The concept of a chemical bond 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.
The variety of atoms and molecules is associated with the emergence of various types of bonds between them. Different classes of molecules are characterized by their own features of the distribution of electrons, and hence their own types of bonds.
Basic concepts
A chemical bond is a set of interactions that lead to the binding of atoms to form stable particles of a more complex structure (molecules, ions, radicals), as well as aggregates (crystals, glasses, etc.). The nature of these interactions is electrical in nature, and they arise during the distribution of valence electrons in approaching atoms.
Valency is usually called the ability of an atom to form a certain number of bonds with other atoms. In ionic compounds, the number of given or attached electrons is taken as the value of valence. ATin covalent compounds, it is equal to the number of common electron pairs.
The oxidation state is understood as the conditional charge that could be on an atom if all polar covalent bonds were ionic.
The bond multiplicity is the number of shared electron pairs between the considered atoms.
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).
Basic communication characteristics
The binding energy is the energy required to break all existing bonds in a molecule. It is also the energy released during bond formation.
The bond length is the distance between adjacent nuclei of atoms in a molecule, at which the forces of attraction and repulsion are balanced.
These two characteristics of the chemical bond of atoms are a measure of its strength: the shorter the length and the greater the energy, the stronger the bond.
The bond angle is usually called the angle between the represented lines passing in the direction of bonding through the nuclei of atoms.
Methods for describing links
The most common two approaches to explaining the chemical bond, borrowed from quantum mechanics:
Method of molecular orbitals. He considers a molecule as a set of electrons and nuclei of atoms, 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 along these orbits. Also, this method is called MO LCAO, which stands for "molecular orbital - a linear combination of atomic orbitals".
Method of valence bonds. 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 formation of a chemical bond is carried out by a pair of electrons with opposite spins, which are located between the two considered atoms. The formed electron pair belongs to two atoms equally.
- The number of bonds formed by an atom is equal to the number of unpaired electrons in the ground and excited state.
- If electron pairs do not take part in bond formation, then they are called lone pairs.
Electronegativity
It is possible to determine the type of chemical bond in substances based on the difference in the electronegativity values of its constituent atoms. Electronegativity is understood as the ability of atoms to attract common electron pairs (electron cloud), which leads to bond polarization.
There are various ways to determine the values of the electronegativity of chemical elements. However, the most commonly used is the 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 pronounced its ionicity. On the contrary, equal or close electronegativity values indicate the covalent nature of the bond. In other words, it is possible to determine which chemical bond is observed in a particular molecule mathematically. To do this, you need to calculate ΔX - the difference in the electronegativity of atoms according to the formula: ΔX=|X 1 -X 2|.
- If ΔХ>1, 7, then the bond is ionic.
- If 0.5≦ΔХ≦1.7, then the covalent bond is polar.
- If ΔХ=0 or close to it, then the bond is covalent non-polar.
Ionic bond
Ionic is such a bond that appears between ions or due to the complete withdrawal of a common electron pair by one of the atoms. In substances, this type of chemical bonding is carried out by forces of electrostatic attraction.
Ions are charged particles formed from atoms as a result of electrons gaining or losing. When an atom accepts electrons, it acquires a negative charge and becomes an anion. If an atom donates valence electrons, 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 main of this process is the aspiration of atoms to acquire stable electronic configurations. And for this, typical metals and non-metals need to give or accept only 1-2 electrons,which they do with ease.
The mechanism of formation of an ionic chemical bond in a molecule is traditionally considered using the example of the interaction of sodium and chlorine. Alkali metal atoms easily donate an electron pulled by a halogen atom. The result is the Na+ cation and the Cl- anion, which are held together by electrostatic attraction.
There is no ideal ionic bond. Even in such compounds, which are often referred to as ionic, the final transfer of electrons from atom to atom does not occur. The formed electron pair still remains in common use. Therefore, they talk about the degree of ionicity of a covalent bond.
Ionic bonding is characterized by two main properties related to each other:
- non-directional, i.e. the electric field around the ion has the shape of a sphere;
- Unsaturation, i.e. the number of oppositely charged ions that can be placed around any ion, is determined by their size.
Covalent chemical bond
The bond formed when the electron clouds of non-metal atoms overlap, that is, carried out by a common electron pair, is called a covalent bond. The number of shared pairs of electrons determines the multiplicity of the bond. Thus, hydrogen atoms are linked by a single H··H bond, and oxygen atoms form a double bond O::O.
There are two mechanisms for its formation:
- Exchange - each atom represents one electron for the formation of a common pair: A +B=A: B, while the connection involves external atomic orbitals, on which one electron is located.
- Donor-acceptor - to form a bond, one of the atoms (donor) provides a pair of electrons, and the second (acceptor) - a free orbital for its placement: A +:B=A:B.
The ways in which electron clouds overlap when a covalent chemical bond is formed are also different.
- Direct. The cloud overlap region lies on a straight imaginary line connecting the nuclei of the considered atoms. 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: s-s, s-p, p-p, s-d or p-d σ-bonds. In a particle (molecule or ion), only one σ-bond can occur between two neighboring atoms.
- Side. It is carried out on both sides of the line connecting the nuclei of atoms. This is how a π-bond is formed, and its varieties are also possible: p-p, p-d, d-d. Separate from the σ-bond, the π-bond is never formed; it can be in molecules containing multiple (double and triple) bonds.
Covalent bond properties
They determine the chemical and physical characteristics of compounds. The main properties of any chemical bond in substances are its directionality, polarity and polarizability, as well as saturation.
The directionality of the bond determines the features of the molecularthe structure of substances and the geometric shape of their molecules. Its essence lies in the fact that the best overlap of electron clouds is possible with a certain orientation in space. The options for the formation of 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 outer orbitals.
The polarity of the bond depends on the difference in the electronegativity values of the atoms. It determines the uniformity of the distribution of electrons between the nuclei of atoms. A covalent bond on this basis can 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 non-polar.
- If the common pair of electrons is shifted to the nucleus of one of the atoms, then a covalent polar chemical bond is formed.
Polarizability is expressed by the displacement of bond electrons under the action of an external electric field, which may 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 in the implementation of a chemical bond. This is necessary to achieve the most effective overlap. There are the following types of hybridization:
- sp3. One s- and three p-orbitals form four"hybrid" orbitals of the same shape. Outwardly, it resembles a tetrahedron with an angle between the axes of 109 °.
- sp2. 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 outer orbitals. As a result, in such chemical elements, the bond between the nucleus and valence electrons is relatively weak and easily broken.
Metal bond is such an interaction between metal atoms-ions, which is carried out with the help of delocalized electrons.
In metal particles, valence electrons can easily leave outer orbitals, as well as occupy vacant places on them. Thus, at different times, the same particle can be an atom and an ion. The electrons torn off from them move freely throughout the entire volume of the crystal lattice and carry out a chemical bond.
This type of bond has similarities with ionic and covalent. As well as for ionic, ions are necessary for the existence of a metallic bond. But if for the implementation of electrostatic interaction in the first case, cations and anions are needed, then in the second, the role of negatively charged particles is played by electrons. If we compare a metallic bond with a covalent bond, then common electrons are needed to form both. However, inunlike a polar chemical bond, they are not localized between two atoms, but belong to all metal particles in the crystal lattice.
Metallic bonds are responsible for the special properties of almost all metals:
- plasticity, present due to the possibility of displacement of layers of atoms in the crystal lattice held by electron gas;
- metallic luster, which is observed due to the reflection of light rays from electrons (in the powder 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, observed due to the ability of electrons to transfer heat.
Hydrogen bond
This type of chemical bond is sometimes called an intermediate between covalent and intermolecular interaction. If a hydrogen atom has a bond with one of the strongly 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 is not more than 40 kJ/mol), but it cannot be neglected. That is why the hydrogen chemical bond in the diagram looks like a dotted line.
The occurrence of a hydrogen bond is possible due to the donor-acceptor electrostatic interaction at the same time. Big difference in valueselectronegativity leads to the appearance of excess electron density on the atoms O, N, F and others, as well as to its lack on the hydrogen atom. In the event that there is no existing chemical bond between such atoms, attractive forces are activated if they are close enough. In this case, the proton is an electron pair acceptor, and the second atom is a donor.
Hydrogen bonding can occur both between neighboring molecules, for example, water, carboxylic acids, alcohols, ammonia, and within a 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 heat capacity, dielectric constant, boiling and melting points, in accordance with the calculations, should be much less than the real ones, which is explained by the bonding of molecules and the need to expend energy to break intermolecular hydrogen bonds.
- Unlike other substances, when the temperature drops, the volume of water increases. This is due to the fact that the molecules occupy a certain position in the crystal 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 protein molecules determines their special structure, and hence their properties. In addition, nucleic acids, making up the DNA double helix, are also connected precisely by hydrogen bonds.
Communications in crystals
The vast majority of solids have a crystal lattice - a speci althe mutual arrangement of the particles that form them. In this case, three-dimensional periodicity is observed, and atoms, molecules or ions are located at the nodes, which are connected by imaginary lines. Depending on the nature of these particles and the bonds between them, all crystal structures are divided into atomic, molecular, ionic and metallic.
There are cations and anions in the nodes of the ionic crystal lattice. 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). They tend to have high melting points and hardness as they require a lot of energy to break.
Molecules of substances formed by a covalent bond are located at the nodes of the molecular crystal lattice (for example, I2). They are connected to 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 valence values. They are connected by strong covalent bonds, which means that the substances have high boiling and melting points 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.
