1.Chemistry as a subject of natural science Chemistry studies that form of movement of matter in which the interaction of atoms occurs with the formation of new specific substances. Chemistry-the science of the composition, structure and properties of substances, their transformations or the phenomena that accompany these transformations. Modern chemistry includes: general, organic, colloidal, analytical, physical, geological, biochemistry, chemistry of building materials. Chemistry subject- chemical elements and their compounds, as well as the laws that govern various chemical reactions. connects physical-mathematical and biological-social sciences.

2.Class of inorganic compounds. Basic Chemical properties acids, bases, salts. According to the properties of inorganic compounds are divided into traces. Classes: oxides, bases, acids, salts. Oxides- a combination of elements with oxygen, in which the latter is a more electronegative element, namely, it exhibits an oxidation state of -2. and only the element O2 is connected. The general formula is CxOy. There are:acidic e-capable of salt formation with basic oxides and bases (SO3+Na2O=Na2SO4; So3+2NaOH=Na2SO4=H2O), main- capable of forming salts with acidic oxides and acids (CaO+CO2=CaCO3; CaO+2HCl=CaCl2+H2O ), amphoteric(to you and basics) and with this and with that (ZnO, BeO, Cr2O3, SnO, PbO, MnO2). and non-salt-forming(CO,NO,N2O) Grounds - substances, during the electrolytic dissociation of which the anion may only hydroxyl group OH. The acidity of a base is the number of OH ions formed during the dissociation of hydroxide. Hydroxides are substances containing the OH group, obtained by combining oxides with water. There are 3 types: basic(bases)acidic(oxygen-containing acids) andamphoteric(ampholytes - exhibit basic and acidic properties Cr(OH)3,Zn(OH)2,Be(OH)2,Al(OH)3) Acids-substances during electrolytic dissociation cat. Cation m.b. only + charged ion H. There are: oxygen-free, oxygen-containing The H number is the basicity of the acid. meta and ortho forms of water molecules. Salts-substances, during the electrolytic dissociation of which the cation can be an ammonium ion (NH4) or a metal ion, and the anion can be any acidic residue There are: medium(complete substitution. consist of an acid residue and a metal ion), sour e (incomplete substitution. presence of unsubstituted H in the composition), basic (incomplete substitution. presence of unsubstituted OH) Not according to composition organic matter are divided into binary– consisting of only two elements, and multi-element– consisting of several elements.

3. Basic provisions of atomic-molecular teaching

1. All substances consist of molecules (corpuscles); during physical phenomena, the molecules are preserved, but during chemical phenomena they are destroyed.

2. Molecules consist of atoms (elements); during chemical reactions, atoms are preserved.

3. Atoms of each type (element) are identical to each other, but differ from atoms of any other type.

4. When atoms interact, molecules are formed: homonuclear (when atoms of one element interact) or heteronuclear (when atoms of different elements interact).

5. Chemical reactions involve the formation of new substances from the same atoms that make up the original substances. + 6. molecules. and the atoms are in continuous motion, and heat consists of internal movement these particles

. Atom- the smallest particle of an element that retains its chemical properties. Atoms differ in nuclear charges, mass and size

Chemical element- type of atoms with the same position. Core charge. Physical properties, characteristic of a simple substance, cannot be attributed to a chemical element. Simple substances- these are substances consisting of atoms of the same chemical element. 4.Basic laws of chemistry (law of conservation, constancy of composition, multiple ratios, Avagadro’s law) Conservation Law: The mass of substances that react is equal to the mass of substances formed as a result of the reaction. Law of Constancy of Composition : (any chemical compound has the same quantitative composition, regardless of the method of its preparation) The ratios between the masses of the elements included in the composition of a given compound are constant and do not depend on the method of obtaining this compound.

Law of Multiples : If two elements form several chemical compounds, then the masses of one of the elements in these compounds per one and the same mass of the other are related to each other as small integers.

Avogadro's law. Equal volumes of any gases taken at the same temperature and the same pressure contain the same number of molecules.

5. Law of Equivalents . Substance equivalent- this is the amount of substance that interacts with 1 mole of a hydrogen atom or displaces the same number of H atoms in a chemical. Reactions. Ve (L/Mol) is the equivalent volume of a substance, that is, the volume of one equivalent of a substance in a gaseous state. LAW: All substances react in chemical reactions and are formed in equivalent quantities. The ratio of equivalent masses, volumes, reacting or forming substances is directly proportional to the ratio of their masses (volumes) or E (simple) = A (atomic mass) / B (valency of the element) E (acids) = M (molar mass) / basic (acid base ) E(Hydroxide)=M/Acid)Acidity of hydroxide) E(salt oxides) = M/a (number of atoms of an element in the sample. Oxide (salts) * in (valency of this element or metal)

6. Structure of atoms. Core. Nuclear reactions. Types of radiation. Rutherford model: 1.almost all the mass is concentrated in the nucleus 2.+ are compensated – 3.charge is equal to the group number. The simplest is H hydrogen The modern concept of chemistry. An element is a type of atom with the same position. According to the nuclear charge, an atom consists of a positively charged nucleus and an electron shell. The electron shell is formed by electrons. The number of electrons is equal to the number of protons, therefore the charge of the atom as a whole is 0. The number of protons, the charge of the nucleus and the number of electrons are numerically equal to the atomic number of the chemical element. Almost all the mass of an atom is concentrated in the nucleus. Electrons move around atomic nucleus, not randomly, but depending on the energy they possess, forming a so-called electron layer. Each electronic layer can contain certain number electrons: on the first - no more than 2, on the second - no more than 8, on the third - no more than 18. The number of electronic layers is determined by the period number. The number of electrons on the last (outer) layer is determined by the group number. In the period, the metallic properties gradually weaken and increase in properties of non-metals A nuclear reaction is the process of formation of new nuclei or particles during collisions of nuclei or particles. Radioactivity called the spontaneous transformation of an unstable isotope of one chemical element into an isotope of another element, accompanied by the emission of elementary particles or nuclei. Types of radiation: alpha, beta (negative and positive) and gamma. Alpha particle is the nucleus of a helium atom 4/2He. When alpha particles are emitted, the nucleus loses two protons and two neutrons, therefore the charge decreases by 2 and the mass number by 4. A negative beta particle is an electron. When an electron is emitted, the nuclear charge increases by one, but the mass number does not change. If the unstable isotope becomes so excited that the emission of the particle does not lead to complete removal of the excitation, then it emits a portion of pure energy called gamma radiation. Atoms with the same nuclear charge, but different mass numbers, are called isotopes (for example, 35/17 Cl and 37/17Cl) Atoms with the same mass numbers, but a different number of protons in the nucleus, are called isobars (for example, 40/19K and 40/20Ca) Half-life (T ½) is the time during which half of the original amount of a radioactive isotope decays.

BASIC CONCEPTS AND LAWS OF CHEMISTRY

Substances and their properties. Chemistry subject

Let's look around. We ourselves and everything that surrounds us consists of substances. There are a lot of substances. Currently, scientists know about 10 million organic and about 100 thousand inorganic substances. And they are all characterized by certain properties. Properties of a substance are the characteristics by which substances differ from each other or are similar to each other..

Every separate species matter, which, under given conditions, has certain physical properties, e.g. aluminum, sulfur, water, oxygen, called substance.

Chemistry studies the composition, structure, properties and transformation of substances. Deep knowledge of chemistry is absolutely necessary for specialists in all industries National economy. Along with physics and mathematics, it forms the basis for the training of highly qualified specialists.

Various changes occur with substances, for example: evaporation of water, melting of glass, combustion of fuel, rusting of metals, etc. These changes with substances can be attributed to physical or to chemical phenomena.

Physical phenomena are those phenomena in which these substances do not transform into others, but usually only their state of aggregation or form changes

Chemical phenomena are those phenomena that result in the formation of other substances from given substances. Chemical phenomena are called chemical transformations or chemical reactions

In chemical reactions, starting substances are transformed into other substances that have different properties. This can be judged by external signs chemical reactions: 1) release of heat (sometimes light); 2) color change; 3) the appearance of odor; 4) formation of sediment; 5) gas release.

Atomic molecular science

In the XVIII – XIX centuries. As a result of the work of M.V. Lomonosov, Dalton, Avogadro and others, a hypothesis was put forward about the atomic-molecular structure of matter. This hypothesis is based on the idea of ​​the real existence of atoms and molecules. In 1860, the International Congress of Chemists clearly defined the concepts atom and molecule. All scientists accepted the atomic-molecular doctrine. Chemical reactions began to be considered from the point of view of atomic-molecular theory. At the end of the 19th and beginning of the 20th centuries. Atomic-molecular teaching turned into a scientific theory. At this time, scientists proved experimentally that atoms and molecules exist objectively, independently of humans.

Currently, it is possible not only to calculate the sizes of individual molecules and their mass, but also to determine the order of connection of atoms in a molecule. Scientists determine the distance between molecules and even photograph some macromolecules. It is also now known that not all substances are made of molecules.

Basic provisions of atomic-molecular teaching can be formulated like this:

1. There are substances with molecular and non-molecular structure.

2. A molecule is the smallest particle of a substance that retains its chemical properties.

3. There are gaps between the molecules, the sizes of which depend on the state of aggregation and temperature. The greatest distances exist between gas molecules. This explains their easy compressibility. Liquids where the spaces between the molecules are much smaller are more difficult to compress. In solids, the spaces between molecules are even smaller, so they hardly compress.

4. Molecules are in continuous motion. The speed of movement of molecules depends on temperature. As temperature increases, the speed of molecular movement increases.

5. Between molecules there are forces of mutual attraction and repulsion. These forces are most expressed in solids, in the smallest – in gases.

6. Molecules are made up of atoms, which, like molecules, are in continuous motion.

7 Atoms are the smallest chemically indivisible particles.

8. Atoms of one type differ from atoms of another type in mass and properties. Each individual type of atom is called a chemical element.

9. During physical phenomena, molecules are preserved; during chemical phenomena, as a rule, they are destroyed. In chemical reactions, a rearrangement of atoms occurs.

Atomic-molecular theory- one of the main theories natural sciences. This theory confirms the material unity of the world.

According to modern concepts, substances in gaseous and vaporous states are made up of molecules. In the solid (crystalline) state, only substances with a molecular structure consist of molecules, for example, organic substances, nonmetals (with a few exceptions), carbon monoxide (IV), and water. Most solid (crystalline) inorganic substances do not have a molecular structure. They do not consist of molecules, but of other particles (ions, atoms) and exist in the form of macrobodies. For example, many salts, oxides and sulfides of metals, diamond, silicon, metals.

In substances with a molecular structure, the chemical bonds between molecules are less strong than between atoms. Therefore they have comparatively low temperatures melting and boiling. In substances with a non-molecular structure, the chemical bond between particles is very strong. Therefore they have high temperatures melting and boiling. Modern chemistry studies the properties of microparticles (atoms, molecules, ions, etc.) and macrobodies.

Molecules and crystals are made up of atoms. Each individual type of atom is called a chemical element.

In total, the existence of (92) different chemical elements has been established in nature (on Earth). Another 22 elements were obtained artificially using nuclear reactors and powerful accelerators.

All substances are divided into simple and complex.

Substances that consist of atoms of one element are called simple.

Sulfur S, hydrogen H2, oxygen O2, ozone O3, phosphorus P, iron Fe are simple substances.

Substances that consist of atoms of different elements are called complex.

For example, water H 2 O consists of atoms of different elements - hydrogen H and oxygen O; chalk CaCO 3 consists of atoms of the elements calcium Ca, carbon C and oxygen O . Water and chalk are complex substances.

The concept of “simple substance” cannot be identified with the concept of “chemical element”. A simple substance is characterized by a certain density, solubility, boiling and melting points, etc. A chemical element is characterized by a certain positive charge of the nucleus ( serial number), oxidation state, isotopic composition, etc. The properties of an element relate to its individual atoms. Complex substances are not made up of simple substances, but from elements. For example, water does not consist of the simple substances hydrogen and oxygen, but of the elements hydrogen and oxygen.

The names of the elements coincide with the names of their corresponding simple substances, with the exception of carbon.

Many chemical elements form several simple substances that differ in structure and properties. This phenomenon is called allotropy, and the formed substances allotropic modifications or modifications. Thus, the element oxygen forms two allotropic modifications: oxygen and ozone; element carbon - three: diamond, graphite and carbine; Several modifications form the element phosphorus.

The phenomenon of allotropy is caused by two reasons: 1) a different number of atoms in the molecule, for example oxygen O 2 and ozone O 3; 2) the formation of various crystalline forms, such as diamond, graphite and carbine.

2. Stoichiometric laws

Stoichiometry- a branch of chemistry that deals with mass and volume relationships between reacting substances. Translated from Greek, the word “stoichiometry” means “component” and “I measure.”

The basis of stoichiometry is stoichiometric laws: conservation of mass of substances, constancy of composition, Avogadro’s law, law of volumetric ratios of gases, law of equivalents. They confirmed the atomic-molecular theory. In turn, atomic-molecular theory explains stoichiometric laws.

Related information.

1. All substances are made up of molecules. Molecule - the smallest particle of a substance that has its chemical properties.

2. Molecules are made up of atoms. Atom - the smallest particle of a chemical element that retains all its chemical properties. Different elements have different atoms.

3. Molecules and atoms are in continuous motion; there are forces of attraction and repulsion between them.

Chemical element - this is a type of atoms characterized by certain nuclear charges and the structure of electronic shells. Currently, 117 elements are known: 89 of them are found in nature (on Earth), the rest are obtained artificially. Atoms exist in free state, in compounds with atoms of the same or other elements, forming molecules. The ability of atoms to interact with other atoms and form chemical compounds is determined by its structure. Atoms consist of a positively charged nucleus and negatively charged electrons moving around it, forming an electrically neutral system that obeys the laws characteristic of microsystems.

Atomic nucleus - central part atom, consisting of Z protons and N neutrons, in which the bulk of the atoms is concentrated.

Core charge - positive, equal in value to the number of protons in the nucleus or electrons in a neutral atom and coincides with the atomic number of the element in the periodic table. The sum of protons and neutrons of an atomic nucleus is called the mass number A = Z + N.

Isotopes - chemical elements with identical nuclear charges, but different mass numbers due to different numbers neutrons in the nucleus.

Mass

Allotropy - the phenomenon of the formation by a chemical element of several simple substances that differ in structure and properties.

Chemical formulas

Any substance can be characterized by its qualitative and quantitative composition. The qualitative composition is understood as a set of chemical elements that form a substance, and the quantitative, in general case, is the relationship between the number of atoms of these elements. The atoms that form a molecule are connected to each other in a certain sequence, this sequence is called the chemical structure of the substance (molecule).

The composition and structure of a molecule can be depicted using chemical formulas. The qualitative composition is written in the form of symbols of chemical elements, the quantitative composition is written in the form of subscripts for the symbol of each element. For example: C 6 H 12 O 6.

Chemical formula - this is a conventional notation of the composition of a substance using chemical symbols (proposed in 1814 by J. Berzelius) and indices (index is the number at the bottom right of the symbol. Indicates the number of atoms in the molecule). The chemical formula shows which atoms of which elements and in what ratio are connected to each other in a molecule.

Chemical formulas are of the following types:

a) molecular - show how many atoms of elements are included in the molecule of a substance, for example H 2 O - one water molecule contains two hydrogen atoms and one oxygen atom.

b) graphical - show in what order the atoms in the molecule are connected, each bond is represented by a dash; for the previous example, the graphical formula will look like this: H-O-H

c) structural - show the relative positions in space and the distances between the atoms that make up the molecule.

It must be borne in mind that only structural formulas allow a substance to be uniquely identified; molecular or graphic formulas can correspond to several or even many substances (especially in organic chemistry).

International unit atomic masses equal to 1/12 of the mass of the 12C isotope - the main isotope of natural carbon.

1 amu = 1/12 m (12C) = 1.66057 10 -24 g

Relative atomic mass (Ar)- a dimensionless quantity equal to the ratio of the average mass of an atom of an element (taking into account the percentage of isotopes in nature) to 1/12 of the mass of a 12C atom.

Average absolute atomic mass (m) equal to the relative atomic mass times the amu.

m (Mg) = 24.312 1.66057 10 -24 = 4.037 10 -23 g

Relative molecular weight (Mr)- a dimensionless quantity showing how many times the mass of a molecule of a given substance is greater than 1/12 the mass of a 12C carbon atom.

Mr = mg / (1/12 mа(12C))

m r is the mass of a molecule of a given substance;

m a (12C) is the mass of the 12C carbon atom.

Mr = S Ar(e). The relative molecular mass of a substance is equal to the sum of the relative atomic masses of all elements, taking into account the indices.

Mr(B 2 O 3) = 2 Ar(B) + 3 Ar(O) = 2 11 + 3 16 = 70

Mr (KAl(SO 4) 2) = 1 Ar(K) + 1 Ar(Al) + 1 2 Ar(S) + 2 4 Ar(O) == 1 39 + 1 27 + 1 2 32 + 2 4 16 = 258

Absolute molecular mass equal to the relative molecular mass multiplied by the amu. The number of atoms and molecules in ordinary samples of substances is very large, therefore, when characterizing the amount of a substance, a special unit of measurement is used - the mole.

Amount of substance, mol . Means a certain number of structural elements (molecules, atoms, ions). Denoted n and measured in moles. A mole is the amount of a substance containing as many particles as there are atoms in 12 g of carbon.

Avogadro's number (N A ). The number of particles in 1 mole of any substance is the same and equals 6.02 · 10 23. (Avogadro's constant has the dimension - mol -1).

How many molecules are there in 6.4 g of sulfur?

The molecular weight of sulfur is 32 g/mol. We determine the amount of g/mol of substance in 6.4 g of sulfur:

n(s) = m(s) / M(s) = 6.4 g / 32 g/mol = 0.2 mol

Let's determine the number of structural units (molecules) using Avogadro's constant NA

N(s) = n(s) NA = 0.2 6.02 1023 = 1.2 1023

Molar mass shows the mass of 1 mole of a substance (denoted M).

The molar mass of a substance is equal to the ratio of the mass of the substance to the corresponding amount of the substance.

The molar mass of a substance is numerically equal to its relative molecular mass, however, the first quantity has the dimension g/mol, and the second is dimensionless.

M = N A m(1 molecule) = N A Mg 1 amu = (N A · 1 amu) Mr = Mr

This means that if the mass of a certain molecule is, for example, 80 amu. (SO 3), then the mass of one mole of molecules is equal to 80 g. Avogadro’s constant is a proportionality coefficient that ensures the transition from molecular relationships to molar ones. All statements regarding molecules remain valid for moles (with replacement, if necessary, of amu by g). For example, the reaction equation: 2Na + Cl 2 2NaCl, means that two sodium atoms react with one chlorine molecule or that the same thing, two moles of sodium react with one mole of chlorine.

Atomic-molecular science was of exceptional importance for the development of chemistry, the cradle of which is Ancient Greece. The atomism of the ancient Greek materialists is separated from us by a 25-century period, however, the logic of the Greeks is so amazing that the philosophical doctrine of the discrete structure of matter, developed by them, involuntarily merges in our consciousness with our today's ideas. How did atomism originate? Main scientific method ancient Greek philosophers were discussion, dispute. To find “root causes” in disputes, many have been discussed logic problems, one of which was the problem about a stone: what happens if you start crushing it?

Most philosophers believed that this process could be continued indefinitely. And only Leucipus (500-440 BC) and his school argued that this process is not endless: when crushed, in the end, a particle will be obtained, the further division of which will be simply impossible. Based on this concept, Leucippus argued: the material world is discrete, it consists of the smallest particles and emptiness. Leucippus' student Democritus (460--370 BC) called tiny particles“indivisible,” which in Greek means “atom.” This is the name we still use today. Democritus developed a new doctrine - “atomism”, ascribed to atoms such “modern” properties as size and shape, the ability to move.

A follower of Democritus, Epicurus (342-270 BC), gave ancient Greek atomism completeness by suggesting that atoms have an internal source of motion, and they themselves are capable of interacting with each other. All the provisions of ancient Greek atomism look surprisingly modern, and they are naturally understandable to us. After all, any of us, referring to the experience of science, can describe many interesting experiments that confirm the validity of any of the concepts put forward. But they were completely incomprehensible 20-25 centuries ago, since the ancient Greek atomists could not provide any experimental evidence confirming the validity of their ideas. So, although the atomism of the ancient Greeks looks surprisingly modern, none of its provisions were proven at that time. Consequently, “atomism, developed by Leucippus, Democritus and Epicurus, was and remains just a guess, a bold assumption, a philosophical concept, but supported by practice. This led to one of the brilliant guesses the human mind was gradually consigned to oblivion.

There were other reasons why the teachings of the atomists were forgotten for a long time. Unfortunately, the atomists did not leave behind systematic works, and the individual records of disputes and discussions that were made only made it difficult to form a correct idea of ​​the teaching as a whole. The main thing is that many concepts of atomism were heretical and the official church could not support them.

The teachings of atomists were not remembered for almost 20 centuries. And only in the 17th century. The ideas of the ancient Greek atomists were revived thanks to the work of the French philosopher Pierre Gassendi (1592-1655). He spent almost 20 years; to restore and bring together the forgotten concepts of ancient Greek philosophers, which he outlined in detail in his works “C) the life, morals and teachings of Epicurus” and “The Code of Philosophy of Epicurus”. These two books, in which the views of ancient Greek materialists were first systematically presented, became a “textbook” for European scientists and philosophers. Before this, the only source that provided information about the views of Democritus - Epicurus was the poem “On the Nature of Things” by the Roman poet Lucretius. The history of science knows many amazing coincidences. Here is one of them: the revival of ancient Greek atomism coincides in time with the establishment by R. Boyle (1627-1691) of a fundamental law describing changes in the volume of a gas depending on its pressure. Only atomism can provide a qualitative explanation for the facts observed by Boyle: if a gas has a discrete structure, that is, it consists of atoms and emptiness, then the ease of its compression is due to the bringing together of atoms as a result of a decrease in the free space between them. The first timid attempt to use atomism to explain quantitatively observed natural phenomena allows us to draw two very important conclusions:

• 1. Having transformed from a philosophical hypothesis into scientific concept, atomism can become a powerful tool that allows us to give only correct interpretation the most diverse natural phenomena.
• 2. To quickly transform atomism from a philosophical hypothesis into a scientific concept, proof of the existence of atoms must, first of all, be sought in the study of gases, and not liquid and solid substances, which were previously studied by chemists. However, it would be about another 100 years before chemists began to seriously study gases. Then a cascade of discoveries of simple substances will follow: hydrogen, oxygen, nitrogen, chlorine. And a little later, gases will help establish those laws that are commonly called the basic laws of chemistry. They will allow us to formulate the main provisions of the atomic-molecular doctrine.

§ 1 M.V. Lomonosov as the founder of atomic-molecular science

Since the 17th century, there has been molecular science in science, which has been used to explain physical phenomena. Practical use molecular theory in chemistry was limited by the fact that its provisions could not explain the essence of the occurrence of chemical reactions, answer the question of how from certain substances during chemical process new ones are formed.

The solution to this issue turned out to be possible on the basis of atomic-molecular theory. In 1741, in the book “Elements of Mathematical Chemistry,” Mikhail Vasilyevich Lomonosov actually formulated the foundations of atomic-molecular science. The Russian scientist-encyclopedist considered the structure of matter not as a specific combination of atoms, but as a combination of larger particles - corpuscles, which, in turn, consist of smaller particles - elements.

Lomonosov's terminology underwent changes over time: what he called corpuscles began to be called molecules, and the term element was replaced by the term atom. However, the essence of the ideas and definitions he expressed brilliantly stood the test of time.

§ 2 History of the development of atomic-molecular science

The history of the development and establishment of atomic-molecular science in science turned out to be very difficult. Working with objects of the microworld caused enormous difficulties: atoms and molecules were impossible to see and, thus, verify their existence, and attempts to measure atomic masses often ended in obtaining erroneous results. 67 years after Lomonosov's discovery, in 1808, the famous English scientist John Dalton put forward the atomic hypothesis. According to it, atoms are the smallest particles of matter that cannot be divided into their component parts or converted into each other. According to Dalton, all atoms of one element have exactly the same weight and are different from the atoms of other elements. By combining the doctrine of atoms with the doctrine of chemical elements, developed by Robert Boyle and Mikhail Vasilievich Lomonosov, Dalton provided a solid foundation for further theoretical research in chemistry. Unfortunately, Dalton denied the existence of molecules in simple substances. He believed that only complex substances consist of molecules. It didn't help further development and application of atomic-molecular teaching.

The conditions for the dissemination of the ideas of atomic-molecular science in natural sciences developed only in the second half of the 19th century. In 1860, at the International Congress of Naturalists in the German city of Karlsruhe, scientific definitions atom and molecule. There was no study of the structure of substances at that time, so it was accepted that all substances consist of molecules. It was believed that simple substances, such as metals, consist of monatomic molecules. Subsequently, such a complete extension of the principle of molecular structure to all substances turned out to be erroneous.

§ 3 Basic provisions of atomic-molecular teaching

1. A molecule is the smallest part of a substance that retains its composition and most important properties.

2. Molecules are made up of atoms. Atoms of one element are similar to each other, but different from atoms of other chemical elements.