Chemistry is the science of substances, their properties, structure and mutual transformations.
Historically, chemistry arose to obtain the substances necessary for human life. To solve this problem, it was necessary to learn how to produce others from some substances, i.e. carry out their qualitative transformations. And since quality is a set of properties of substances, it was necessary to find out what these properties depend on. This was the reason for the appearance of theoretical chemistry.
The subject of chemistry is chemical elements and their compounds, as well as the laws governing various chemical reactions.
Chemical reactions are the processes of formation of more complex substances from simple in composition, the transition of some complex substances to others and the decomposition of complex substances into substances of simpler composition.
Modern chemistry is engaged in obtaining substances with desired properties and identifying ways to control the properties of a substance. This is the main problem of chemistry and its backbone as a science.
On the basis of the studied objects (substances), chemistry is usually divided into inorganic and organic. Explaining the essence chemical phenomena and the establishment of their general laws on the basis of physical principles and experimental data is engaged in physical chemistry, including quantum chemistry, electrochemistry, chemical thermodynamics, chemical kinetics. Analytical and colloidal chemistry are also independent sections.
The combination of chemistry with other funny natural sciences is biochemistry, geochemistry, photochemistry, etc.
We emphasized that matter exists in two physical forms - matter and field.
Substances are different types of moving matter, the rest mass of which is not equal to zero. All substances are corpuscular. The processes taking place in a chemical substance or in mixtures of various substances are chemical reactions.
When chemical reactions proceed, new substances are always formed. For example, when magnesium (silvery white metal) is heated in molecular oxygen (colorless gas), magnesium oxide (white powder) is formed:
Chemical reactions are always accompanied by physical effects: absorption and release of energy, for example, in the form of heat transfer, a change in the state of aggregation of reagents, a change in the color of the reaction mixture, etc. It is by these physical effects that chemical reactions are often judged.
In chemical processes (chemical reactions), new substances are obtained with properties different from reagents, but atoms of new elements are never formed.
The conditions for the occurrence of chemical processes include, first of all, thermodynamic factors that characterize the dependence of reactions on temperature, pressure and some other conditions. To an even greater extent, the nature and especially the rate of reactions depend on the kinetic conditions, which are determined by the presence of catalysts and other additives to the reactions, as well as the influence of solvents and other conditions.
In chemistry, a distinction is made between simple and complex substances. Simple substances consist of atoms of one type of element, i.e. they are singleton. Complex substances are composed of atoms of carved elements, i.e. they are multi-element. Complex substances are otherwise called chemical compounds. This term means that substances can be obtained using chemical reactions of a compound from simple substances - chemical synthesis or separated into elements in free form (simple substances) using chemical decomposition reactions - chemical analysis.
2Hg + O 2 = 2HgO
simple chemical
substance compound
2HgO = 2Hg + O 2
chemical simple
compound substance
The smallest chemical particles, which are the limit of the chemical decomposition of any substance, are atoms. A simple substance (if it is not monoatomic, such as, for example, helium He) decomposes into atoms of one type, a complex substance - into atoms different types... Atoms are chemically indivisible.
The mass of atoms of different types is about 10 -24 - 10 -22 g, the size (diameter) of atoms varies within 1 * 10 -10 - 5 * 10 -10 m. Therefore, atoms are considered the smallest chemical particles.
A chemical element is a kind of atoms with a certain positive nuclear charge. All chemical elements are listed in the Periodic Table of the Elements of D.I.Mendeleev. Each element has its own ordinal (atomic) number in the Periodic Table. The value of the ordinal number of an element and the value of the charge of the nucleus of an atom of the same element coincide. This means that a chemical element is a collection of atoms with the same serial number.
Today, there are 109 elements in the Periodic Table of chemical elements with serial numbers from 1 to 109. Of these, 88 have been found in nature. Such elements as technetium Tc, promethium Pm, astatine At and francium Fr with serial numbers 43, 61, 85, 87 and all the elements following uranium U (serial number 92) were obtained artificially for the first time.
Oxygen and silicon are the most abundant chemical elements in the earth's envelope. These elements, together with the elements aluminum, iron, calcium, sodium, potassium, magnesium, hydrogen and titanium, make up more than 99% of the mass of the earth's shell (the lithosphere is taken as the earth's shell - solid Earth's crust, extending to a depth of 17 km, the hydrosphere is the water of the seas and oceans and the atmosphere is an air envelope, extending to a height of 15 km).
Chemical compounds, consisting of atoms of at least two elements, have as the smallest constituent parts of the molecule - electrically neutral groups of atoms, or ions - electrically charged atoms or groups of atoms. Most complex chemical substances are not molecules, but ions. For example, all salts are ionic.
Example. Sodium chloride NaCl consists of Na + and Cl - ions.
Chemical compounds are formed through chemical bonds. There are three main types of chemical bonds: covalent, ionic and metallic.
The covalent bond is carried out due to the formation of electron pairs, equally belonging to both atoms. Ionic bond is an electrostatic attraction between ions formed by the complete displacement of an electron pair to one of the atoms. A metallic bond is a bond between positive ions in metal crystals, carried out due to the attraction of electrons freely moving around the crystal.
A chemical bond is an interaction that binds individual atoms into molecules, ions, crystals, i.e. structural levels of the organization of matter, which are studied by chemical science.
The nature of the chemical bond, according to modern concepts, is explained by the interaction of electric fields created by electrons and atomic nuclei that participate in the formation of a chemical compound.
Each chemical element has its own symbol. Symbols of chemical elements are international designations of elements. They are placed in the Periodic Table of the Elements of D.I. Mendeleev. Modern symbols of chemical elements were introduced in 1813 by the Swedish chemist Berzelius.
Each substance is designated by a chemical formula inherent only to it.
A chemical formula is an image of the qualitative and quantitative composition of a substance using symbols of chemical elements, as well as numerical, alphabetic and other signs. For example, the formula H 2 O shows that water contains the elements hydrogen H and oxygen O in a 2: 1 atomic ratio.
Any chemical reaction is written in the form of a chemical reaction equation, for example,
2Na + Cl 2 = 2NaCl
The selection of coefficients in the equation of a chemical reaction is based on the fact that the sum of the atoms of each element does not change during the course of a chemical reaction.
The most important feature chemical reactions due to the fact that their course is accompanied by changes in energy. Most of the energy produced in modern society comes from chemical reactions, mainly from the combustion of coal, oil products and natural gas.
In order to optimally carry out the course of a chemical process, it is necessary to know the general laws governing the transformation of energy during the chemical interaction of substances. The thermodynamic method has become widespread in the practice of chemistry to establish mutual connections between phenomena and generalize experimental material. Before proceeding to the presentation of the foundations of chemical thermodynamics, we will try to give a definition of the initial concepts and the object of application of the thermodynamic method - a thermodynamic system.
A system is understood as a body or a group of bodies mentally separated from environment... Let's imagine that it is required to determine the heat of combustion of liquid benzene. The experiment is carried out in a calorimetric bomb, which can be regarded as a system.
Depending on the phenomenon under consideration, the system can be complex and of various sizes, but it must always consist of a large number of particles, i.e. be macroscopic. Only for macroscopic systems it is possible to operate with such concepts as temperature, pressure, heat, and some others. Based on the nature of the interaction of various systems with the environment, they are divided into open, closed and isolated systems.
An open system is a system that can exchange energy and matter with the environment. An open system, for example, is a glass with an aqueous sugar solution. As a result of the gradual evaporation of water from the solution into the environment and heat exchange, both the mass of the system and its energy will change.
A closed system is a system in which there is no exchange of matter with the environment, but the exchange of energy with it is possible. An example of such a system is a sugar solution placed in a glass with a stopper. When the glass is closed with a stopper, the process in the solution will be carried out at a constant volume. If the temperature of the solution T 1 differs from the temperature T 2 of the environment, then when T 1 is greater than T 2, part of the energy from the solution will be transferred to the environment, and vice versa, when T 1 is less than T 2, the energy of the system will increase due to the transition of some part of the energy from the environment into the solution. In this case, the mass of the system will not change.
An isolated system is called one, the volume of which remains constant, and which does not exchange energy and matter with the environment. This type of system can be attributed to an aqueous solution of sugar, placed in a closed vessel, the walls of which are made of ideal heat-insulating material. The concept of an "isolated system" is an ideal (abstract) concept, since in practice there is no material that does not conduct heat at all.
The system can be homogeneous (homogeneous) or heterogeneous (heterogeneous).
A system is called homogeneous if it consists of one phase. A heterogeneous system necessarily contains several phases.
The totality of all the chemical and physical properties of a system is called the state of the system. Those properties are usually considered. which can be uniquely expressed in terms of temperature functions. pressure and concentration of substances in the system. Such properties are called thermodynamic (heat capacity, internal energy, enthalpy, etc.), they are part of the general properties (physical and chemical) of the system. For full description the state of the system, it is enough to know the smallest number of thermodynamic properties, which are most easily determined experimentally (pressure P, volume V, temperature T and concentration (C 1) components). The parameters of the state of the system are related to each other by a relationship called the equation of state. If the system consists of one substance and the parameters are pressure, volume and temperature, then the equation of state in general view can be written like this:
For n ideal gas models, the equation of state is the Mendeleev-Clapeyron equation:
Applying the basic concepts, let us consider the energetics of chemical processes.
Topic 2. The role of chemistry in the development of society
2.1. System of modern knowledge in the field of chemistry
Basic concepts and laws of chemistry. The levels of organization of matter studied in chemistry: atom, chemical element, ion, molecule, chemical substance. Periodic law and its significance for modern science
All chemical elements are ordered in the periodic table. In 1869, D. Mendeleev first discovered that some properties of elements are periodically repeated, namely, elements in the same column (group) of the periodic table have similar properties. Members of the same group in the table have the same number of electrons on the outer shells of their atoms and form bonds of the same type, usually with the same valence. Inert gases with a filled outer electron shell do not form bonds at all. Thus, the periodic table reflects a deep understanding of chemical bonding and chemical behavior. In addition, it made it possible to predict the existence of new elements, many of which were later discovered or synthesized.
Chemical substances in nature: simple and complex, organic and inorganic. Oil and natural gas as sources of organic matter
Oil and natural gas are the most important primary fossil fuels. The first well for oil production was drilled in the middle of the 19th century. With the invention of the automobile, this hydrocarbon fuel was used as a source of gasoline. Since then, oil and its products have been used as fuels for heat generation, for land, air and sea transport, for thermal power plants and as a source of oil products and lubricants. Oil is recovered from drilled wells, transported by pipelines or tankers to refineries, where it is converted into fuel and petrochemicals. Oil and natural gas, produced primarily in Saudi Arabia, USA and Russia, today account for about 60% of global energy consumption. At present rates of oil consumption, its known reserves will be depleted by the middle of the 21st century.
Crude oil commonly occurs with natural gas, a hydrocarbon made up primarily of methane and ethane. Natural gas is extracted from wells and then transported either in its natural gaseous state through pipelines or after liquefaction by refrigerated tankers. Liquefied gas occupies about 1/600 of the volume of the gaseous product. The use of natural gas as a source of energy is constantly growing.
Classification and basic chemical properties of organic and inorganic compounds
Organic compounds are hydrocarbons. Due to the ability of carbon atoms to form chemical bonds with each other and with the atoms of most elements, the number of organic compounds is very large and exceeds 4 million. They are characterized by the ability to complex and diverse transformations, which are studied in organic chemistry. Natural organic compounds, such as nucleic acids, proteins, lipids, hormones, vitamins, play a major role in the structure and life of plant and living organisms.
The properties of organic compounds depend on the amount of CH 2 -groups in the carbon chain (for example, methane CH 4, ethane C 2 H 6, propane C 3 H 8, etc.).
Inorganic compounds are metal alloys, minerals, salts, acids, alkalis. For the synthesis of inorganic compounds, methods of physical influence are used - ultra-high temperatures and pressures, ultrasound, vibrations, strong light radiation, magnetic fields, shock waves and centrifugal forces. Low and ultra-low temperatures, ultra-deep vacuum, and the study of processes in zero gravity are often used.
Aqueous and non-aqueous solutions.
Water is an excellent solvent for many substances. This is due to the ability of its molecules to form chemical bonds with other molecules. Non-aqueous solvents such as acetonitrile or acetic acid are also widely used.
Many chemical reactions take place in liquid solutions, incl. technical and vital. A solvent is a component whose concentration is higher than the concentration of other components. The solvent retains its state during the formation of solutions. The boiling point of the solution is higher than the boiling point of the solvent, and the freezing point of the solution is below the freezing point of the pure solvent.
A solution in which an equilibrium is established between dissolution and the formation (precipitation, crystallization) of a substance is called saturated, and the concentration of such a solution is called solubility. Solubility is influenced by the nature of the solvent. For organic compounds, the substance dissolves better in solvents that are chemically similar to it, for example, hydrocarbons in hydrocarbon solvents.
Structure and unique properties of water
Water is the simplest chemical compound and common substance on Earth. She accompanies every moment of our life. But do we know what secret the amazing element keeps, where it came from, who and why gave it to our planet?
Water is more than a physical substance, the very idea of life is connected with water. There is nothing softer and more pliable than water, but it wears away stones and cuts the hardest rocks. Water has physical properties, but no scientist can explain why the density of water increases at minus temperatures and decreases at plus temperatures. When cooled, any substance contracts, while water, on the contrary, expands. Being in pores and capillaries, water can create enormous pressure. In grain, for example, at the moment of germination, it can reach 400 atmospheres. That is why the sprout can penetrate the asphalt with ease.
Until now, science does not know why water can be in three states, why water has the highest surface tension, why water is the most powerful solvent on Earth, and how water can rise up the trunks of huge trees, overcoming pressure of tens of atmospheres. Numerous experiments carried out in various countries have led to the discovery that water has memory; she perceives and captures any impact that occurs in the surrounding space. By capturing information, water acquires new properties, while it chemical composition(H 2 O) remains the same. It turned out that the structure of water, i.e. how the molecules are organized is much more important than the chemical composition.
The most powerful moment of impact on water is human emotions (positive or negative). Numerous studies have revealed that waves of love, tenderness, care combined molecules into strict, beautiful combinations (flowers) and, on the contrary, fear, aggression, hatred created torn, shapeless connections. The same clearly directed changes occur in the structure of water when it first acts on the music of Bach, Mozart, Beethoven, and then hard rock and similar sounds generated by vague souls.
Love increases the energy of water, structures it, and aggression sharply lowers it. This is why structured water is the greatest blessing for all life on Earth. In Nature, rivers and streams flow along a smoothly bending channel, while household water, overcoming numerous right angles, loses most of its energy. The water structure of each person's body is identical to the structure of water where he was born.
In all world religions (Christianity, Islam, Judaism), it is customary to read a prayer or consecrate food on religious holidays before taking food. It turned out that the oscillation frequency of any prayer in any language is 8 hertz, which corresponds to the oscillation frequency of the Earth's magnetic field. Hence the advice - do not sit down at the table with bad thoughts and do not eat food in a bad state (in this case, it becomes poisonous), but for purification it is better to drink water. The one who sends negative thoughts pollutes his own water, of which 75-90% of the body is composed, negatively charges it. Is it not for this reason that the most serious crimes are committed in those areas where people most often use foul language.
In Japan, experiments were carried out and it was determined that the water reacted to the words of religious content, forming beautiful, geometrically regular crystals. The same reaction followed when the words love, hope, soul were applied to the laboratory cups of water. This means, according to the statement of Japanese scientists, that the concept of our Nature is the same with every religion. However, the words “I hate you”, “you are disgusting to me” resulted in ugly, torn connections.
Numerous experiments to find the word that has the strongest beneficial effect on water, have shown that it is not one, but a combination of two: love and gratitude.
Fresh water accounts for less than 1% of all its reserves. More than 1 billion people on Earth do not have access to safe drinking water. If this situation remains unchanged, then water can become a source of international conflicts. Now they are fighting for oil and gas, but they will fight for water. Water is increasingly acquiring the status of the main resource, which is beginning to figure in the dialogue between countries.
Thus, we draw two conclusions. Firstly, water must be protected as the most precious property of Nature, and secondly, in each of us there is a drop of water from the primordial ocean, and every our action, thought, word, emotion is remembered by water!
Hydrogen index (pH) as a measure of the acidity of the medium
One of the main indicators on which our health depends is acid-base balance or pH. The pH is 7.41. Even a slight decrease in acid-base balance towards acidification causes a sharp drop in the activity of intracellular processes. The organs and systems of the body begin to work with great stress, the state of health worsens, the efficiency decreases. Otherwise, the more dirt in the body, the weaker the immune system works.
Animal food oxidizes, and plant food alkalizes the body up to 80%. The more acidified the environment of the body, the more pathogenic microflora, fungi, viruses are activated in it. Placed in an acidic environment, they begin to develop rapidly, and placed in an alkaline environment, they die. Conclusion - there are more vegetables and fruits and less animal food (meat, fish, eggs, milk). Thus, pH serves as a criterion for the strength of an acid or base. The deviation of pH from the norm significantly leads to a breakdown in the activity of the body. Soil pH has a significant effect on yield, and water pH on the ecology of the reservoir.
The concept of maximum permissible concentration (MPC)
Waste industrial production pollute land, water and air. The maximum amount of a harmful substance in a unit of volume or mass, which, with daily exposure for an unlimited time, does not cause any painful changes in the body and adverse hereditary changes in offspring, is considered the maximum permissible concentration (MPC). For each substance, its own MPC level is legally established, and the MPC of the same substance is different for different objects of the environment. For example, for lead and its organic compounds in the water of reservoirs for drinking and cultural purposes, the MPC is 0.005 mg / l, in the air of industrial premises - 0.01 mg / m 3, and in the atmosphere - 0.007 mg / m 3 ...
Chemical analytical control and diagnostics of the state of the environment
Air is the mixture of gases that make up the Earth's atmosphere. The most important are nitrogen (78%) and oxygen (21%). The air also contains small amounts of argon, neon, helium, methane and other gases. Water vapor, ozone and carbon monoxide are present in varying concentrations. The air also contains traces of ammonia and hydrogen sulfide. All of these gases are essential for maintaining human life and health. Water vapor is an important source of precipitation. Carbon monoxide (CO 2) is required for photosynthesis and infrared radiation.
Ozone in the stratosphere (the layer of the atmosphere lying at an altitude of 10 to 50 km, called the ozone layer) is a kind of shield from solar ultraviolet radiation. However, at ground level, it is the main constituent of smog - aerosols of smoke, fog and dust that arise in the atmosphere of industrial cities. Each of the inhabitants, especially large cities, feels how polluted the air is due to an automobile or industrial nature.
To reduce such a negative impact on human health: the requirements for the composition of the exhaust gases of cars are being tightened (the quality of fuel is increasing and the fuel combustion system is being improved); carried over industrial enterprises outside the city; industrial and household waste, etc. are collected centrally.
Physicists, especially chemists, should turn in darkness, not knowing the inner, insensitive particles of the structure.
M. V. Lomonosov
Currently chemical element is called a substance, all atoms of which have the same nuclear charge,
although they differ in their mass, as a result of which the atomic weights of the elements are not expressed in whole numbers.
Molecule is still called the smallest particle of a substance, which determines its properties and can exist independently. However, various other quantum-mechanical systems (ionic, atomic single crystals, polymers and other macromolecules) are now also referred to as molecules. The latter is especially important for a clear understanding of the structure from the point of view of a systematic approach, where under structure imply an ordered connection and interaction between the elements of the system, due to which its new integral properties arise. In such a chemical system as a molecule, it is the specific nature of the interaction of its constituent atoms that determines the properties of the molecule.
Chemistry studies the processes of transformation of molecules during interactions and when exposed to external factors (heat, light, electric current, magnetic field), during which new chemical bonds are formed. Under chemical bond the result of interaction between atoms is understood, expressed in the creation of a certain configuration of atoms, which distinguishes one type of molecule from another. Chemical bonds give rise to the interaction of the electronic shells of atoms. If the atomic configurations fit together, one round structure appears, somewhat larger than before each atom was separately. Thus, a saturated molecule is obtained, and it is almost impossible to attach another atom to it, that is, chemical bonds are distinguished by their saturation. With the introduction of the concept of valence, it began to explain the structure and chemical properties of molecules. The most common are four types of chemical bonds: ionic, covalent, metallic, and hydrogen. The chemical bond, carried out due to the formation of common electron pairs for interacting atoms, is called a covalent bond. The chemical bond, which is based on the electrostatic interaction of ions, is called ionic. A chemical bond based on the socialization of the valence electrons of all atoms in a crystal is called
metal. The chemical bond caused by the interaction of polar molecules, one of which is hydrogen, is called hydrogen. Chemical bonds can be viewed from the point of view of energy conversion: if, when creating a molecule, its energy is less than the sum of the energies of its constituent isolated atoms, then it can exist, that is, its bond is stable.
Each substance is characterized by certain physical and chemical properties. When some simple substance enters into a chemical reaction and forms a new substance, then it loses most of its properties. For example, iron, combining with sulfur, loses its metallic luster, malleability, magnetic properties, etc. Therefore, iron sulfide does not contain iron, as we know it as a simple substance. But since metallic iron can be obtained from iron sulfide (FeS) using chemical reactions, they say that the element iron is included in the composition of iron sulfide, meaning by this the material that makes up metallic iron. In the same way, hydrogen (H) and oxygen (O), which are part of water, are contained in water not in the form of gaseous hydrogen and oxygen with their characteristic properties, but in the form of elements - hydrogen and oxygen. If the elements are in a "free state", that is, they are not chemically bonded with any other element, then they form simple substances.
For a long time, no distinction was made between an element and a simple substance. The concept of "element" as a scientific term was first used by R. Boyle in 1661. Since Boyle's time, any simple substance that can be obtained as a result of decomposition of complex substances, but which is not capable of further decomposition into even simpler substances, was considered an element.
The phlogiston theory of metal oxidation was also refuted by numerous experiments of M.V. Lomonosov. According to this theory, the metal oxidation process was considered as a decomposition reaction: the metal was considered a complex substance, and the scale was simple, that is, iron - "scale + phlogiston.
MV Lomonosov, conducting experiments in sealed retorts, found that the mass of a vessel with calcined iron does not change if weighed without opening it. The French scientist A. Lavoisier also showed that combustion is the reaction of combining a substance with oxygen in the air. Lavoisier put on his feet all the chemistry, which in its phlogistonic form stood on the head.
Early XIX v. was marked by the discovery of new quantitative laws. The development of atomic-molecular theory allowed Dalton to formulate an atomic hypothesis and introduce into chemistry the concept of the relative atomic weight of elements and determine the atomic weights of some elements. According to Dalton, an element can be defined as a kind of atoms, characterized by a certain value of atomic weight, and simple substances consist of a certain type of atoms, therefore, simple substances are elements. The confusion was cleared up later when it was found that many simple substances are formed from molecules rather than atoms. For the first time, Mendeleev in this regard pointed out the need to clearly distinguish between two concepts: an element and a simple substance, or a simple body. If a simple substance (body) corresponds to the concept of a particle, then to an element - about an atom. Carbon is an element, but coal, graphite and diamond are simple bodies.
Using the concept of chemical elements, we can say that the most important task of chemistry is to study the properties of elements in finding general patterns in their behavior and in relation to each other. By the middle of the XIX century. there were already 63 elements and enough experimental material was accumulated concerning their physical and chemical properties, and group general properties were established. Information was also accumulated on such characteristics as the atomic mass of elements and their valence, that is, the ability to form various forms of compounds. First of all, it was necessary to solve the main question: are the chemical elements scattered, independent, or are they naturally interconnected into a single system.
The first attempts to solve this problem date back to the first half of the 19th century. Debereiner (1829) grouped the elements into triads; Odling (1857) placed 48 elements in a single table of 13 groups of similar elements; Newlands and de Charcuntois (1863) distributed 63 elements in ascending order of their atomic mass, a table of elements was published by the German chemist L. Meyer, in which boron, aluminum and hydrogen were absent. In total, there were at least fifty attempts at classification, and all were essentially unsuccessful. The failure was based on their metaphysical way of thinking. Finally, in 1869, DI Mendeleev proposed a periodic systematization of the properties of elements.
The dialectical-materialist approach to the systematization of elements is the main reason for the success of D.I. Mendeleev. The periodic table of elements had a great influence on the subsequent development of chemistry; it was a powerful tool for further research. On the basis of the periodic law, DI Mendeleev predicted the existence of 12 new elements, and for three of them (gallium - Ga, germanium - Ge and scandium - Sc) described their properties in detail. For half a century, almost all elements up to uranium have been discovered in nature. A guiding thread for finding and establishing chemical nature elements was the periodic law and the prediction method used by D. I. Mendeleev. The periodic law and the periodic system received their full confirmation and further development when establishing the structure of the atoms of the elements. Now the factual data in chemistry has grown thousands of times. There is information about 8 million individual chemical compounds of constant composition and billions of compounds of variable composition.
The modern formulation of the periodic law is as follows: all properties of an element and its position in the periodic system depend on the value of the positive charge of the atomic nucleus. The theory of the structure of the atom explains the periodic change in the properties of elements during the transition from one period to another: with an increase in Z, the structure of the electron shells of atoms is repeated.
This is especially true of the outer energy levels at which the valence electrons are located. Within one period, with an increase in the nuclear charge, the outer layers are filled gradually, reaching their completion in the atoms of noble gases. This sequence is repeated in each period, as a result of which there is a transition from metals at the beginning of the period to non-metals and a noble gas at its end. In the light of the theory of the structure of the atom, the periodic law received a modern formulation: the properties of simple substances, as well as the forms and properties of compounds of elements, are periodically dependent on the magnitude of the charge of the atomic nucleus.
The atomic weight of an element is defined as the arithmetic mean of the masses of the isotopes that make up the element. Atoms that have the same nuclear charge (and, therefore, identical chemical properties), but a different number of neutrons, are called isotopes. For example, chlorine consists of two isotopes with mass numbers 75.53% from the 35 Cl isotope and 24.47% from 37 Cl, as a result the average atomic mass of chlorine is 35.453. The discovery of isotopes required a revision of the concept of a "chemical element". A chemical element is a type of atom characterized by a certain amount of positive nuclear charge. The existence of a chemical element in the form of several simple substances is called allotropy. Graphite, diamond, coal are allotropic modifications of the element carbon.
With the development of quantitative research methods in chemistry, experimental facts were accumulated, the generalization of which led to the discovery of the so-called stoichiometric laws - the law of constancy of composition, the law of equivalents and the law of multiple ratios. It was these laws that contributed to the final approval of the atomic-molecular doctrine in chemistry. The basis of chemical science is the atomic-molecular doctrine, the law of conservation of matter, the periodic law of D. I. Mendeleev and the theory of chemical structure.
The main provisions of atomic-molecular doctrine are as follows:
1. Substances are made up of molecules; molecules of various substances differ in chemical composition, size, physical and chemical properties.
2. Molecules are in continuous motion; there is mutual attraction and repulsion between them. The speed of movement of molecules depends on the state of aggregation of substances.
3. When physical phenomena the composition of molecules remains unchanged, with chemical ones - they undergo qualitative and quantitative changes and others are formed from some molecules.
4. Molecules are made up of atoms. Atoms are characterized by specific sizes and masses. The properties of the atoms of the same element are the same and differ from the properties of the atoms of other elements.
The mass of an atom, expressed in atomic mass units (amu), is called the relative atomic mass. 1 amu = = 1,667 10 -27 kg.
The elements, combining in different quantitative ratios with each other, form chemical compounds - complex substances. What is a chemical compound? Does a complex substance have a variable or constant composition?
The famous French chemist J. Proust, in contrast to C. Berthollet, believed that any chemically pure compound, regardless of the method of its preparation, has a well-defined composition. It was on this law, which received the name the law of constancy of composition, J. Proust explained the difference between chemical compounds and mixtures. For example, CO 2 (carbon dioxide) can be obtained in several ways:
but pure CO 2 always contains 27.29% C and 72.71% O 2 by weight.
Many elements, combining with each other, can form different substances, each of which is characterized by
a certain ratio between the masses of these elements. So, carbon and oxygen form carbon monoxide - CO and CO 2 - carbon dioxide. Studying such compounds, the English scientist D. Dalton established the law of multiple relations: if two elements form several compounds with each other, then the masses of one of the elements, corresponding to the same mass of the other in these compounds, relate to each other as small numbers.
Dalton adhered to the atomic theory of the structure of matter; studying the properties of gases, he discovered the law of partial pressures of gases. The law directly testified that the elements are included in the composition of compounds only in certain portions, which indicates the discontinuous structure of matter. Developing the atomic-molecular theory, Dalton introduced a concept close to the modern one about atoms and about the relative atomic masses of elements. But unlike the law of conservation of mass, the validity of which was fully confirmed by the discoveries made after its establishment, the laws of constancy of composition and multiple ratios turned out to be not so universal. In connection with the discovery of isotopes, it became clear that the ratio between the masses of the elements that make up a given substance is constant only if the isotopic composition of these elements is constant. For example, heavy water contains 20% (mass) hydrogen, while ordinary water only 11%.
At the beginning of the XX century. (more than 100 years later) the Russian scientist NS Kurnakov, studying metal alloys, discovered compounds of variable composition, in which a different mass of another element can fall on a unit mass of a given element. For many compounds of variable composition, limits have been established within which their composition can change, and the formula TiO 2 more accurately expresses its composition in the form of TiO 1. 9 _ 2.0. Of course, formulas of this kind do not indicate the composition of the molecule (substances have an atomic structure), but only reflect the boundaries of the composition of the substance. The periodic table is an example of an ordered finite countable set of chemical elements. Is it possible to arrange in a similar way a multitude of chemical compounds, the number of which, although large, is not unlimited? And so
it turned out that substances with the same sums of atomic numbers, molecular masses and densities have extremely similar physicochemical properties. It is enough to know the chemical composition of a substance and its density in order to predict all its other properties. NS Kurnakov suggested calling the compounds bertholloids in honor of K. Berthollet, who was the first to predict the existence of substances of variable composition.
Thus, there is an extensive class of compounds that do not obey stoichiometric compounds, laws, that is, violation of laws is associated with a completely definite state of aggregation of matter.
In principle, there is no clear boundary between compounds of constant and variable composition from the point of view of modern physics. A compound can also be formed from atoms of one chemical element - a simple substance. A complex substance is formed from atoms of a different nature, that is, various elements are included in the composition of a molecule of complex substances. Water is formed by atoms of hydrogen and oxygen, and the substance is oxygen only from the molecules of one element - oxygen. But one element oxygen forms two allotropic modifications of simple substances oxygen and ozone, which differ in structure, structure, physical and chemical properties.
