As you know, chemistry studies the structure and properties of substances, as well as their mutual transformations. An important place in the characterization of chemical compounds is occupied by the question of what kind of particles they consist of. It can be atoms, ions or molecules. In solids, they enter the nodes of crystal lattices. The molecular structure has a relatively small number of compounds in the solid, liquid and gaseous state.
In our article, we will give examples of substances that are characterized by molecular crystal lattices, and also consider several types of intermolecular interactions characteristic of solids, liquids and gases.
Why you need to know the structure of chemical compounds
In each branch of human knowledge, one can single out a group of fundamental laws on which the further development of science is based. in chemistry- this is the theory of M. V. Lomonosov and J. D alton, explaining the atomic and molecular structure of matter. As scientists have established, knowing the internal structure, it is possible to predict both the physical and chemical properties of the compound. The whole huge amount of organic substances artificially synthesized by man (plastics, drugs, pesticides, etc.) have predetermined characteristics and properties that are most valuable for his industrial and household needs.
Knowledge about the features of the structure and properties of compounds is in demand when conducting control sections, tests and exams in the course of chemistry. For example, in the proposed list of substances, find the correct answers: what substance has a molecular structure?
- Zinc.
- Magnesium oxide.
- Diamond.
- Naphthalene.
The correct answer is: zinc has a molecular structure, as well as naphthalene.
Forces of intermolecular interaction
It has been experimentally established that the molecular structure is characteristic of substances with low melting points and low hardness. How can one explain the fragility of the crystal lattices of these compounds? As it turned out, everything depends on the strength of the joint influence of the particles located in their nodes. It has an electrical nature and is called intermolecular interaction or van der Waals forces, which are based on the influence of oppositely charged molecules - dipoles - on each other. It turned out that there are several mechanisms for their formation,depending on the nature of the substance itself.
Acids as compounds of molecular composition
Solutions of most acids, both organic and inorganic, contain polar particles that are oriented relative to each other with oppositely charged poles. For example, in a solution of hydrochloric acid HCl, there are dipoles, between which orientational interactions occur. With an increase in temperature, the molecules of hydrochloric, hydrobromic (HBr) and other halogen-containing acids have a decrease in the orientation effect, since the thermal motion of the particles interferes with their mutual attraction. In addition to the above substances, sucrose, naphthalene, ethanol and other organic compounds have a molecular structure.
How induced charged particles are produced
Earlier, we considered one of the mechanisms of action of Van der Waals forces, called orientational interaction. In addition to organic substances and halogen-containing acids, the molecular structure has hydrogen oxide - water. In substances consisting of non-polar, but prone to the formation of dipoles, molecules, such as carbon dioxide CO2, one can observe the appearance of induced charged particles - dipoles. Their most important property is the ability to attract each other due to the appearance of electrostatic attraction forces.
Molecular structure of gas
In the previous subheading, we mentioned the compound carbon dioxide. Each of its atoms creates an electric field around itself, which inducespolarization per atom of a nearby carbon dioxide molecule. It changes into a dipole, which, in turn, becomes capable of polarizing other CO2 particles. As a result, the molecules are attracted to each other. The inductive interaction can also be observed in substances consisting of polar particles, however, in this case it is much weaker than the orientational van der Waals forces.
Dispersion interaction
Both the atoms themselves and the particles that make up them (nucleus, electrons) are capable of continuous rotational and oscillatory motion. It leads to the appearance of dipoles. According to the research of quantum mechanics, the occurrence of instantaneous doubly charged particles occurs both in solids and in liquids synchronously, so that the ends of the molecules located nearby turn out to be with opposite poles. This leads to their electrostatic attraction, called the dispersion interaction. It is characteristic of all substances, except those that are in a gaseous state, and whose molecules are monatomic. However, van der Waals forces can arise, for example, during the transition of inert gases (helium, neon) into the liquid phase at low temperatures. Thus, the molecular structure of bodies or liquids determines their ability to form various types of intermolecular interaction: orientational, induced or dispersion.
What is sublimation
Molecular structure of a solid, such as iodine crystals,causes such an interesting physical phenomenon as sublimation - volatilization of I2 molecules in the form of violet vapors. It occurs from the surface of a substance in the solid phase, bypassing the liquid state.
This visually spectacular experiment is often done in high school chemistry classrooms to illustrate the structure of molecular crystal lattices and related properties of compounds. Usually this is low hardness, low melting and boiling points, poor thermal and electrical conductivity, volatility.
Practical use of knowledge about the structure of substances
As we have seen, a certain correlation can be established between the type of crystal lattice, structure and properties of the compound. Therefore, if the characteristics of a substance are known, then it is quite easy to predict the features of its structure and composition of particles: atoms, molecules or ions. The information obtained can also be useful if in tasks in chemistry it is necessary to correctly select substances that have a molecular structure from a certain group of compounds, excluding those that have atomic or ionic types of lattices.
Summing up, we can conclude the following: the molecular structure of a solid body, and its spatial structure of crystal lattices, and the arrangement of polarized particles in liquids and gases are fully responsible for its physical and chemical properties. In theoretical terms, the properties of compounds,containing dipoles depend on the magnitude of the forces of intermolecular interaction. The higher the polarity of the molecules and the smaller the radius of the atoms that make up them, the stronger the orientation forces that arise between them. On the contrary, the larger the atoms that make up the molecule, the higher its dipole moment, and, therefore, the more significant the dispersion forces. Thus, the molecular structure of a solid also affects the forces of interaction between its particles - dipoles.
