> A: B

For this reason, the homeopolar relationship is directional. The pair of electrons that perform the bond belongs simultaneously to both bonded atoms, for example:

Types of covalent bond

There are three types of covalent chemical bond, differing in the mechanism of its formation:

1. Simple covalent bond. For its formation, each atom provides one unpaired electron. When a simple covalent bond is formed, the formal charges of the atoms remain unchanged. If the atoms forming a simple covalent bond are the same, then the true charges of the atoms in the molecule are also the same, since the atoms forming the bond equally own a shared electron pair, such a bond is called a non-polar covalent bond. If the atoms are different, then the degree of possession of a shared pair of electrons is determined by the difference in the electronegativity of the atoms, the atom with greater electronegativity in to a greater extent possesses a pair of bonding electrons, and therefore its true charge has a negative sign; an atom with lower electronegativity acquires a charge of the same magnitude, but with a positive sign.

Sigma (σ)-, pi (π)-bonds are an approximate description of the types of covalent bonds in molecules of organic compounds; the σ-bond is characterized by the fact that the density of the electron cloud is maximum along the axis connecting the nuclei of atoms. When a π bond is formed, the so-called lateral overlap of electron clouds occurs, and the density of the electron cloud is maximum “above” and “below” the σ bond plane. For example, take ethylene, acetylene and benzene.

In the ethylene molecule C 2 H 4 there is a double bond CH 2 = CH 2, its electronic formula: H:C::C:H. The nuclei of all ethylene atoms are located in the same plane. The three electron clouds of each carbon atom form three covalent bonds with other atoms in the same plane (with angles between them of approximately 120°). The cloud of the fourth valence electron of the carbon atom is located above and below the plane of the molecule. Such electron clouds of both carbon atoms, partially overlapping above and below the plane of the molecule, form a second bond between the carbon atoms. The first, stronger covalent bond between carbon atoms is called a σ bond; the second, weaker covalent bond is called a π bond.

In a linear acetylene molecule

N-S≡S-N (N: S::: S: N)

there are σ bonds between carbon and hydrogen atoms, one σ bond between two carbon atoms, and two π bonds between the same carbon atoms. Two π-bonds are located above the sphere of action of the σ-bond in two mutually perpendicular planes.

All six carbon atoms of the cyclic benzene molecule C 6 H 6 lie in the same plane. There are σ bonds between carbon atoms in the plane of the ring; Each carbon atom has the same bonds with hydrogen atoms. To make these bonds, carbon atoms spend three electrons. Clouds of fourth valence electrons of carbon atoms, shaped like figures of eight, are located perpendicular to the plane of the benzene molecule. Each such cloud overlaps equally with the electron clouds of neighboring carbon atoms. In a benzene molecule, not three separate π bonds are formed, but a single π electron system of six electrons, common to all carbon atoms. The bonds between the carbon atoms in the benzene molecule are exactly the same.

A covalent bond is formed as a result of the sharing of electrons (to form common electron pairs), which occurs during the overlap of electron clouds. The formation of a covalent bond involves the electron clouds of two atoms. There are two main types of covalent bonds:

• A covalent nonpolar bond is formed between atoms of a nonmetal of the same chemical element. Simple substances, for example O 2, have such a connection; N 2; C 12.
• A polar covalent bond is formed between atoms of different nonmetals.

see also

Literature

• "Chemical encyclopedic Dictionary", M., " Soviet encyclopedia", 1983, p. 264.

Wikimedia Foundation. 2010.

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

The formation of a hydrogen molecule from atoms can be schematically 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 view 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 on its outer energy level two paired and three unpaired electrons:

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;

- directionality;

- polarity and polarizability.

Thanks to which molecules of inorganic and organic substances are formed. A chemical bond appears through 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 reacting atoms decreases. In the process of bond formation, atoms try to complete their electron shell.

Types of communication

In chemistry, there are several types of bonds: ionic, covalent, metallic. Covalent chemical bonds have two types: polar and non-polar.

What is the mechanism for its creation? A covalent nonpolar chemical bond is formed between atoms of identical nonmetals that have the same electronegativity. In this case, common electron pairs are formed.

Non-polar bond

Examples of molecules that have a nonpolar covalent chemical bond include halogens, hydrogen, nitrogen, and oxygen.

This connection was first discovered in 1916 by the American chemist Lewis. At first he put forward a hypothesis, and it was confirmed only after experimental confirmation.

Covalent chemical bonding is related to electronegativity. For non-metals it has a high value. During the chemical interaction of atoms, the transfer of electrons from one atom to another is not always possible; as a result, they combine. A genuine covalent chemical bond appears between the atoms. 8th grade regular school curriculum involves a detailed consideration of several types of communication.

Substances that have this type of bond under normal conditions are liquids, gases, as well as solids that have a low melting point.

Types of covalent bond

Let's look in more detail at this issue. What are the types of chemical bonds? Covalent bonds exist in exchange and donor-acceptor versions.

The first type is characterized by the donation of one unpaired electron by each atom to the formation of a common electronic bond.

Electrons combined into a common bond must have opposite spins. As an example of this type of covalent bond, consider hydrogen. When its atoms come closer, their electron clouds penetrate into each other, which in science is called overlapping of electron clouds. As a result, the electron density between the nuclei increases, and the energy of the system decreases.

At a minimum distance, the hydrogen nuclei repel each other, resulting in a certain optimal distance.

In the case of the donor-acceptor type of covalent bond, one particle has electrons and is called a donor. The second particle has a free cell in which a pair of electrons will be located.

Polar molecules

How are covalent polar chemical bonds formed? They arise in situations where the nonmetal atoms being bonded have different electronegativity. IN similar cases shared electrons are located closer to the atom whose electronegativity value is higher. As an example of a covalent polar bond, we can consider the bonds that arise in the hydrogen bromide molecule. Here the public electrons, which are responsible for forming 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 covalent bonds

How to define covalent polar chemical bonds? To do this, you need to know the composition of the molecules. If it contains atoms different elements, there is a polar covalent bond in the molecule. Nonpolar molecules contain atoms of one chemical element. Among the tasks offered as part of school course chemistry, there are also those that involve identifying the type of connection. Tasks of this type are included in the final certification tasks in chemistry in grade 9, as well as in the tests of the unified state exam in chemistry in grade 11.

Ionic bond

What is the difference between covalent and ionic chemical bonds? If covalent bonds are characteristic of nonmetals, then ionic bond is formed between atoms that have significant differences by electronegativity. For example, this is typical for compounds of elements of the first and second groups of the main subgroups of the PS (alkali and alkaline earth metals) and elements of the 6th and 7th groups 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.

Features of ionic bonding

Since the force fields of oppositely charged ions are distributed evenly in all directions, each of them is capable of attracting particles of opposite sign. This characterizes the non-directionality of the ionic bond.

The interaction of two ions with opposite signs does not imply complete mutual compensation of individual force fields. This helps to maintain 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 to itself a number of others of opposite sign to form a crystal lattice of an ionic nature. There are no molecules in such a crystal. Each ion is surrounded in a substance by a certain number of ions of a different sign.

Metal connection

This type chemical bond has certain individual characteristics. Metals have an excess number of valence orbitals and a deficiency of electrons.

When individual atoms come together, their valence orbitals overlap, which facilitates the free movement of electrons from one orbital to another, creating a bond between all metal atoms. These free electrons are the main feature of a 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, ductility, malleability, thermal conductivity, opacity.

Type of covalent bond

It is formed between a hydrogen atom and an element that has high electronegativity. There are intra- and intermolecular hydrogen bonds. This type of covalent bond is the weakest; 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 the atom with high electronegativity.

Among characteristic properties covalent bonds are distinguished: saturation, directionality, polarizability, polarity. Each of these indicators has a specific meaning for the compound being formed. For example, directionality is determined by the geometric shape of the molecule.

The term “covalent bond” itself comes from two Latin words: “co” - together and “vales” - having force, since this is a bond occurring due to a pair of electrons belonging simultaneously to both (or in simpler terms, a bond between atoms due to pairs of electrons that are common to them). The formation of a covalent bond occurs exclusively among non-metal atoms, and it can appear both in the atoms of molecules and crystals.

Covalent was first discovered back in 1916 by the American chemist J. Lewis and existed for some time as a hypothesis, an idea, only then was it confirmed experimentally. What did chemists find out about it? And the fact that the electronegativity of non-metals can be quite large and during the chemical interaction of two atoms the transfer of electrons from one to another may be impossible, it is at this moment that the electrons of both atoms unite, a real covalent bond of atoms arises between them.

Types of covalent bond

In general, there are two types of covalent bonds:

• exchange,
• donor-acceptance.

In the exchange type of covalent bond between atoms, each of the connecting atoms contributes one unpaired electron to form an electronic bond. In this case, these electrons must have opposite charges (spins).

An example of such a covalent bond would be the bonds occurring in a hydrogen molecule. When hydrogen atoms come together, their electron clouds penetrate each other, in science this is called electron cloud overlap. As a result, the electron density between the nuclei increases, they themselves are attracted to each other, and the energy of the system decreases. However, when approaching too closely, the nuclei begin to repel each other, and thus a certain optimal distance between them appears.

This is shown more clearly in the picture.

As for the donor-acceptor type of covalent bond, it occurs when one particle, in in this case the donor represents its electron pair for bonding, and the second, the acceptor, represents a free orbital.

Also speaking about the types of covalent bonds, we can distinguish non-polar and polar covalent bonds; we will write about them in more detail below.

Covalent nonpolar bond

The definition of a covalent nonpolar bond is simple; it is a bond that forms between two identical atoms. For an example of the formation of a nonpolar covalent bond, see the diagram below.

Scheme of a covalent nonpolar bond.

In molecules with a covalent nonpolar bond, common electron pairs are located at equal distances from the atomic nuclei. For example, in a molecule (in the diagram above), the atoms acquire an eight-electron configuration, while they share four pairs of electrons.

Substances with covalent nonpolar bonds are usually gases, liquids, or relatively low-melting solids.

Covalent polar bond

Now let’s answer the question: which bond is polar covalent? So, a polar covalent bond is formed when the covalently bonded atoms have different electronegativity and the shared electrons are not shared equally by the two atoms. Most of the time, public electrons are closer to one atom than to another. An example of a covalent polar bond is the bonds that arise in the hydrogen chloride molecule, where the public electrons responsible for the formation of a covalent bond are located closer to the chlorine atom than to the hydrogen atom. The thing is that the electronegativity of chlorine is greater than that of hydrogen.

This is what the diagram of a polar covalent bond looks like.

A striking example of a substance with a polar covalent bond is water.

How to Define a Covalent Bond

Well, now you know the answer to the question of how to determine a polar covalent bond, and a non-polar one, for this it is enough to know the properties and chemical formula molecules, if this molecule consists of atoms of different elements, then the bond will be polar, if from one element, then non-polar. It is also important to remember that covalent bonds in general can only occur among non-metals, this is due to the very mechanism of covalent bonds described above.

Covalent bond video

And finally, a video lecture on the topic of our article, covalent bonds.