Covalent, ionic and metallic - the three main types 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 approach each other up to a certain distance, their orbitals partially overlap (see Fig.), 
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, there is a strong increase in the forces of attraction between the molecular cloud and the nuclei.
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 the atoms approaching to touch is 0.106 nm, then after the overlap of the electron clouds it will be 0.074 nm. The greater the overlap of electron orbitals, the stronger the chemical bond.
covalent called chemical bonding carried out by electron pairs. Compounds with a covalent bond are called homeopolar or atomic.
Exist two types of covalent bond: polar And non-polar.
With non-polar covalent bond formed by a common pair of electrons, the electron cloud is distributed symmetrically with respect to the nuclei of both atoms. An example can be diatomic molecules that consist of one element: Cl 2, N 2, H 2, F 2, O 2 and others, in which the electron pair belongs to both atoms equally.
At polar In a covalent bond, the electron cloud is displaced towards the atom with a higher relative electronegativity. For example, molecules of volatile inorganic compounds such as H 2 S, HCl, H 2 O and others.
The formation of the 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 shifts towards the chlorine atom.
In addition to the exchange mechanism for the formation of a covalent bond - 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 a two-electron cloud of one atom (donor) and a 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 it as .
In the process of ammonium ion formation, the two-electron cloud of nitrogen becomes common for nitrogen and hydrogen atoms, which means it is converted into a molecular electron cloud. Therefore, a fourth covalent bond appears. The process of ammonium formation can be represented as follows: 
The charge of the hydrogen ion is dispersed among all atoms, and the two-electron cloud that belongs to nitrogen becomes common with hydrogen.
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In which one of the atoms donated an electron and became a cation, and the other atom accepted an electron and became an anion.
Characteristic properties covalent bond - directionality, saturation, polarity, polarizability - determine the chemical and physical properties connections.
The direction of the bond is due to the molecular structure of the substance and geometric shape their molecules. The angles between two bonds are called bond angles.
Saturation - 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 the 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 with respect 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 the 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 another reacting particle. Polarizability is determined by electron mobility. The polarity and polarizability of covalent bonds determine the reactivity of molecules with respect to 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 single-electron chemical bond is realized in the molecular ion hydrogen H 2 + .
The molecular hydrogen ion H 2 + contains two protons and one electron. The single electron of the molecular system compensates for the electrostatic repulsion of two protons and keeps them at a distance of 1.06 Å (the length of the H 2 + chemical bond). The center of the electron density of the electron cloud of the molecular system is equidistant from both protons by the Bohr radius α 0 =0.53 A and is the center of symmetry of the molecular hydrogen ion H 2 + .
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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 is 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 (MOs): binding MO And antibonding (loosening) MO. Shared electrons are located on a lower energy binding MO.
Formation of a bond during the recombination of atoms
However, the mechanism of interatomic interaction remained unknown for a long time. Only in 1930, F. London introduced the concept of dispersion attraction - the interaction between instantaneous and induced (induced) dipoles. At present, the attractive forces due to the interaction between 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 sixth power of the distance between two atoms or molecules.
Bond formation by the donor-acceptor mechanism
In addition to the homogeneous mechanism for the formation of a covalent bond described in the previous section, there is a heterogeneous mechanism - the interaction of oppositely charged ions - the proton H + and the negative hydrogen ion H -, called the hydride ion:
H + + H - → H 2
When the ions approach, the two-electron cloud (electron pair) of the hydride ion is attracted to the proton and eventually becomes common to both hydrogen nuclei, that is, it turns into a binding 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. Such a mechanism for the formation of a covalent bond is called donor-acceptor.
H + + H 2 O → H 3 O +
The proton attacks the lone electron pair of the water molecule and forms a stable cation that exists in aqueous solutions acids.
Similarly, a proton is attached to an ammonia molecule with the formation of a complex ammonium cation:
NH 3 + H + → NH 4 +
In this way (according to the donor-acceptor mechanism for the formation of a covalent bond), a large class of onium compounds is obtained, which includes ammonium, oxonium, phosphonium, sulfonium and other compounds.
A hydrogen molecule can act as an electron pair donor, which, upon contact with a proton, leads to the formation of a molecular hydrogen ion H 3 + :
H 2 + H + → H 3 +
The binding 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 that differ in the mechanism of formation:
1. Simple covalent bond. For its formation, each of the atoms provides one unpaired electron. When a simple covalent bond is formed, the formal charges of the atoms remain unchanged.
- If the atoms that form a simple covalent bond are the same, then the true charges of the atoms in the molecule are also the same, since the atoms that form the bond equally own a shared electron pair. Such a connection is called non-polar covalent bond. Simple substances have such a connection, for example: 2, 2, 2. But not only non-metals of the same type can form a covalent non-polar bond. Non-metal elements whose electronegativity is of equal value can also form a covalent non-polar bond, for example, in the PH 3 molecule, the bond is covalent non-polar, since the EO of hydrogen is equal to the EO of phosphorus.
- If the atoms are different, then the degree of ownership of a socialized pair of electrons is determined by the difference in the electronegativity of the atoms. An atom with greater electronegativity attracts a pair of bond electrons to itself more strongly, and its true charge becomes negative. An atom with less electronegativity acquires, respectively, the same positive charge. If a compound is formed between two different non-metals, then such a compound is called polar covalent bond.
In the ethylene molecule C 2 H 4 there is a double bond CH 2 \u003d CH 2, its electronic formula: N:S::S:N. The nuclei of all ethylene atoms are located in the same plane. Three electron clouds of each carbon atom form three covalent bonds with other atoms in the same plane (with angles between them of about 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 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.
In a linear acetylene molecule
H-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 (\displaystyle \pi ) bonds between the same carbon atoms. Two π (\displaystyle \pi )-bonds are located above the sphere of action of the σ-bond in two mutually perpendicular planes.
All six carbon atoms of the C 6 H 6 cyclic benzene molecule lie in the same plane. σ-bonds act between carbon atoms in the plane of the ring; the same bonds exist for each carbon atom with hydrogen atoms. Each carbon atom spends three electrons to make these bonds. Clouds of the fourth valence electrons of carbon atoms, having the shape of eights, are located perpendicular to the plane of the benzene molecule. Each such cloud overlaps equally with the electron clouds of neighboring carbon atoms. In the benzene molecule, not three separate π (\displaystyle \pi )-connections, but a single π (\displaystyle \pi ) dielectrics or semiconductors. Typical examples of atomic crystals (the atoms in which are interconnected by covalent (atomic) bonds) are
covalent chemical bond occurs between atoms with close or equal values of electronegativity. Suppose that chlorine and hydrogen tend to take electrons and take on the structure of the nearest noble gas, then neither of them will give up an electron to the other. How are they connected anyway? It's simple - they will share with each other, a common electron pair is formed.
Now consider the distinctive features of a 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, the covalent bond is characteristic saturability– the formation of a limited number of bonds.
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 orientation.
If a covalent bond in a molecule is formed by identical atoms or atoms with equal electronegativity, then such a bond has no polarity, i.e. the electron density is distributed symmetrically. It's called non-polar covalent bond ( H 2 , Cl 2 , O 2 ). Bonds can be single, double or triple.
If the electronegativity of the atoms differ, then when they combine, the electron density is distributed unevenly between the atoms and forms covalent polar bond(HCl, H 2 O, CO), the multiplicity of which can also be different. When this type of bond is formed, a more electronegative atom acquires a partial negative charge, and an atom with a lower 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 compound is all the more pronounced, the greater the dipole moment. Molecules will be non-polar if the dipole moment is zero.
In connection with the above features, it can be concluded that covalent compounds are volatile and have low melting and boiling points. Electricity cannot pass through these connections, hence they are poor conductors and good insulators. When heat is applied, many covalently bonded compounds ignite. For the most part, these are hydrocarbons, as well as oxides, sulfides, halides of non-metals and transition metals.
Categories ,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 if we listen to the teacher in the lesson and learn new material or we brew tea, which in general is also 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 force.
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 various 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 chemical reaction.
Examples of covalent polar, substances with 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 polar electrons were located closer to one atom, then at non-polar bond, the electrons are arranged symmetrically, that is, equally with respect to both.
It is noteworthy that a non-polar arises between the atoms of a non-metal of one chemical element.
Eg, substances with a non-polar covalent bond:
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.
covalent bond(from the Latin "with" jointly and "vales" valid) is carried out by an electron pair belonging to both atoms. Formed between atoms of non-metals.
The electronegativity of non-metals is quite large, so that during the chemical interaction of two non-metal atoms, the complete transfer of electrons from one to the other (as in the case) is impossible. In this case, electron pooling is necessary to perform.
As an example, let's discuss the interaction of hydrogen and chlorine atoms:
H 1s 1 - one electron
Cl 1s 2 2s 2 2 p6 3 s2 3 p5 - seven electrons in the outer level
Each of the two atoms lacks one electron in order to have a complete outer electron shell. And each of the atoms allocates “for common use” one electron. Thus, the octet rule is satisfied. The best way to represent this is with the Lewis formulas:
Formation of a covalent bond
The shared electrons now belong to both atoms. The hydrogen atom has two electrons (its own and the shared electron of the chlorine atom), and the chlorine atom has eight electrons (its own plus the shared electron of the hydrogen atom). These two shared electrons form a covalent bond between the hydrogen and chlorine atoms. The particle formed when two atoms bond is called molecule.
Non-polar covalent bond
A covalent bond can form between two the same atoms. For example:
This diagram explains why hydrogen and chlorine exist as diatomic molecules. Thanks to the pairing and socialization of two electrons, it is possible to fulfill the octet rule for both atoms.
In addition to single bonds, a double or triple covalent bond can be formed, as, for example, in oxygen O 2 or nitrogen N 2 molecules. Nitrogen atoms each have five valence electrons, so three more electrons are required to complete the shell. This is achieved by sharing three pairs of electrons, as shown below:
Covalent compounds - usually gases, liquids, or relatively low melting points solids. One of the rare exceptions is diamond, which melts above 3,500°C. This is due to the structure of diamond, which is a continuous lattice of covalently bonded carbon atoms, and not a collection of individual molecules. In fact, any diamond crystal, regardless of its size, is one huge molecule.
A covalent bond occurs when the electrons of two nonmetal atoms join together. The resulting structure is called a molecule.
Polar covalent bond
In most cases, two covalently bonded atoms have different electronegativity and shared electrons do not belong equally to two atoms. Most of the time they are closer to one atom than to another. In a molecule of hydrogen chloride, for example, the electrons that form a covalent bond are located closer to the chlorine atom, since its electronegativity is higher than that of hydrogen. However, the difference in the ability to attract electrons is not so great that there is a complete transfer of an electron from a hydrogen atom to a chlorine atom. Therefore, the bond between hydrogen and chlorine atoms can be viewed as a cross between an ionic bond (full electron transfer) and a non-polar covalent bond (symmetrical arrangement of a pair of electrons between two atoms). The partial charge on atoms is denoted by the Greek letter δ. Such a connection is called polar covalent bond, and the hydrogen chloride molecule is said to be polar, that is, it has a positively charged end (hydrogen atom) and a negatively charged end (chlorine atom).
The table below lists the main types of bonds and examples of substances:
Exchange and donor-acceptor mechanism of covalent bond formation
1) Exchange mechanism. Each atom contributes one unpaired electron to a shared electron pair.
2) Donor-acceptor mechanism. One atom (donor) provides an electron pair, and another atom (acceptor) provides an empty orbital for this pair.
