In which compound is covalent? Covalent polar bond: formula, properties, features

Covalent chemical bond occurs between atoms with similar or equal electronegativity values. Suppose that chlorine and hydrogen tend to take away electrons and take on the structure of the nearest noble gas, which means that neither of them will give an electron to the other. How are they still connected? It's simple - they share with each other, a common electron pair is formed.

Now let's consider distinctive features covalent bond.

Unlike ionic compounds, the molecules of covalent compounds are held together by “intermolecular forces,” which are much weaker than chemical bonds. In this regard, covalent bonds are characterized saturability– formation of a limited number of connections.

It is known that atomic orbitals are oriented in space in a certain way, therefore, when a bond is formed, the overlap of electron clouds occurs in a certain direction. Those. such a property of a covalent bond is realized as direction.

If a covalent bond in a molecule is formed by identical atoms or atoms with equal electronegativity, then such a bond has no polarity, that is, the electron density is distributed symmetrically. It's called non-polar covalent bond ( H2, Cl2, O2 ). Bonds can be single, double, or triple.

If the electronegativity of atoms differs, then when they combine, the electron density is distributed unevenly between the atoms and forms covalent polar connection (HCl, H 2 O, CO), the multiplicity of which can also be different. During education of this type bond, the more electronegative atom acquires a partial negative charge, and the atom with less electronegativity acquires a partial positive charge (δ- and δ+). An electric dipole is formed in which charges of opposite sign are located at a certain distance from each other. The dipole moment is used as a measure of bond polarity:

The polarity of the connection is more pronounced, the greater the dipole moment. The molecules will be non-polar if the dipole moment is zero.

In connection with the above features, we can conclude that covalent compounds are volatile and have low temperatures melting and boiling. Electricity cannot pass through these connections, hence they are poor conductors and good insulators. When heat is applied, many compounds with covalent bonds ignite. For the most part these are hydrocarbons, as well as oxides, sulfides, halides of non-metals and transition metals.

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Covalent, ionic, and metallic are the three main types of chemical bonds.

Let's get to know more about covalent chemical bond. Let's consider the mechanism of its occurrence. Let's take the formation of a hydrogen molecule as an example:

A spherically symmetric cloud formed by a 1s electron surrounds the nucleus of a free hydrogen atom. When atoms come close to a certain distance, their orbitals partially overlap (see figure), as a result, a molecular two-electron cloud appears between the centers of both nuclei, which has a maximum electron density in the space between the nuclei. With an increase in the density of the negative charge, a strong increase in the forces of attraction between the molecular cloud and the nuclei occurs.

So, we see that a covalent bond is formed by overlapping electron clouds of atoms, which is accompanied by the release of energy. If the distance between the nuclei of atoms approaching before touching is 0.106 nm, then after the electron clouds overlap it will be 0.074 nm. The greater the overlap of electron orbitals, the stronger the chemical bond.

Covalent called chemical bond carried out by electron pairs. Compounds with covalent bonds are called homeopolar or atomic.

Exist two types of covalent bonds: polar And non-polar.

For non-polar In a covalent bond, the electron cloud formed by a common pair of electrons is distributed symmetrically relative to the nuclei of both atoms. An example is diatomic molecules that consist of one element: Cl 2, N 2, H 2, F 2, O 2 and others, the electron pair in which belongs to both atoms equally.

At polar In a covalent bond, the electron cloud is shifted toward the atom with higher relative electronegativity. For example, molecules of volatile inorganic compounds such as H 2 S, HCl, H 2 O and others.

The formation of an HCl molecule can be represented as follows:

Because the relative electronegativity of the chlorine atom (2.83) is greater than that of the hydrogen atom (2.1), the electron pair is shifted to the chlorine atom.

In addition to the exchange mechanism of covalent bond formation - due to overlap, there is also donor-acceptor the mechanism of its formation. This is a mechanism in which the formation of a covalent bond occurs due to the two-electron cloud of one atom (donor) and the free orbital of another atom (acceptor). Let's look at an example of the mechanism for the formation of ammonium NH 4 +. In the ammonia molecule, the nitrogen atom has a two-electron cloud:

The hydrogen ion has a free 1s orbital, let's denote this as .

During the formation of the ammonium ion, the two-electron cloud of nitrogen becomes common to the nitrogen and hydrogen atoms, which means it is converted into a molecular electron cloud. Consequently, a fourth covalent bond appears. You can imagine the process of ammonium formation with the following diagram:

The charge of the hydrogen ion is dispersed between all atoms, and the two-electron cloud that belongs to nitrogen becomes shared with hydrogen.

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And two-electron three-center communication.

Taking into account the statistical interpretation of the M. Born wave function, the probability density of finding bonding electrons is concentrated in the space between the nuclei of the molecule (Fig. 1). The theory of electron pair repulsion considers the geometric dimensions of these pairs. Thus, for elements of each period there is a certain average radius of an electron pair (Å):

0.6 for elements up to neon; 0.75 for elements up to argon; 0.75 for elements up to krypton and 0.8 for elements up to xenon.

Characteristic properties of a covalent bond

Characteristic properties covalent bond - directionality, saturation, polarity, polarizability - determine chemical and physical properties connections.

The angles between two bonds are called bond angles.

  • Saturability is the ability of atoms to form a limited number of covalent bonds. The number of bonds formed by an atom is limited by the number of its outer atomic orbitals.
  • The polarity of the bond is due to the uneven distribution of electron density due to differences in the electronegativity of the atoms.

On this basis, covalent bonds are divided into non-polar and polar (non-polar - a diatomic molecule consists of identical atoms (H 2, Cl 2, N 2) and the electron clouds of each atom are distributed symmetrically relative to these atoms; polar - a diatomic molecule consists of atoms of different chemical elements , and the general electron cloud shifts towards one of the atoms, thereby forming an asymmetry in the distribution of electric charge in the molecule, generating a dipole moment of the molecule).

  • The polarizability of a bond is expressed in the displacement of the bond electrons under the influence of an external electric field, including that of another reacting particle. Polarizability is determined by electron mobility. The polarity and polarizability of covalent bonds determines the reactivity of molecules towards polar reagents.

However, twice winner Nobel Prize L. Pauling pointed out that “in some molecules there are covalent bonds due to one or three electrons instead of a common pair.” A one-electron chemical bond is realized in the molecular hydrogen ion H 2 +.

The molecular hydrogen ion H2+ contains two protons and one electron. The single electron of the molecular system compensates for the electrostatic repulsion of the two protons and holds them at a distance of 1.06 Å (the length of the H 2+ chemical bond). The center of electron density of the electron cloud of the molecular system is equidistant from both protons at the Bohr radius α 0 = 0.53 A and is the center of symmetry of the molecular hydrogen ion H 2 + .

History of the term

The term "covalent bond" was first coined by Nobel Prize winner Irving Langmuir in 1919. The term referred to a chemical bond due to the shared possession of electrons, as opposed to a metallic bond, in which the electrons were free, or an ionic bond, in which one of the atoms gave up an electron and became a cation, and the other atom accepted an electron and became an anion.

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A covalent bond is formed by a pair of electrons shared between two atoms, and these electrons must occupy two stable orbitals, one from each atom.

A + + B → A: B

As a result of socialization, electrons form a filled energy level. A bond is formed if their total energy at this level is less than in the initial state (and the difference in energy will be nothing more than the bond energy).

According to the theory of molecular orbitals, the overlap of two atomic orbitals leads, in the simplest case, to the formation of two molecular orbitals (MO): linking MO And anti-binding (loosening) MO. The shared electrons are located on the lower energy bonding MO.

Bond formation during recombination of atoms

However, the mechanism of interatomic interaction for a long time remained unknown. Only in 1930 F. London introduced the concept of dispersion attraction - the interaction between instantaneous and induced (induced) dipoles. Currently, the attractive forces caused by the interaction between the fluctuating electric dipoles of atoms and molecules are called “London forces”.

The energy of such an interaction is directly proportional to the square of the electronic polarizability α and inversely proportional to the distance between two atoms or molecules to the sixth power.

Bond formation by donor-acceptor mechanism

In addition to the homogeneous mechanism of covalent bond formation outlined in the previous section, there is a heterogeneous mechanism - the interaction of oppositely charged ions - the H + proton and negative ion hydrogen H -, called hydride ion:

H + + H - → H 2

As the ions approach, the two-electron cloud (electron pair) of the hydride ion is attracted to the proton and ultimately becomes common to both hydrogen nuclei, that is, it turns into a bonding electron pair. The particle that supplies an electron pair is called a donor, and the particle that accepts this electron pair is called an acceptor. This mechanism of covalent bond formation is called donor-acceptor.

H + + H 2 O → H 3 O +

A proton attacks the lone electron pair of a water molecule and forms a stable cation that exists in aqueous solutions of acids.

Similarly, a proton is added to an ammonia molecule to form a complex ammonium cation:

NH 3 + H + → NH 4 +

In this way (according to the donor-acceptor mechanism of covalent bond formation), a large class of onium compounds is obtained, which includes ammonium, oxonium, phosphonium, sulfonium and other compounds.

A hydrogen molecule can act as a donor of an electron pair, which, upon contact with a proton, leads to the formation of a molecular hydrogen ion H 3 +:

H 2 + H + → H 3 +

The bonding electron pair of the molecular hydrogen ion H 3 + belongs simultaneously to three protons.

Types of covalent bond

There are three types of covalent chemical bonds, differing in the mechanism of 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. This connection is called non-polar covalent bond. Simple substances have such a connection, for example: 2, 2, 2. But not only nonmetals of the same type can form a covalent nonpolar bond. Non-metal elements whose electronegativity is of equal importance can also form a covalent nonpolar bond, for example, in the PH 3 molecule the bond is covalent nonpolar, since the EO of hydrogen is equal to the EO of phosphorus.
  • 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. An atom with greater electronegativity attracts a pair of bonding electrons more strongly toward itself, and its true charge becomes negative. An atom with lower electronegativity acquires, accordingly, a positive charge of the same magnitude. If a compound is formed between two different non-metals, then such a compound is called covalent polar bond.

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 π (\displaystyle \pi )- communication.

A covalent bond is the bonding of atoms using common (shared between them) electron pairs. In the word “covalent,” the prefix “co-” means “joint participation.” And “valens” translated into Russian means strength, ability. In this case, we mean the ability of atoms to bond with other atoms.

When a covalent bond is formed, atoms combine their electrons as if into a common “piggy bank” - a molecular orbital, which is formed from the atomic shells of individual atoms. This new shell contains as complete a number of electrons as possible and replaces the atoms with their own incomplete atomic shells.

Ideas about the mechanism of formation of the hydrogen molecule were extended to more complex molecules. The theory of chemical bonding developed on this basis was called valence bond method (VS method). The BC method is based on the following provisions:

1) A covalent bond is formed by two electrons with opposite spins, and this electron pair belongs to two atoms.

2) The stronger the covalent bond, the more the electron clouds overlap.

Combinations of two-electron two-center bonds, reflecting the electronic structure of the molecule, are called valence schemes. Examples of constructing valence circuits:

Valence schemes most clearly embody the representations Lewis on the formation of a chemical bond by sharing electrons with the formation of an electron shell of a noble gas: for hydrogen– of two electrons (shell He), For nitrogen– of eight electrons (shell Ne).

29. Non-polar and polar covalent bonds.

If a diatomic molecule consists of atoms of one element, then the electron cloud is distributed in space symmetrically relative to the atomic nuclei. Such a covalent bond is called nonpolar. If a covalent bond is formed between atoms of different elements, then the common electron cloud is shifted towards one of the atoms. In this case, the covalent bond is polar.

As a result of the formation of a polar covalent bond, the more electronegative atom acquires a partial negative charge, and the atom with less electronegativity acquires a partial positive charge. These charges are usually called the effective charges of the atoms in the molecule. They may have a fractional value.

30. Methods of expressing covalent bonds.

There are two main ways of education covalent bond * .

1) An electron pair forming a bond can be formed due to unpaired electrons, available in unexcited atoms. An increase in the number of covalent bonds created is accompanied by the release of more energy than is expended on excitation of the atom. Since the valence of an atom depends on the number of unpaired electrons, excitation leads to an increase in valence. For nitrogen, oxygen, and fluorine atoms, the number of unpaired electrons does not increase, because there are no vacancies within the second level orbitals*, and the movement of electrons to the third quantum level requires significantly more energy than that which would be released during the formation of additional bonds. Thus, when an atom is excited, transitions of electrons to freeorbitals possible only within one energy level.

2) Covalent bonds can be formed due to paired electrons present in the outer electron layer of the atom. In this case, the second atom must have a free orbital on the outer layer. An atom that provides its electron pair to form a covalent bond * is called a donor, and an atom that provides an empty orbital is called an acceptor. A covalent bond formed in this way is called a donor-acceptor bond. In the ammonium cation, this bond is absolutely identical in its properties to the other three covalent bonds formed by the first method, therefore the term “donor-acceptor” does not mean any special type of communication, but only the method of its formation.

It's no secret that chemistry is a rather complex and also diverse science. A bunch of various reactions, reagents, chemicals and other complex and confusing terms - they all interact with each other. But the main thing is that we deal with chemistry every day, it doesn’t matter whether we listen to the teacher in class and learn new material or we brew tea, which in general is also a chemical process.

It can be concluded that you just need to know chemistry, understanding it and knowing how our world or some of its parts work is interesting, and, moreover, useful.

Now we have to deal with such a term as a covalent bond, which, by the way, can be either polar or non-polar. By the way, the word “covalent” itself is derived from the Latin “co” - together and “vales” - having force.

Appearances of the term

Let's start with the fact that The term “covalent” was first introduced in 1919 by Irving Langmuir - Nobel Prize Laureate. The concept of "covalent" implies chemical bond, in which both atoms have electrons, which is called shared possession. Thus, it differs, for example, from a metallic one, in which electrons are free, or from an ionic one, where one completely gives electrons to another. It should be noted that it is formed between non-metals.

Based on the above, we can draw a small conclusion about what this process is like. It arises between atoms due to the formation of common electron pairs, and these pairs arise on the external and pre-external sublevels of electrons.

Examples, substances with polarity:

Types of covalent bond

There are also two types: polar and, accordingly, nonpolar bonds. We will analyze the features of each of them separately.

Covalent polar - formation

What does the term “polar” mean?

What usually happens is that two atoms have different electronegativity, therefore the electrons they share do not belong equally, but are always closer to one than to the other. For example, a hydrogen chloride molecule, in which the electrons of the covalent bond are located closer to the chlorine atom, since its electronegativity is higher than that of hydrogen. However, in reality, the difference in electron attraction is small enough for complete electron transfer from hydrogen to chlorine to occur.

As a result, when polar, the electron density shifts to a more electronegative one, and a partial negative charge appears on it. In turn, the nucleus whose electronegativity is lower develops, accordingly, a partial positive charge.

We conclude: polar occurs between different nonmetals that differ in their electronegativity values, and the electrons are located closer to the nucleus with greater electronegativity.

Electronegativity is the ability of some atoms to attract electrons from others, thereby forming chemical reaction.

Examples of covalent polar, substances with a polar covalent bond:

Formula of a substance with a polar covalent bond

Covalent nonpolar, difference between polar and nonpolar

And finally, non-polar, we will soon find out what it is.

The main difference between non-polar and polar- this is symmetry. If in the case of a polar bond the electrons were located closer to one atom, then in a non-polar bond the electrons were located symmetrically, that is, equally relative to both.

It is noteworthy that nonpolar arises between atoms of a nonmetal of the same chemical element.

Eg, substances with non-polar covalent bonds:

Also, a collection of electrons is often called simply an electron cloud, based on this we conclude that the electronic cloud of communication, which forms a common pair of electrons, is distributed in space symmetrically, or evenly in relation to the nuclei of both.

Examples of a covalent nonpolar bond and a scheme for the formation of a covalent nonpolar bond

But it is also useful to know how to distinguish between covalent polar and nonpolar.

Covalent nonpolar- these are always atoms of the same substance. H2. CL2.

This article has come to an end, now we know what this chemical process is, we know how to define it and its varieties, we know the formulas for the formation of substances, and in general a little more about our complex world, success in chemistry and the formation of new formulas.