All the bodies that surround us are made up of atoms. Atoms, in turn, assemble into a molecule. It is due to the difference in the molecular structure that one can talk about substances that are different from each other, based on their properties and parameters. Molecules and atoms are always in a state of dynamics. Moving, they still do not scatter in different directions, but are held in a certain structure, which we owe to the existence of such a huge variety of substances in the whole world around us. What are these particles and what are their properties?
General concepts
If we start from the theory of quantum mechanics, then the molecule does not consist of atoms, but their nuclei and electrons, which constantly interact with each other.
For some substances, a molecule is the smallest particle that has the composition and chemical properties of the substance itself. So, the properties of molecules from the point of view of chemistry are determined by its chemical structure andcomposition. But only for substances with a molecular structure, the rule works: the chemical properties of substances and molecules are the same. For some polymers, such as ethylene and polyethylene, the composition does not match the molecular composition.
It is known that the properties of molecules are determined not only by the number of atoms, their type, but also by the configuration, the order of connection. A molecule is a complex architectural structure, where each element stands in its place and has its specific neighbors. The atomic structure can be more or less rigid. Each atom vibrates about its equilibrium position.
Configuration and parameters
It happens that some parts of the molecule rotate in relation to other parts. So, in the process of thermal motion, a free molecule takes on bizarre shapes (configurations).
Basically, the properties of molecules are determined by the bond (its type) between atoms and the architecture of the molecule itself (structure, shape). Thus, first of all, the general chemical theory considers chemical bonds and is based on the properties of atoms.
With a strong polarity, the properties of molecules are difficult to describe with two- or three-constant correlations, which are excellent for non-polar molecules. Therefore, an additional parameter with a dipole moment was introduced. But this method is not always successful, since polar molecules have individual characteristics. Parameters have also been proposed to account for quantum effects, which are important at low temperatures.
What do we know about the molecule of the most common substance on Earth?
Of all the substances on our planet, the most common is water. It, in the literal sense, provides life for everything that exists on Earth. Only viruses can do without it, the rest of the living structures in their composition for the most part have water. What properties of the water molecule, characteristic only of it, are used in the economic life of man and the wildlife of the Earth?
After all, this is a truly unique substance! No other substance can boast of a set of properties inherent in water.
Water is the main solvent in nature. All reactions occurring in living organisms, one way or another, occur in the aquatic environment. That is, substances enter into reactions while in a dissolved state.
Water has excellent heat capacity, but low thermal conductivity. Thanks to these properties, we can use it as a heat transport. This principle is included in the cooling mechanism of a large number of organisms. In the nuclear power industry, the properties of the water molecule gave rise to the use of this substance as a coolant. In addition to the possibility of being a reactive medium for other substances, water itself can enter into reactions: photolysis, hydration, and others.
Natural pure water is an odorless, colorless and tasteless liquid. But at a layer thickness greater than 2 meters, the color becomes bluish.
The whole water molecule is a dipole (two opposite poles). It is the dipole structure inmainly determines the unusual properties of this substance. The water molecule is a diamagnet.
Metal water has another interesting property: its molecule acquires the structure of the golden ratio, and the structure of the substance acquires the proportions of the golden section. Many of the properties of the water molecule have been established by analyzing the absorption and emission of striped spectra in the gas phase.
Science and molecular properties
All substances, except chemical ones, have the physical properties of the molecules that make up their structure.
In physical science, the concept of molecules is used to explain the properties of solids, liquids and gases. The ability of all substances to diffuse, their viscosity, thermal conductivity and other properties are determined by the mobility of molecules. When the French physicist Jean Perrin was studying Brownian motion, he experimentally proved the existence of molecules. All living organisms exist due to a finely balanced internal interaction in the structure. All chemical and physical properties of substances are of fundamental importance for natural science. The development of physics, chemistry, biology and molecular physics gave rise to such a science as molecular biology, which studies the basic phenomena in life.
Using statistical thermodynamics, the physical properties of molecules, which are determined by molecular spectroscopy, in physical chemistry determine the thermodynamic properties of substances necessary to calculate chemical equilibria and the rates of its establishment.
What is the difference between the properties of atoms and molecules?
First of all, atoms do not occur in the free state.
Molecules have richer optical spectra. This is due to the lower symmetry of the system and the emergence of the possibility of new rotations and oscillations of the nuclei. For a molecule, the total energy consists of three energies that are different in order of magnitude of the components:
- electronic shell (optical or ultraviolet radiation);
- vibrations of nuclei (infrared part of the spectrum);
- rotation of the molecule as a whole (radio frequency range).
Atoms emit characteristic line spectra, while molecules emit striped spectra consisting of many closely spaced lines.
Spectral analysis
Optical, electrical, magnetic and other properties of a molecule are also determined by the connection with the wave functions. Data on the states of molecules and the likely transition between them show molecular spectra.
Transitions (electronic) in molecules show chemical bonds and the structure of their electron shells. Spectra with more connections have long-wavelength absorption bands that fall into the visible region. If a substance is built from such molecules, it has a characteristic color. These are all organic dyes.
Properties of molecules of the same substance are the same in all states of aggregation. This means that in the same substances, the properties of the molecules of liquid, gaseous substances do not differ from the properties of the solid. The molecule of one substance always has the same structure, regardless ofaggregate state of matter itself.
Electrical data
The way a substance behaves in an electric field is determined by the electrical characteristics of the molecules: polarizability and permanent dipole moment.
Dipole moment is the electrical asymmetry of a molecule. Molecules that have a center of symmetry like H2 do not have a permanent dipole moment. The ability of the electron shell of a molecule to move under the influence of an electric field, as a result of which an induced dipole moment is formed in it, is polarizability. To find the value of polarizability and dipole moment, it is necessary to measure the permittivity.
The behavior of a light wave in an alternating electric field is characterized by the optical properties of a substance, which are determined by the polarizability of a molecule of this substance. Directly related to polarizability are: scattering, refraction, optical activity and other phenomena of molecular optics.
One can often hear the question: “On what, besides molecules, do the properties of a substance depend?” The answer is quite simple.
Properties of substances, except for isometry and crystal structure, are determined by the temperature of the environment, the substance itself, pressure, the presence of impurities.
Chemistry of molecules
Before the formation of the science of quantum mechanics, the nature of chemical bonds in molecules was an unsolved mystery. Classical physics explain directionality andsaturation of valence bonds could not. After the creation of basic theoretical information about the chemical bond (1927) using the example of the simplest H2 molecule, the theory and calculation methods began to be gradually improved. For example, based on the widespread use of the method of molecular orbitals, quantum chemistry, it became possible to calculate interatomic distances, the energy of molecules and chemical bonds, the distribution of electron density and other data that completely coincided with experimental data.
Substances with the same composition, but different chemical structure and different properties, are called structural isomers. They have different structural formulas, but the same molecular formulas.
Different types of structural isomerism are known. The differences lie in the structure of the carbon skeleton, the position of the functional group, or the position of the multiple bond. In addition, there are still spatial isomers in which the properties of a substance molecule are characterized by the same composition and chemical structure. Therefore, both structural and molecular formulas are the same. The differences lie in the spatial shape of the molecule. Special formulas are used to represent different spatial isomers.
There are compounds that are called homologues. They are similar in structure and properties, but differ in composition by one or more CH2 groups. All substances similar in structure and properties are combined into homologous series. Having studied the properties of one homologue, one can reason about any other of them. The set of homologs is a homologous series.
When transforming the structures of matterthe chemical properties of molecules change dramatically. Even the simplest compounds serve as an example: methane, when combined with even one oxygen atom, becomes a poisonous liquid called methanol (methyl alcohol - CH3OH). Accordingly, its chemical complementarity and effect on living organisms become different. Similar but more complex changes occur when modifying the structures of biomolecules.
Chemical molecular properties strongly depend on the structure and properties of molecules: on the energy bonds in it and the geometry of the molecule itself. This is especially true in biologically active compounds. Which competing reaction will be predominant is often determined only by spatial factors, which in turn depend on the initial molecules (their configuration). One molecule with an "uncomfortable" configuration will not react at all, while another with the same chemical composition but a different geometry may react instantly.
A large number of biological processes observed during growth and reproduction are associated with the geometric relationships between the reaction products and the starting materials. For your information: the action of a considerable number of new drugs is based on a similar molecular structure of a compound that is harmful from a biological point of view for the human body. The drug takes the place of the harmful molecule and makes it difficult to act.
With the help of chemical formulas, the composition and properties of the molecules of different substances are expressed. Based on the molecular weight, chemical analysis, the atomic ratio is established and compiledempirical formula.
Geometry
Determination of the geometric structure of a molecule is made taking into account the equilibrium arrangement of atomic nuclei. The energy of interaction of atoms depends on the distance between the nuclei of atoms. At very large distances, this energy is zero. As the atoms approach each other, a chemical bond begins to form. Then the atoms are strongly attracted to each other.
If there is a weak attraction, then the formation of a chemical bond is not necessary. If the atoms begin to approach at closer distances, electrostatic repulsive forces begin to act between the nuclei. An obstacle to the strong approach of atoms is the incompatibility of their internal electron shells.
Sizes
It is impossible to see molecules with the naked eye. They are so small that even a microscope with 1000x magnification will not help us to see them. Biologists observe bacteria as small as 0.001 mm. But molecules are hundreds and thousands of times smaller.
Today, the structure of molecules of a certain substance is determined by diffraction methods: neutron diffraction, X-ray diffraction analysis. There is also vibrational spectroscopy and the electron paramagnetic method. The choice of method depends on the type of substance and its condition.
The size of a molecule is a conditional value, taking into account the electron shell. The point is the distances of electrons from atomic nuclei. The larger they are, the less likely it is to find the electrons of the molecule. In practice, the size of molecules can be determined by taking into account the equilibrium distance. This is the interval for which the molecules themselves can approach each other when densely packed in a molecular crystal and in a liquid.
Large distances have molecules to attract, and small ones, on the contrary, to repulsion. Therefore, X-ray diffraction analysis of molecular crystals helps to find the dimensions of the molecule. Using the coefficient of diffusion, thermal conductivity and viscosity of gases, as well as the density of a substance in a condensed state, one can determine the order of magnitude of molecular sizes.
