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 the crystal lattices. The molecular structure has a relatively small number of compounds in a 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 interaction characteristic of solids, liquids, and gases.
Why you need to know the structure of chemical compounds
In each branch of human knowledge, one can distinguish 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. Dalton, explaining the atomic-molecular structure of matter. As scientists established, knowing the internal structure, one can predict both the physical and chemical properties of the compound. All the huge number of organic substances artificially synthesized by humans (plastics, pharmaceuticals, pesticides, etc.) have predefined characteristics and properties that are most valuable for their industrial and household needs.
Knowledge of the structural features and properties of the compounds is in demand when conducting control sections, tests and exams in a chemistry course. For example, in the proposed list of substances to find the correct answer: what substance has a molecular structure?
- Zinc.
- Magnesium Oxide
- Diamond.
- Naphthalene.
The correct answer is: zinc has a molecular structure, as well as naphthalene.
The forces of intermolecular interaction
It has been experimentally established that the molecular structure is characteristic of substances with low melting points and low hardness. What can 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 of 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 by oppositely charged poles. For example, dipoles are present in the HCI chloride solution, between which orientational interactions occur. With an increase in temperature, the molecules of hydrochloric, hydrobromic (HBr) and other halogen-containing acids decrease in the orientation effect, since the thermal motion of the particles prevents their mutual attraction. In addition to the above substances, the molecular structure has sucrose, naphthalene, ethanol and other organic compounds.
How do induced charged particles arise?
Earlier, we examined one of the mechanisms of action of the Van der Waals forces, called orientational interaction. In addition to organic substances and halogen-containing acids, the molecular structure has hydrogen oxide - water. For substances consisting of non-polar, but prone to dipoles, molecules, such as carbon dioxide CO 2 , one can observe the appearance of induced charged particles - dipoles. Their most important property is the ability to be attracted to each other, due to the appearance of electrostatic attraction forces.
Molecular structure of gas
In the previous subheading, we mentioned a compound such as carbon dioxide. Each of its atoms creates an electric field around itself, which induces polarization on the atom of a nearby carbon dioxide molecule. It modifies into a dipole, which, in turn, becomes capable of polarizing other CO 2 particles. As a result, the molecules are attracted to each other. Induction interaction can also be observed in substances consisting of polar particles, although 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 their composition (nuclei, electrons) are capable of continuous rotational and vibrational motion. It leads to the appearance of dipoles. According to the studies of quantum mechanics, the appearance of instantly doubly charged particles occurs both in solids and in liquids simultaneously, so that the ends of the molecules located next to each other end up 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) to 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 dispersive.
What is sublimation?
The molecular structure of a solid, for example, iodine crystals, causes such an interesting physical phenomenon as sublimation - the volatilization of I 2 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 carried out in chemistry classrooms to illustrate the structural features of molecular crystal lattices and the related properties of the compounds. Usually this is low hardness, low melting and boiling points, poor heat and electrical conductivity, and 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, it is easy enough to predict the features of its structure and particle composition: atoms, molecules or ions. The information obtained can also be useful if, in chemistry tasks, it is necessary to select substances with a molecular structure from a certain group of compounds, excluding those that have atomic or ionic types of lattices.
To summarize, we can draw the following conclusion: the molecular structure has a solid body, and its spatial structure of crystal lattices, and the features of the location 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 their composition, the stronger the orientational forces that arise between them. On the contrary, the more atoms that make up a molecule, the higher its dipole moment, and, therefore, the more significant dispersion forces. Thus, the molecular structure of a solid also affects the forces of interaction between its particles - dipoles.