The concept of chemical bonding is of no small importance in various fields of chemistry as a science. This is due to the fact that it is with its help that individual atoms are able to combine into molecules, forming all kinds of substances, which, in turn, are the subject of chemical research.
With the variety of atoms and molecules associated with the emergence of various types of bonds between them. Different classes of molecules are characterized by their own characteristics of the distribution of electrons, and hence their types of bonds.
Basic concepts
A chemical bond is a set of interactions that lead to the bonding of atoms with the formation of stable particles of a more complex structure (molecules, ions, radicals), as well as aggregates (crystals, glasses, etc.). The nature of these interactions is electric in nature, and they arise during the distribution of valence electrons in approaching atoms.
The valency is called the ability of an atom to form a certain number of bonds with other atoms. In ionic compounds, the number of electrons donated or attached is taken as the valency value. In covalent compounds, it is equal to the number of total electron pairs.
By the degree of oxidation is meant a conditional charge that could be on an atom if all polar covalent bonds were ionic in nature.
The multiplicity of a bond is the number of socialized electron pairs between the atoms in question.
The bonds considered in various branches of chemistry can be divided into two types of chemical bonds: those that lead to the formation of new substances (intramolecular) , and those that arise between molecules (intermolecular).
Key Communication Features
Bond energy is the energy that is required to break all available bonds in a molecule. It is also the energy released during bond formation.
The bond length is the distance between adjacent atomic nuclei in a molecule at which the forces of attraction and repulsion are balanced.
These two characteristics of the chemical bonding of atoms are a measure of its strength: the smaller the length and the more energy, the stronger the bond.
The valence angle is the angle between the represented lines passing in the direction of communication through the nuclei of atoms.
Relationship Description Methods
The most common are two approaches to explaining chemical bonds, borrowed from quantum mechanics:
The method of molecular orbitals. He considers the molecule as a combination of electrons and atomic nuclei, with each individual electron moving in the field of action of all other electrons and nuclei. The molecule has an orbital structure, and all its electrons are distributed in these orbits. Also, this method is called MO LKAO, which stands for "molecular orbital - a linear combination of atomic orbitals."
The valence bond method. Represents a molecule as a system of two central molecular orbitals. Moreover, each of them corresponds to one bond between two adjacent atoms in the molecule. The method is based on the following provisions:
- The chemical bond is formed by a pair of electrons having opposite spins that are located between the two atoms in question. The formed electron pair belongs to two atoms equally.
- The number of bonds formed by a particular atom is equal to the number of unpaired electrons in the ground and excited states.
- If electronic pairs do not participate in the formation of communication, then they are called lone.
Electronegativity
The type of chemical bond in substances can be determined based on the difference in the values of the electronegativities of its constituent atoms. By electronegativity is understood the ability of atoms to pull back common electron pairs (the electron cloud), which leads to polarization of the bond.
There are various methods for determining the electronegativity of chemical elements. However, the most applicable is a scale based on thermodynamic data, which was proposed back in 1932 by L. Pauling.
The greater the difference in the electronegativity of atoms, the more its ionicity manifests itself. On the contrary, equal or close values of electronegativity indicate the covalent nature of the bond. In other words, it is possible to determine mathematically what chemical bond is observed in a particular molecule. To do this, you need to calculate ΔX - the difference of electronegativity of atoms by the formula: ΔX = | X 1 -X 2 |.
- If ΔX> 1.7, then the bond is ionic.
- If 0.5≤ΔX≤1.7, then the covalent bond is polar.
- If ΔX = 0 or is close to it, then the bond refers to non-polar covalent.
Ionic bond
An ion is a bond that appears between ions or due to the complete pulling of a common electron pair by one of the atoms. In substances, this type of chemical bond is carried out by electrostatic attraction.
Ions are charged particles formed from atoms as a result of the attachment or recoil of electrons. If an atom receives electrons, it acquires a negative charge and becomes an anion. If the atom gives up valence electrons, then it becomes a positively charged particle, called a cation.
It is characteristic of compounds formed by the interaction of atoms of typical metals with atoms of typical non-metals. The core of this process is the desire of atoms to acquire stable electronic configurations. And for typical metals and non-metals for this you need to give or receive only 1-2 electrons, which they easily do.
The mechanism of the formation of an ionic chemical bond in a molecule is traditionally considered by the example of the interaction of sodium and chlorine. Atoms of an alkali metal easily give off an electron drawn by a halogen atom. As a result, the Na + cation and the Cl - anion are formed , which are held nearby by electrostatic attraction.
An ideal ionic bond does not exist. Even in such compounds, which are often referred to as ionic, the final transition of electrons from atom to atom does not occur. An educated electron pair still remains in common use. Therefore, they speak of the degree of ionicity of the covalent bond.
The ionic bond is characterized by two main properties related to each other:
- non-directionality, i.e., the electric field around the ion has the shape of a sphere;
- unsaturation, i.e., the number of oppositely charged ions, which can be placed around any ion, is determined by their size.
Covalent chemical bond
The bond formed by the overlapping of electronic clouds of non-metal atoms, that is, by a common electron pair, is called a covalent bond. The number of socialized electron pairs determines the multiplicity of the bond. So, hydrogen atoms are connected by a single H ··· H bond, and oxygen atoms form an O :: O double bond.
There are two mechanisms of its formation:
- Exchange - each atom represents, for the formation of a common pair, one electron: A · + · B = A: B, while external atomic orbitals, on which one electron is located, participate in the bonding process.
- Donor-acceptor - to form a bond, one of the atoms (donor) provides a pair of electrons, and the second (acceptor) provides a free orbital for its placement: A +: B = A: B.
The methods for overlapping electron clouds in the formation of a covalent chemical bond are also different.
- Direct. The region of overlapping clouds lies on a straight imaginary line connecting the nuclei of the atoms in question. In this case, σ bonds are formed. The type of chemical bond that occurs in this case depends on the type of electron clouds undergoing overlap: ss, sp, pp, sd or pd σ-bonds. In a particle (molecule or ion) between two neighboring atoms, only one σ-bond can be realized.
- Lateral. It is carried out on both sides of the line connecting the nuclei of atoms. Thus, the π – bond is formed, and its varieties are also possible: pp, pd, dd. Apart from the σ-bond, the π-bond never forms; it can be in molecules containing multiple (double and triple) bonds.
Properties of covalent bond
They determine the chemical and physical characteristics of the compounds. The main properties of any chemical bond in substances is its directivity, polarity and polarizability, as well as saturation.
The orientation of the bond is due to the peculiarities of the molecular structure of substances and the geometric shape of their molecules. Its essence is that the best overlapping of electronic clouds is possible with a certain orientation in space. The options for the formation of the σ and π bonds have already been considered above.
Saturation is understood as the ability of atoms to form a certain number of chemical bonds in a molecule. The number of covalent bonds for each atom is limited by the number of external orbitals.
The bond polarity depends on the difference in the values of electronegativity of atoms. The uniformity of the distribution of electrons between the nuclei of atoms depends on it. The covalent bond on this basis may be polar or non-polar.
- If the common electron pair equally belongs to each of the atoms and is located at the same distance from their nuclei, then the covalent bond is nonpolar.
- If the total pair of electrons is shifted to the nucleus of one of the atoms, then a covalent polar chemical bond is formed.
The polarizability is expressed by the displacement of the bond electrons under the action of an external electric field, which can belong to another particle, neighboring bonds in the same molecule, or come from external sources of electromagnetic fields. So, a covalent bond under their influence can change its polarity.
Under the hybridization of orbitals understand the change in their forms during the implementation of chemical bonds. This is necessary to achieve the most effective overlap. The following types of hybridization are available:
- sp 3 . One s- and three p-orbitals form four “hybrid” orbitals of the same shape. Outwardly resembles a tetrahedron with an angle between the axes of 109 °.
- sp 2 . One s- and two p-orbitals form a flat triangle with an angle between the axes of 120 °.
- sp. One s- and one p-orbital form two “hybrid” orbitals with an angle between their axes of 180 °.
Metal bond
A feature of the structure of metal atoms is a rather large radius and the presence of a small number of electrons in external orbitals. As a result of this, in such chemical elements, the bond between the nucleus and valence electrons is relatively weak and breaks easily.
A metal bond refers to such an interaction between metal atoms-ions, which is carried out using delocalized electrons.
In metal particles, valence electrons can easily leave external orbitals, as, by the way, can occupy vacant places on them. Thus, at different points in time, the same particle can be an atom and an ion. Electrons that have separated from them move freely throughout the entire volume of the crystal lattice and carry out chemical bonding.
This type of bond has similarities with ionic and covalent. As well as for ionic, ions are necessary for the existence of a metal bond. But if, in the first case, cations and anions are needed for electrostatic interaction, then in the second, the role of negatively charged particles is played by electrons. If we compare the metallic bond with the covalent bond, then common electrons are necessary for the formation of both. However, unlike the polar chemical bond, they are not localized between two atoms, but belong to all metal particles in the crystal lattice.
A metal bond determines the special properties of almost all metals:
- plasticity, due to the possibility of displacement of atomic layers in the crystal lattice held by an electron gas;
- metallic luster, which is observed due to the reflection of light rays from electrons (in a powdered state there is no crystal lattice and, therefore, electrons moving along it);
- electrical conductivity, which is carried out by a stream of charged particles, and in this case small electrons move freely among large metal ions;
- thermal conductivity is observed due to the ability of electrons to transfer heat.
Hydrogen bond
This type of chemical bond is sometimes called the intermediate between covalent and intermolecular interactions. If a hydrogen atom has a bond with one of the highly electronegative elements (such as phosphorus, oxygen, chlorine, nitrogen), then it is able to form an additional bond, called hydrogen.
It is much weaker than all the types of bonds considered above (energy no more than 40 kJ / mol), but it cannot be neglected. That is why the hydrogen chemical bond in the diagram looks like a dashed line.
The occurrence of a hydrogen bond is possible due to the donor-acceptor electrostatic interaction at the same time. A large difference in the values of electronegativity leads to the appearance of excess electron density at the O, N, F atoms and others, as well as its lack on the hydrogen atom. In the event that there is no existing chemical bond between such atoms, when they are close enough, the attractive forces are activated. In this case, the proton is the acceptor of the electron pair, and the second atom is the donor.
Hydrogen bonding can occur both between adjacent molecules, for example, water, carboxylic acids, alcohols, ammonia, and inside the molecule, for example, salicylic acid.
The presence of a hydrogen bond between water molecules explains a number of its unique physical properties:
- The values of its specific heat, permittivity, boiling and melting temperatures, in accordance with the calculations, should be much lower than real values, which is explained by the bonding of molecules and the need to expend energy on breaking intermolecular hydrogen bonds.
- Unlike other substances, with decreasing temperature, the volume of water increases. This is due to the fact that the molecules occupy a certain position in the crystalline structure of ice and move away from each other by the length of the hydrogen bond.
This connection plays a special role for living organisms, since its presence in the molecules of proteins determines their special structure, and hence their properties. In addition, nucleic acids, making up the double helix of DNA, are also connected precisely by hydrogen bonds.
Bonds in crystals
The vast majority of solids has a crystal lattice - a special mutual arrangement of the particles forming them. In this case, three-dimensional periodicity is observed, and atoms, molecules or ions are located in the nodes, which are connected by imaginary lines. Depending on the nature of these particles and the bonds between them, all crystalline structures are divided into atomic, molecular, ionic and metallic.
At the sites of the ionic crystal lattice are cations and anions. Moreover, each of them is surrounded by a strictly defined number of ions with only the opposite charge. A typical example is sodium chloride (NaCl). High melting points and hardness are usual for them, since their destruction requires a lot of energy.
At the nodes of the molecular crystal lattice are molecules of substances formed by a covalent bond (for example, I 2 ). They are connected with each other by a weak van der Waals interaction, and therefore, such a structure is easy to destroy. Such compounds have low boiling and melting points.
The atomic crystal lattice is formed by atoms of chemical elements with high valency values. They are connected by strong covalent bonds, which means that the substances are characterized by high boiling, melting and high hardness. An example is a diamond.
Thus, all types of bonds found in chemicals have their own characteristics, which explain the intricacies of the interaction of particles in molecules and substances. The properties of the compounds depend on them. They determine all processes occurring in the environment.