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:
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Covalent bond(atomic bond, homeopolar bond) - a bond 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:
| .. | .. | .. | |||||||||
| : | Cl | : | Cl | : | H | : | O | : | H | ||
| .. | .. | .. |
Types of covalent bond
There are three types of covalent chemical bonds, differing in the mechanism of their 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.
| Organic chemistry |
|---|
| List of organic compounds |
| Structural chemistry | |
|---|---|
| Chemical bond: | Aromaticity | Covalent bond| Ionic bond | Metal connection | Hydrogen bond | Donor-acceptor bond | Tautomerism |
| Structure display: | Functional group | Structural formula | Chemical formula | Ligand |
| Electronic properties: | Electronegativity | Electron affinity | Ionization energy | Dipole | Octet rule |
| Stereochemistry: | Asymmetric atom | Isomerism | Configuration | Chirality | Conformation |
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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 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 - laureate Nobel Prize. The term "covalent" implies a chemical bond in which both atoms share electrons, which is called shared ownership. 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 polar:
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 a chemical reaction.
Examples of covalent polar, substances with a polar covalent bond:
Formula of a substance with a polar covalent 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- 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 non-polar occurs between non-metal atoms of one 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.
A chemical bond is the interaction of particles (ions or atoms), which occurs in the process of exchanging electrons located at the last electronic level. There are several types of such bonds: covalent (it is divided into non-polar and polar) and ionic. In this article we will dwell in more detail on the first type of chemical bonds - covalent ones. And to be more precise, in its polar form.
A polar covalent bond is a chemical bond between the valence electron clouds of neighboring atoms. The prefix “ko-” means in in this case“together”, and the stem “valence” is translated as strength or ability. Those two electrons that bond with each other are called an electron pair.
Story
The term was first used in a scientific context by Nobel Prize winner chemist Irving Lenngrum. This happened in 1919. In his work, the scientist explained that a bond in which electrons common to two atoms are observed is different from a metallic or ionic one. This means it requires a separate name.Later, already in 1927, F. London and W. Heitler, taking as an example the hydrogen molecule as the chemically and physically simplest model, described a covalent bond. They took on the matter from the other end, and substantiated their observations using quantum mechanics.
The essence of the reaction
The process of converting atomic hydrogen into molecular hydrogen is a typical chemical reaction, the qualitative sign of which is the large release of heat when two electrons combine. It looks something like this: two helium atoms approach each other, each having one electron in their orbit. Then these two clouds come closer and form a new one, similar to a shell of helium, in which two electrons already rotate.
Completed electron shells are more stable than incomplete ones, so their energy is significantly lower than that of two separate atoms. When a molecule is formed, excess heat is dissipated into the environment.
Classification
In chemistry, there are two types of covalent bonds:
- A covalent nonpolar bond formed between two atoms of the same nonmetallic element, such as oxygen, hydrogen, nitrogen, carbon.
- A polar covalent bond occurs between atoms of different nonmetals. A good example could be a hydrogen chloride molecule. When atoms of two elements combine with each other, the unpaired electron from hydrogen partially transfers to the last electron level of the chlorine atom. Thus, a positive charge is formed on the hydrogen atom, and a negative charge on the chlorine atom.
Donor-acceptor bond is also a type of covalent bond. It lies in the fact that one atom of the pair provides both electrons, becoming a donor, and the atom receiving them, accordingly, is considered an acceptor. When a bond is formed between atoms, the charge of the donor increases by one, and the charge of the acceptor decreases.
Semipolar connection - e e can be considered a subtype of donor-acceptor. Only in this case do atoms unite, one of which has a complete electron orbital (halogens, phosphorus, nitrogen), and the second - two unpaired electrons (oxygen). The formation of a connection takes place in two stages:
- first, one electron is removed from the lone pair and added to the unpaired ones;
- the union of the remaining unpaired electrodes, that is, a covalent polar bond is formed.
Properties
A polar covalent bond has its own physical and chemical properties, such as directionality, saturation, polarity, polarizability. They determine the characteristics of the resulting molecules.
The direction of the bond depends on the future molecular structure of the resulting substance, namely on the geometric shape that the two atoms form upon joining.
Saturation shows how many covalent bonds one atom of a substance can form. This number is limited by the number of outer atomic orbitals.
The polarity of a molecule occurs because the electron cloud formed from two different electrons is uneven around its entire circumference. This occurs due to the difference in negative charge in each of them. It is this property that determines whether a bond is polar or nonpolar. When two atoms of the same element combine, the electron cloud is symmetrical, which means the covalent bond is nonpolar. And if atoms unite different elements, then an asymmetric electron cloud is formed, the so-called dipole moment of the molecule.
Polarizability reflects how actively the electrons in a molecule are displaced under the influence of external physical or chemical agents, for example, an electric or magnetic field, or other particles.
The last two properties of the resulting molecule determine its ability to react with other polar reagents.
Sigma bond and pi bond
The formation of these bonds depends on the electron density distribution in the electron cloud during the formation of the molecule.
A sigma bond is characterized by the presence of a dense accumulation of electrons along the axis connecting the nuclei of atoms, that is, in the horizontal plane.
The pi bond is characterized by the compaction of electron clouds at the point of their intersection, that is, above and below the atomic nucleus.
Visualization of the relationship in the formula record
For example, we can take the chlorine atom. Its outermost electronic level contains seven electrons. In the formula, they are arranged in three pairs and one unpaired electron around the symbol of the element in the form of dots.
If you write a chlorine molecule in the same way, you will see that two unpaired electrons have formed a pair common to two atoms; it is called shared. In this case, each of them received eight electrons.
Octet-doublet rule
The chemist Lewis, who proposed how a polar covalent bond is formed, was the first of his colleagues to formulate a rule explaining the stability of atoms when they are combined into molecules. Its essence lies in the fact that chemical bonds between atoms are formed when a sufficient number of electrons are shared to form an electronic configuration that is similar to the atoms of noble elements.
That is, during the formation of molecules, in order to stabilize them, it is necessary that all atoms have a complete external electronic level. For example, hydrogen atoms, combining into a molecule, repeat the electronic shell of helium, chlorine atoms become similar at the electronic level to the argon atom.
Link length
A covalent polar bond, among other things, is characterized by a certain distance between the nuclei of the atoms that form the molecule. They are at such a distance from each other that the energy of the molecule is minimal. In order to achieve this, it is necessary that the electron clouds of atoms overlap each other as much as possible. There is a directly proportional pattern between the size of atoms and the length of the bond. The larger the atom, the longer the bond between the nuclei.
It is possible that an atom forms not one, but several covalent polar bonds. Then so-called bond angles are formed between the nuclei. They can be from ninety to one hundred and eighty degrees. They determine geometric formula molecules.
Rarely chemical substances consist of individual, unrelated atoms of chemical elements. Under normal conditions, only a small number of gases called noble gases have this structure: helium, neon, argon, krypton, xenon and radon. Most often, chemical substances do not consist of isolated atoms, but of their combinations into various groups. Such associations of atoms can number a few, hundreds, thousands, or even more atoms. The force that holds these atoms in such groups is called chemical bond.
In other words, we can say that a chemical bond is an interaction that provides the connection of individual atoms into more complex structures (molecules, ions, radicals, crystals, etc.).
The reason for the formation of a chemical bond is that the energy of more complex structures is less than the total energy of the individual atoms that form it.
So, in particular, if the interaction of atoms X and Y produces a molecule XY, this means that the internal energy of the molecules of this substance is lower than the internal energy of the individual atoms from which it was formed:
E(XY)< E(X) + E(Y)
For this reason, when chemical bonds are formed between individual atoms, energy is released.
Electrons of the outer electron layer with the lowest binding energy with the nucleus, called valence. For example, in boron these are electrons of the 2nd energy level - 2 electrons per 2 s- orbitals and 1 by 2 p-orbitals:
When a chemical bond is formed, each atom tends to obtain the electronic configuration of noble gas atoms, i.e. so that there are 8 electrons in its outer electron layer (2 for elements of the first period). This phenomenon is called the octet rule.
It is possible for atoms to achieve the electron configuration of a noble gas if initially single atoms share some of their valence electrons with other atoms. In this case, common electron pairs are formed.
Depending on the degree of sharing of electrons, covalent, ionic and metallic bonds can be distinguished.
Covalent bond
Covalent bonds most often occur between atoms of nonmetal elements. If the nonmetal atoms forming a covalent bond belong to different chemical elements, such a bond is called a polar covalent bond. The reason for this name lies in the fact that atoms of different elements also have different abilities to attract a common electron pair. Obviously, this leads to a displacement of the common electron pair towards one of the atoms, as a result of which a partial negative charge is formed on it. In turn, a partial positive charge is formed on the other atom. For example, in a hydrogen chloride molecule the electron pair is shifted from the hydrogen atom to the chlorine atom:
Examples of substances with polar covalent bonds:
CCl 4, H 2 S, CO 2, NH 3, SiO 2, etc.
A covalent nonpolar bond is formed between nonmetal atoms of the same chemical element. Since the atoms are identical, their ability to attract shared electrons is also the same. In this regard, no displacement of the electron pair is observed:
The above mechanism for the formation of a covalent bond, when both atoms provide electrons to form common electron pairs, is called exchange.
There is also a donor-acceptor mechanism.
When a covalent bond is formed by the donor-acceptor mechanism, a shared electron pair is formed due to the filled orbital of one atom (with two electrons) and the empty orbital of another atom. An atom that provides a lone pair of electrons is called a donor, and an atom with a vacant orbital is called an acceptor. Atoms that have paired electrons, for example N, O, P, S, act as donors of electron pairs.
For example, according to the donor-acceptor mechanism, the formation of the fourth covalent N-H connections in the ammonium cation NH 4 +:
In addition to polarity, covalent bonds are also characterized by energy. Bond energy is the minimum energy required to break a bond between atoms.
The binding energy decreases with increasing radii of bonded atoms. Since we know that atomic radii increase down the subgroups, we can, for example, conclude that the strength of the halogen-hydrogen bond increases in the series:
HI< HBr < HCl < HF
Also, the bond energy depends on its multiplicity - the greater the bond multiplicity, the greater its energy. Bond multiplicity refers to the number of shared electron pairs between two atoms.
Ionic bond
An ionic bond can be considered as an extreme case of a polar covalent bond. If in a covalent-polar bond the common electron pair is partially shifted to one of the pair of atoms, then in an ionic bond it is almost completely “given” to one of the atoms. The atom that donates electron(s) acquires a positive charge and becomes cation, and the atom that has taken electrons from it acquires a negative charge and becomes anion.
Thus, ionic bond is a bond formed due to the electrostatic attraction of cations to anions.
The formation of this type of bond is typical during the interaction of atoms of typical metals and typical non-metals.
For example, potassium fluoride. The potassium cation is formed by the removal of one electron from a neutral atom, and the fluorine ion is formed by the addition of one electron to the fluorine atom:
An electrostatic attraction force arises between the resulting ions, resulting in the formation of an ionic compound.
When a chemical bond was formed, electrons from the sodium atom passed to the chlorine atom and oppositely charged ions were formed, which have a completed external energy level.
It has been established that electrons from the metal atom are not completely detached, but are only shifted towards the chlorine atom, as in a covalent bond.
Most binary compounds that contain metal atoms are ionic. For example, oxides, halides, sulfides, nitrides.
Ionic bonding also occurs between simple cations and simple anions (F −, Cl −, S 2-), as well as between simple cations and complex anions (NO 3 −, SO 4 2-, PO 4 3-, OH −). Therefore, ionic compounds include salts and bases (Na 2 SO 4, Cu(NO 3) 2, (NH 4) 2 SO 4), Ca(OH) 2, NaOH)
Metal connection
This type of bond is formed in metals.
Atoms of all metals have electrons in their outer electron layer that have a low binding energy with the nucleus of the atom. For most metals, the process of losing outer electrons is energetically favorable.
Due to such a weak interaction with the nucleus, these electrons in metals are very mobile and the following process continuously occurs in each metal crystal:
М 0 — ne − = M n + ,
where M 0 is a neutral metal atom, and M n + a cation of the same metal. The figure below provides an illustration of the processes taking place.
That is, electrons “rush” across a metal crystal, detaching from one metal atom, forming a cation from it, joining another cation, forming a neutral atom. This phenomenon was called “electron wind,” and the collection of free electrons in a crystal of a nonmetal atom was called “electron gas.” This type of interaction between metal atoms is called a metallic bond.
Hydrogen bond
If a hydrogen atom in a substance is bonded to an element with high electronegativity (nitrogen, oxygen, or fluorine), that substance is characterized by a phenomenon called hydrogen bonding.
Since a hydrogen atom is bonded to an electronegative atom, a partial positive charge is formed on the hydrogen atom, and a partial negative charge is formed on the atom of the electronegative element. In this regard, electrostatic attraction becomes possible between a partially positively charged hydrogen atom of one molecule and an electronegative atom of another. For example, hydrogen bonding is observed for water molecules:
It is the hydrogen bond that explains the abnormally high melting point of water. In addition to water, strong hydrogen bonds are also formed in substances such as hydrogen fluoride, ammonia, oxygen-containing acids, phenols, alcohols, and amines.
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
Schematically, the formation of a hydrogen molecule from atoms can be represented as follows (a dot means an electron, a line means a pair of electrons):
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.