All the bodies that surround us are made up of atoms. Atoms, in turn, assemble into a molecule. Due to the difference in molecular structure, we 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 having 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 and composition. But only for substances with a molecular structure does the rule work: the chemical properties of substances and molecules are the same. For some polymers, for example, ethylene and polyethylene, the composition is not molecular.
It is known that the properties of molecules are determined not only by the number of atoms, their type, but also by the configuration and order of connection. A molecule is a complex architectural structure, where each element stands in its place and has its own specific neighbors. The atomic structure may be more or less rigid. Each atom oscillates relative to its equilibrium position.
Configuration and Options
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 the 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 pronounced polarity, the properties of the molecules are difficult to describe by 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 for accounting for quantum effects, which are important at low temperatures.
What do we know about the molecule of the most abundant substance on Earth?
Of all the substances on our planet, the most common is water. In the literal sense, it provides life to everything on earth. Only viruses can do without it, other living structures in their composition for the most part have water. What properties of a water molecule, characteristic only of it, are used in the economic life of man and the living nature of the Earth?
After all, this is truly a unique substance! A set of properties inherent in water, no other substance can boast.
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 react when they are in a dissolved state.
Water has excellent heat capacity, but low thermal conductivity. Thanks to these properties, we can use it as a heat transfer. This principle is part of the cooling mechanism of a large number of organisms. In atomic energy, the properties of a water molecule gave rise to the use of this substance as a coolant. In addition to being a reactive medium for other substances, water itself can enter into reactions: photolysis, hydration, and others.
Natural pure water is a liquid that does not have a smell, color or taste. 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 that mainly determines the unusual properties of this substance. The water molecule is a diamagnet.
Another interesting property is melt water: its molecule acquires the structure of the golden ratio, and the structure of the substance - the proportion of the golden ratio. Many of the properties that a water molecule possesses are established by analyzing the absorption and emission of striped spectra in the gas phase.
Natural sciences and molecular properties
All substances, except chemicals, 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 the molecules. When the French physicist Jean Perrin studied Brownian motion, he experimentally proved the existence of molecules. All living organisms exist thanks to a finely balanced internal interaction in the structure. All chemical and physical properties of substances are fundamental to natural science. The development of physics, chemistry, biology and molecular physics has led to the emergence of 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 the methods of molecular spectroscopy, in physical chemistry determine the thermodynamic properties of substances necessary for calculating chemical equilibria and the rates of its establishment.
What is the difference between the properties of atoms and molecules among themselves?
First of all, atoms do not occur in a free state.
In molecules, the optical spectra are richer. This is due to the lower symmetry of the system and the emergence of the possibility of new rotations and vibrations of nuclei. In a molecule, the total energy consists of three energies that are different in order of magnitude of the components:
- electron shell (optical or ultraviolet radiation);
- nuclear vibrations (infrared part of the spectrum);
- rotation of the molecule as a whole (radio frequency range).
Atoms emit characteristic line spectra, and molecules emit striped ones, consisting of many closely spaced lines.
Spectral analysis
Optical, electrical, magnetic and other properties of a molecule are also determined by the relationship with wave functions. Data on the states of molecules and the probable transition between them show molecular spectra.
Transitions (electronic) in molecules show chemical bonds and the structure of their electron shells. Spectra with a larger number of bonds 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.
The properties of the molecules of the same substance are the same in all aggregate states. This means that for the same substances the properties of the molecules of liquid, gaseous substances do not differ from the properties of solid. The molecule of one substance always has the same structure, regardless of the state of aggregation of the substance itself.
Electrical specifications
The way a substance behaves in an electric field is determined by the electrical characteristics of the molecules: polarizability and constant dipole moment.
The dipole moment is the electrical asymmetry of the molecule. Molecules that have a center of symmetry like H 2 do not have a constant 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 dielectric constant.
The behavior in an alternating electric field of a light wave is characterized by the optical properties of a substance, which are determined by the polarizability of the molecule of this substance. Directly related to polarizability are: scattering, refraction, optical activity, and other molecular optics phenomena.
You can often hear the question: “On what, apart from molecules, do the properties of a substance depend?” The answer to it is quite simple.
The properties of substances, in addition to isometry and crystal structure, are determined by the temperature of the environment, the substance itself, pressure, and the presence of impurities.
Chemistry of molecules
Before the formation of a science such as quantum mechanics, the nature of chemical bonds in molecules was an undisclosed secret. Classical physics could not explain the direction and saturation of valence bonds. After the creation of basic theoretical information on chemical bonds (1927) using the simplest H2 molecule as an example, the theory and methods of calculation began to improve gradually. 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, which completely coincided with the 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.
Various types of structural isomerism are known. The differences are 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 molecule of a substance are characterized by the same composition and chemical structure. Therefore, their structural and molecular formulas are the same. The differences are in the spatial form of the molecule. For the image of different spatial isomers, special formulas are used.
There are compounds 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 in homologous series. Having studied the properties of one homologue, we can talk about any other of them. The totality of homologues is a homologous series.
When transforming the structures of matter, the chemical properties of molecules change dramatically. An example is even the simplest compounds: methane, connecting even with 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 the structures of biomolecules are modified.
Chemical molecular properties strongly depend on the structure and properties of molecules: on energy bonds in it and the geometry of the molecule itself. This especially works in biologically active compounds. Which competing reaction will be predominant is often determined only by spatial factors, which, in turn, depend on the starting molecules (their configuration). One molecule with an “uncomfortable” configuration will not react at all, and another, with the same chemical composition but different geometry, can react to the reaction instantly.
A large number of biological processes observed during growth and reproduction are associated with geometric relationships between the reaction products and the starting materials. Note: the effect of a considerable number of new drugs is based on the similar structure of the molecules of a compound that is harmful from the biological point of view to the human body. The medicine takes the place of a harmful molecule and impedes its action.
Using chemical formulas express the composition and properties of molecules of different substances. Based on molecular weight, chemical analysis , an atomic ratio is established and an empirical formula is drawn up.
Geometry
The geometric structure of the molecule is determined taking into account the equilibrium arrangement of atomic nuclei. The atomic interaction energy depends on the distance between the nuclei of atoms. At very large distances, this energy is zero. When atoms come closer together, a chemical bond begins to form. Then the atoms are strongly attracted to each other.
If a weak attraction is observed, then the formation of a chemical bond is not necessary. If atoms begin to come closer at closer distances, electrostatic repulsive forces begin to act between the nuclei. An obstacle to the strong convergence of atoms is the incompatibility of their internal electron shells.
Dimensions
It is impossible to see molecules with the naked eye. They are so small that even a microscope with a 1000x magnification will not help us make them out. Biologists observe bacteria of 0.001 mm in size. But molecules are hundreds and thousands of times smaller than them.
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 electron paramagnetic method. The choice of method depends on the type of substance and its condition.
The size of the molecule is a conditional value, given the electronic 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 the molecules can be determined taking into account the equilibrium distance. This is the gap over which the molecules themselves can come close when densely packed in a molecular crystal and in a liquid.
Large distances tend to attract molecules, and small, on the contrary, to repulsion. Therefore, x-ray analysis of molecular crystals helps to find the size 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, it is possible to determine the order of magnitude of the molecular dimensions.