It's no secret that chemistry is a rather complex and diverse science. Many different reactions, reagents, chemicals and other complex and incomprehensible terms - they all interact with each other. But the main thing is that we deal with chemistry every day, no matter whether we listen to the teacher in the lesson and learn new material or brew tea, which in general is also a chemical process.
It can be concluded that chemistry is a must, to understand it and to know how our world or some of its separate parts works is interesting, and, moreover, useful.
Now we have to deal with such a term as a covalent bond, which, by the way, can be both polar and non-polar. By the way, the very word "covalent" is formed from the Latin "co" - together and "vales" - having power.
Term occurrences
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 a chemical bond in which both atoms share electrons, which is called co-ownership. Thus, it differs, for example, from a metallic one, in which electrons are free, or from an ionic one, where one gives electrons to another. It should be noted that it is formed between non-metals.
Based on the foregoing, we can draw a small conclusion about what this process is. It arises between atoms due to the formation of common electron pairs, and these pairs arise on the outer and pre-outer sublevels of electrons.
Examples, substances with a polar:
Types of covalent bond
Two types are also distinguished - these are polar, and, accordingly, non-polar bonds. We will analyze the features of each of them separately.
Covalent polar - education
What is the term "polar"?
It usually happens that two atoms have different electronegativity, therefore, common electrons do not belong to them equally, but they are always closer to one than to the other. For example, a molecule of hydrogen chloride, 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 transfer of an electron from hydrogen to chlorine.
As a result, at polarity, the electron density shifts to a more electronegative one, and a partial negative charge arises on it. In turn, the nucleus, whose electronegativity is lower, has, accordingly, a partial positive charge.
We conclude: polar arises between different non-metals, which differ in the value of electronegativity, and electrons are located closer to the nucleus with greater electronegativity.
Electronegativity - the ability of some atoms to attract the electrons of others, thereby forming a chemical reaction.
Examples of covalent polar, substances with a covalent polar bond:
The formula of a substance with a covalent polar bond
Covalent non-polar, difference between polar and non-polar
And finally, non-polar, we will soon find out what it is.
The main difference between non-polar and polar is symmetry. If, in the case of a polar bond, the electrons were located closer to one atom, then with a non-polar bond, the electrons are arranged symmetrically, that is, equally with respect to both.
It is noteworthy that non-polar arises between non-metal atoms of one chemical element.
For example, substances with non-polar covalent bonds:
Also, a set of electrons is often called simply an electron cloud, based on this we conclude that the electron cloud of communication, which forms a common pair of electrons, is distributed in space symmetrically, or evenly with respect to the nuclei of both.
Examples of a covalent non-polar bond and a scheme for the formation of a covalent non-polar bond
But it is also useful to know how to distinguish between covalent polar and non-polar.
covalent non-polar 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 determine 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.
Rice. 2.1. The formation of molecules from atoms is accompanied by redistribution of electrons of valence orbitals and leads to gain in energy because the energy of molecules is less than the energy of non-interacting atoms. The figure shows a diagram of the formation of a non-polar covalent chemical bond between hydrogen atoms.
§2 Chemical bond
Under normal conditions, the molecular state is more stable than the atomic state. (fig.2.1). The formation of molecules from atoms is accompanied by a redistribution of electrons in valence orbitals and leads to a gain in energy, since the energy of molecules is less than the energy of non-interacting atoms(Appendix 3). The forces that hold atoms in molecules have received a generalized name chemical bond.
The chemical bond between atoms is carried out by valence electrons and has an electrical nature . There are four main types of chemical bonding: covalent,ionic,metal and hydrogen.
1 Covalent bond
A chemical bond carried out by electron pairs is called atomic, or covalent. . Compounds with covalent bonds are called atomic, or covalent. .
When a covalent bond occurs, an overlap of electron clouds of interacting atoms occurs, accompanied by energy release (Fig. 2.1). In this case, a cloud with an increased negative charge density arises between positively charged atomic nuclei. Due to the action of the Coulomb forces of attraction between opposite charges, an increase in the negative charge density favors the approach of the nuclei.
A covalent bond is formed by unpaired electrons in the outer shells of atoms . In this case, electrons with opposite spins form electron pair(Fig. 2.2), common to interacting atoms. If one covalent bond has arisen between atoms (one common electron pair), then it is called single, two-double, etc.
Energy is a measure of the strength of a chemical bond. E sv spent on the destruction of the bond (gain in energy during the formation of a compound from individual atoms). Usually this energy is measured per 1 mol substances and are expressed in kilojoules per mol (kJ ∙ mol -1). The energy of a single covalent bond is in the range of 200–2000 kJmol–1.
Rice. 2.2. A covalent bond is the most general type of chemical bond that occurs due to the socialization of an electron pair through an exchange mechanism. (a), when each of the interacting atoms supplies one electron, or through the donor-acceptor mechanism (b) when an electron pair is shared by one atom (donor) to another atom (acceptor).
A covalent bond has properties satiety and focus . The saturation of a covalent bond is understood as the ability of atoms to form a limited number of bonds with their neighbors, determined by the number of their unpaired valence electrons. The directionality of a covalent bond reflects the fact that the forces that hold atoms near each other are directed along the straight line connecting the atomic nuclei. Besides, covalent bond can be polar or non-polar .
When non-polar In a covalent bond, an electron cloud formed by a common pair of electrons is distributed in space symmetrically with respect to the nuclei of both atoms. A non-polar covalent bond is formed between atoms of simple substances, for example, between identical atoms of gases that form diatomic molecules (O 2, H 2, N 2, Cl 2, etc.).
When polar covalent bond electron cloud bond is shifted to one of the atoms. The formation of a polar covalent bond between atoms is characteristic of complex substances. Molecules of volatile inorganic compounds can serve as an example: HCl, H 2 O, NH 3, etc.
The degree of displacement of the common electron cloud to one of the atoms during the formation of a covalent bond (degree of polarity of a bond ) determined mainly by the charge of atomic nuclei and the radius of interacting atoms .
The greater the charge of the atomic nucleus, the stronger it attracts a cloud of electrons. At the same time, the larger the atomic radius, the weaker the outer electrons are held near the atomic nucleus. The cumulative effect of these two factors is expressed in the different ability of different atoms to "pull" the cloud of covalent bonds towards themselves.
The ability of an atom in a molecule to attract electrons to itself is called electronegativity. . Thus, electronegativity characterizes the ability of an atom to polarize a covalent bond: the greater the electronegativity of an atom, the more the electron cloud of a covalent bond is shifted towards it .
A number of methods have been proposed to quantify electronegativity. At the same time, the method proposed by the American chemist Robert S. Mulliken, who determined the electronegativity an atom as half the sum of its energy E e electron and energy affinities E i atom ionization:
.
(2.1)
Ionization energy of an atom is called the energy that needs to be expended in order to “tear off” an electron from it and remove it to an infinite distance. The ionization energy is determined by photoionization of atoms or by bombarding atoms with electrons accelerated in an electric field. That smallest value of the energy of photons or electrons, which becomes sufficient for the ionization of atoms, is called their ionization energy E i. Usually this energy is expressed in electron volts (eV): 1 eV = 1.610 -19 J.
Atoms are the most willing to give away their outer electrons. metals, which contain a small number of unpaired electrons (1, 2 or 3) on the outer shell. These atoms have the lowest ionization energy. Thus, the value of the ionization energy can serve as a measure of the greater or lesser "metallicity" of the element: the lower the ionization energy, the stronger must be expressed metalproperties element.
In the same subgroup of the periodic system of elements of D.I. Mendeleev, with an increase in the ordinal number of the element, its ionization energy decreases (Table 2.1), which is associated with an increase in the atomic radius (Table 1.2), and, consequently, with a weakening of the bond of external electrons with a core. For elements of the same period, the ionization energy increases with increasing serial number. This is due to a decrease in the atomic radius and an increase in the nuclear charge.
Energy E e, which is released when an electron is attached to a free atom, is called electron affinity(expressed also in eV). The release (rather than absorption) of energy when a charged electron is attached to some neutral atoms is explained by the fact that atoms with filled outer shells are the most stable in nature. Therefore, for those atoms in which these shells are “slightly unfilled” (i.e., 1, 2, or 3 electrons are missing before filling), it is energetically beneficial to attach electrons to themselves, turning into negatively charged ions 1 . Such atoms include, for example, halogen atoms (Table 2.1) - elements of the seventh group (main subgroup) of the periodic system of D.I. Mendeleev. The electron affinity of metal atoms is usually zero or negative, i.e. it is energetically unfavorable for them to attach additional electrons, additional energy is required to keep them inside atoms. The electron affinity of non-metal atoms is always positive and the greater, the closer to the noble (inert) gas the non-metal is located in the periodic system. This indicates an increase non-metallic properties as we approach the end of the period.
From all that has been said, it is clear that the electronegativity (2.1) of atoms increases in the direction from left to right for elements of each period and decreases in the direction from top to bottom for elements of the same group of the Mendeleev periodic system. It is not difficult, however, to understand that in order to characterize the degree of polarity of a covalent bond between atoms, it is not the absolute value of the electronegativity that is important, but the ratio of the electronegativity of the atoms forming the bond. So in practice, they use the relative values of electronegativity(Table 2.1), taking the electronegativity of lithium as a unit.
To characterize the polarity of a covalent chemical bond, the difference in the relative electronegativity of atoms is used. Usually the bond between atoms A and B is considered purely covalent, if | A – B|0.5.
For the first time about such a concept as covalent bond chemical scientists started talking after the discovery of Gilbert Newton Lewis, who described it as the socialization of two electrons. Later studies made it possible to describe the very principle of covalent bonding. Word covalent can be considered within the framework of chemistry as the ability of an atom to form bonds with other atoms.
Let's explain with an example:
There are two atoms with slight differences in electronegativity (C and CL, C and H). As a rule, these are which are as close as possible to the structure of the electron shell of noble gases.
When these conditions are met, the nuclei of these atoms are attracted to the electron pair common to them. In this case, the electron clouds do not simply overlap each other, as in the case of a covalent bond, which ensures a reliable connection of two atoms due to the fact that the electron density is redistributed and the energy of the system changes, which is caused by the "drawing" of one atom of the electron cloud of another into the internuclear space. The more extensive the mutual overlap of electron clouds, the stronger the connection is considered.
From here, covalent bond- this is a formation that has arisen by the mutual socialization of two electrons belonging to two atoms.
As a rule, substances with a molecular crystal lattice are formed through a covalent bond. Characteristics are melting and boiling at low temperatures, poor solubility in water and low electrical conductivity. From this we can conclude: the basis of the structure of such elements as germanium, silicon, chlorine, hydrogen is a covalent bond.
Properties characteristic of this type of connection:
- Saturability. This property is usually understood as the maximum number of bonds that they can establish specific atoms. This number is determined by the total number of those orbitals in the atom that can participate in the formation of chemical bonds. The valency of an atom, on the other hand, can be determined by the number of orbitals already used for this purpose.
- Orientation. All atoms tend to form the strongest possible bonds. The greatest strength is achieved in the case of the coincidence of the spatial orientation of the electron clouds of two atoms, since they overlap each other. In addition, it is precisely such a property of a covalent bond as directionality that affects the spatial arrangement of molecules, that is, is responsible for their "geometric shape".
- Polarizability. This position is based on the idea that there are two types of covalent bonds:
- polar or asymmetrical. A bond of this type can only be formed by atoms of different types, i.e. those whose electronegativity differs significantly, or in cases where the shared electron pair is not symmetrically separated.
- arises between atoms, the electronegativity of which is almost equal, and the distribution of electron density is uniform.
In addition, there are certain quantitative:
- Bond energy. This parameter characterizes the polar bond in terms of its strength. Energy is understood as the amount of heat that was necessary to break the bond of two atoms, as well as the amount of heat that was released when they were combined.
- Under bond length and in molecular chemistry, the length of a straight line between the nuclei of two atoms is understood. This parameter also characterizes the bond strength.
- Dipole moment- a value that characterizes the polarity of the valence bond.
The 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 electric current. 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. Substances such as graphite, silicon (Si), silicon dioxide (SiO 2), silicon carbide (SiC) and others also have an atomic structure.
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.
Don't Miss: Mechanism of Education, Case Studies.
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 hydrochloric acid molecule.
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.
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 in the chemical interaction of two atoms of non-metals, 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 (complete 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.
