In this lesson, you will learn about the Periodic Law of Mendeleev, which describes the change in the properties of simple bodies, as well as the shape and properties of compounds of elements, depending on the value of their atomic masses. Consider how a chemical element can be described by position in the Periodic Table.

Topic: Periodic Law andPeriodic system chemical elements D.I. Mendeleev

Lesson: Description of an element by position in the Periodic Table of Elements by D. I. Mendeleev

In 1869, D.I. Mendeleev, on the basis of accumulated data on chemical elements, formulated his own periodic law. Then it sounded like this: "The properties of simple bodies, as well as the shapes and properties of compounds of elements, are periodically dependent on the magnitude of the atomic masses of the elements." For a very long time, the physical meaning of Mendeleev's law was incomprehensible. Everything fell into place after the discovery of the structure of the atom in the 20th century.

The modern formulation of the periodic law:"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 charge of the nucleus of an atom is equal to the number of protons in the nucleus. The number of protons is balanced by the number of electrons in the atom. Thus, the atom is electrically neutral.

Nuclear charge of an atom in the Periodic Table is the ordinal number of the element.

Period number shows number of energy levels, on which the electrons rotate.

Group number shows the number of valence electrons. For elements of the main subgroups, the number of valence electrons is equal to the number of electrons at the external energy level. It is the valence electrons that are responsible for the formation chemical bonds element.

Chemical elements of group 8 - inert gases have 8 electrons on the outer electron shell. Such an electron shell is energetically favorable. All atoms tend to fill their outer electron shell up to 8 electrons.

What characteristics of an atom change periodically in the Periodic Table?

The structure of the external electronic level is repeated.

The radius of the atom changes periodically. In a group radius increases with an increase in the period number, as the number of energy levels increases. In the period from left to right the atomic nucleus will grow, but the attraction to the nucleus will be greater and therefore the radius of the atom decreases.

Each atom seeks to complete the last energy level of the elements of group 1 at the last layer 1 electron. Therefore, it is easier for them to give it away. And it is easier for the elements of the 7th group to attract 1 electron missing to the octet. In a group, the ability to donate electrons will increase from top to bottom, as the radius of the atom increases and the attraction to the nucleus is less. In the period from left to right, the ability to donate electrons decreases, because the radius of the atom decreases.

The easier an element gives up electrons from the external level, the greater its metallic properties, and its oxides and hydroxides have greater basic properties. This means that the metallic properties in groups increase from top to bottom, and in periods from right to left. With non-metallic properties, the opposite is true.

Rice. 1. Position of magnesium in the table

In the group, magnesium is adjacent to beryllium and calcium. Fig. 1. Magnesium is lower than beryllium, but higher than calcium in the group. Magnesium has more metallic properties than beryllium, but less than calcium. The basic properties of its oxides and hydroxides change as well. During the period, sodium is to the left, and aluminum to the right of magnesium. Sodium will exhibit more metallic properties than magnesium, and magnesium more than aluminum. Thus, you can compare any element with its neighbors in the group and period.

Acidic and non-metallic properties change opposite to basic and metallic properties.

Characterization of chlorine by its position in the periodic system of D.I. Mendeleev.

Rice. 4. Position of chlorine in the table

. Meaning serial number 17 shows the number of protons17 and electrons17 in an atom. Fig. 4. Atomic mass 35 will help you calculate the number of neutrons (35-17 = 18). Chlorine is in the third period, which means the number of energy levels in the atom is 3. It is in the 7 -A group, refers to the p-elements. It is non-metal. We compare chlorine with its neighbors in the group and by period. Chlorine has more non-metallic properties than sulfur, but less than argon. Chlorine ob-la-da-e has less non-metallic properties than fluorine and more than bromine. Let's distribute electrons to energy levels and write an electronic formula. The general distribution of electrons will look like this. See Fig. 5

Rice. 5. Distribution of electrons of the chlorine atom by energy levels

Determine the highest and lowest oxidation state of chlorine. The highest oxidation state is +7, since it can donate 7 electrons from the last electronic layer. The lowest oxidation state is -1 because chlorine needs 1 electron to complete. The formula of the higher oxide Cl 2 O 7 ( acid oxide), hydrogen compound HCl.

In the process of giving or attachment of electrons, the atom acquires conditional charge... This conditional charge is called .

- Simple substances have an oxidation state equal to zero.

Items may exhibit maximum oxidation state and minimal. Maximum an element exhibits an oxidation state when gives away all its valence electrons from the external electronic level. If the number of valence electrons is equal to the group number, then the maximum oxidation state is equal to the group number.

Rice. 2. Position of arsenic in the table

Minimum an element will exhibit an oxidation state when it will accept all possible electrons to complete the electron layer.

Let's look at the example of element # 33, the values ​​of the oxidation states.

This is arsenic As, which is in the fifth main subgroup. Fig. 2. At the last electronic level, it has five electrons. This means that, giving them away, it will have an oxidation state of +5. Until the completion of the electron layer, the As atom lacks 3 electrons. By attracting them, it will have an oxidation state of -3.

The position of the elements of metals and non-metals in the Periodic Table of D.I. Mendeleev.

Rice. 3. Position of metals and non-metals in the table

V collateral subgroups are all metals ... If you mentally hold diagonal from boron to astatine , then above of this diagonal in the main subgroups will be all non-metals , a below this diagonal - all metals ... Fig. 3.

1.No. 1-4 (p. 125) Rudzitis G.Ye. Inorganic and organic chemistry. Grade 8: textbook for educational institutions: basic level / G.E. Rudzitis, F.G. Feldman. M .: Education. 2011 176s.: Ill.

2. What characteristics of the atom change periodicity?

3. Give a characteristic of the chemical element oxygen by its position in the Periodic Table of DI Mendeleev.

The nineteenth century in the history of mankind is a century in which many sciences were reformed, including chemistry. It was at this time that Mendeleev's periodic system appeared, and with it the periodic law. It was he who became the basis of modern chemistry. The periodic table of D.I.Mendeleev is a systematization of elements, which establishes the dependence of chemical and physical properties from the structure and charge of the atom of the substance.

History

The beginning of the periodical was laid by the book "Correlation of properties with the atomic weight of elements", written in the third quarter of the 17th century. It reflected the basic concepts of the known chemical elements (at that time there were only 63 of them). In addition, for many of them the atomic masses were determined incorrectly. This greatly interfered with the discovery of D.I. Mendeleev.

Dmitry Ivanovich began his work by comparing the properties of elements. First of all, he took up chlorine and potassium, and only then moved on to work with alkali metals. Armed with special cards on which chemical elements were depicted, he repeatedly tried to assemble this "mosaic": he laid it out on his table in search of the necessary combinations and coincidences.

After much effort, Dmitry Ivanovich nevertheless found the pattern he was looking for, and lined up the elements in periodic rows. As a result, having received empty cells between the elements, the scientist realized that not all chemical elements are known to Russian researchers, and that it was he who should give this world the knowledge in the field of chemistry that had not yet been given by his predecessors.

Everyone knows the myth that Mendeleev periodic table appeared in a dream, and from memory he collected the elements into a single system. This is, roughly speaking, a lie. The fact is that Dmitry Ivanovich worked on his work for a long time and with concentration, and it was very exhausting him. While working on the system of elements, Mendeleev once fell asleep. Waking up, he realized that he had not finished the table, and rather continued filling empty cells... His acquaintance, a certain Inostrantsev, a university teacher, decided that Mendeleev had dreamed the table in a dream and spread this rumor among his students. This is how this hypothesis appeared.

Notoriety

Chemical elements of Mendeleev is a reflection of the periodic law created by Dmitry Ivanovich in the third quarter of the 19th century (1869). It was in 1869 at a meeting of the Russian chemical community that Mendeleev's notice about the creation of a certain structure was read out. And in the same year the book "Fundamentals of Chemistry" was published, in which Mendeleev's periodic table of chemical elements was first published. And in the book “ Natural system elements and its use to indicate the qualities of undiscovered elements "D. I. Mendeleev first mentioned the concept of" periodic law ".

Structure and rules for placing elements

The first steps in the creation of the periodic law were taken by Dmitry Ivanovich back in 1869-1871, at that time he worked hard to establish the dependence of the properties of these elements on the mass of their atom. Modern version is a two-dimensional table of elements.

The position of an element in the table has a certain chemical and physical meaning. By the location of an element in the table, you can find out what valency it has, determine other chemical features. Dmitry Ivanovich tried to establish a connection between elements, both similar in properties and different.

He based the classification of the chemical elements known at that time on the valence and atomic mass. Comparing the relative properties of elements, Mendeleev tried to find a pattern that would combine all known chemical elements into one system. Having arranged them, based on the increase in atomic masses, he nevertheless achieved periodicity in each of the rows.

Further development of the system

The periodic table, which appeared in 1969, has been revised more than once. With the advent of noble gases in the 1930s, it turned out to reveal the newest dependence of elements - not on mass, but on the serial number. Later it was possible to establish the number of protons in atomic nuclei, and it turned out that it coincides with the ordinal number of the element. Scientists of the 20th century studied the electronic one.It turned out that it also affects the frequency. This greatly changed the idea of ​​the properties of elements. This item was reflected in the later editions of the periodic table of Mendeleev. Each new discovery of the properties and features of the elements fit organically into the table.

Characteristics of the periodic table of Mendeleev

The periodic table is divided into periods (7 lines arranged horizontally), which, in turn, are subdivided into large and small. The period begins with an alkali metal, and ends with an element with non-metallic properties.
Dmitry Ivanovich's table is vertically divided into groups (8 columns). Each of them in the periodic system consists of two subgroups, namely, the main and the secondary. After long disputes, at the suggestion of DI Mendeleev and his colleague U. Ramzai, it was decided to introduce the so-called zero group. It includes inert gases (neon, helium, argon, radon, xenon, krypton). In 1911, the scientist F. Soddy was proposed to place indistinguishable elements, the so-called isotopes, in the periodic table - separate cells were allocated for them.

Despite the fidelity and accuracy of the periodic system, the scientific community did not want to recognize this discovery for a long time. Many great scientists ridiculed the activities of D.I. Mendeleev and believed that it was impossible to predict the properties of an element that had not yet been discovered. But after the alleged chemical elements were discovered (and these were, for example, scandium, gallium and germanium), Mendeleev's system and his periodic law became the science of chemistry.

Table in modern times

Mendeleev's Periodic Table of Elements is the basis of most of the chemical and physical discoveries associated with atomic-molecular science. Modern concept element was formed precisely thanks to the great scientist. The appearance of the periodic table of Mendeleev introduced drastic changes in the idea of various connections and simple substances. The creation of the periodic system by scientists had a huge impact on the development of chemistry and all sciences related to it.

The periodic table of chemical elements is a classification of chemical elements based on certain structural features of the atoms of chemical elements. It was compiled on the basis of the Periodic Law, discovered in 1869 by D.I.Mendeleev. At that time, the Periodic Table included 63 chemical elements and was different in appearance from the modern one. Now the Periodic Table includes about one hundred and twenty chemical elements.

The periodic table is compiled in the form of a table, in which the chemical elements are arranged in a specific order: as their atomic masses grow. Now there are many types of images of the Periodic Table. The most common image is in the form of a table with the arrangement of elements from left to right.

All chemical elements in the Periodic Table are combined into periods and groups. The periodic system includes seven periods and eight groups. Periods are called horizontal rows of chemical elements, in which the properties of the elements change from typical metallic to non-metallic. Vertical columns of chemical elements, which contain elements with similar chemical properties, form groups of chemical elements.

The first, second and third periods are called small, since they contain a small number of elements (the first - two elements, the second and third - eight elements each). Elements of the second and third periods are called typical, their properties naturally change from a typical metal to inert gas.

All other periods are called large (the fourth and fifth contain 18 elements each, the sixth - 32 and the seventh - 24 elements). Elements that are within large periods at the end of each even row exhibit a special similarity in properties. These are the so-called triads: Ferum - Cobalt - Nichol, forming the family of iron, and two others: Ruthenium - Rhodium - Palladium and Osmium - Iridium - Platinum, which form the family of platinum metals (platinoids).

At the bottom of DI Mendeleev's table are the chemical elements that form the family of lanthanides and the family of actinides. All these elements formally belong to the third group and come after the chemical elements lanthanum (number 57) and actinium (number 89).

The periodic table of elements contains ten rows. Small periods (first, second and third) consist of one row, large periods (fourth, fifth and sixth) contain two rows each. There is one row in the seventh period.

Each major period consists of odd and even rows. Paired rows contain metal elements; in odd rows, the properties of elements change as in standard elements, i.e. from metallic to pronounced non-metallic.

Each group of DI Mendeleev's table consists of two subgroups: main and secondary. The main subgroups include elements of both small and large periods, that is, the main subgroups begin with either the first or second period. The side subgroups include elements of only large periods, i.e. side subgroups begin only from the fourth period.

The brilliant Russian chemist D.I.Mendeleev was distinguished throughout his life by the desire to learn the unknown. This desire, as well as the deepest and most extensive knowledge, combined with an unerring scientific intuition, allowed Dmitry Ivanovich to develop a scientific classification of chemical elements - the Periodic Table in the form of his famous table.

The periodic table of chemical elements of D.I.Mendeleev can be represented as big house, in which absolutely all chemical elements "live together", known to man... To be able to use the Periodic Table, it is necessary to study the chemical alphabet, that is, the signs of chemical elements.

With their help, you will learn how to write words - chemical formulas, and on their basis you will be able to write sentences - equations of chemical reactions. Each chemical element is designated by its own chemical sign, or symbol, which, along with the name of the chemical element, is recorded in the table of D.I. Mendeleev. The initial letters of the Latin names of chemical elements were adopted in most cases as symbols at the suggestion of the Swedish chemist J. Berzelius. So, hydrogen (the Latin name Hydrogenium - hydrogenium) is designated by the letter H (read "ash"), oxygen (the Latin name Oxygenium - oxygenium) - by the letter O (read "o"), carbon (Latin name Сarboneum - carbononeum) - by the letter C ( read "tse").

The letter C begins the Latin names of several more chemical elements: calcium (

Calcium), copper (Cuprum), cobalt (Cobaltum), etc. To distinguish them, I. Berzelius suggested adding one of the subsequent letters of the name to the initial letter of the Latin name. So, the chemical sign of calcium is written down by the symbol Ca (read "calcium"), copper - Cu (read "cuprum"), cobalt - Co (read "cobalt").

The names of some chemical elements reflect the most important properties of the elements, for example, hydrogen - giving rise to water, oxygen - giving rise to acids, phosphorus - carrying light (Fig. 20), etc.

Rice. twenty.
Etymology of the name of element No. 15 of the Periodic Table of D. I. Mendeleev

Other elements are named after celestial bodies or planets Solar system- selenium and tellurium (Fig. 21) (from the Greek. Selena - Moon and Telluris - Earth), uranium, neptunium, plutonium.

Rice. 21.
Etymology of the name of element No. 52 of the Periodic Table of D. I. Mendeleev

Some names are borrowed from mythology (Fig. 22). For example, tantalum. That was the name of the beloved son of Zeus. For crimes against the gods, Tantalus was severely punished. He stood up to his throat in water, and over him hung branches with juicy, fragrant fruits... However, as soon as he wanted to get drunk, the water flowed away from him, barely wanted to satisfy his hunger and stretched out his hand for the fruits - the branches deviated to the side. Trying to isolate tantalum from ores, chemists experienced no less torment.

Rice. 22.
Etymology of the name of element No. 61 of the Periodic Table of D. I. Mendeleev

Several elements have been named after different states or parts of the world. For example, germanium, gallium (Gaul is the old name of France), polonium (in honor of Poland), scandium (in honor of Scandinavia), francium, ruthenium (Ruthenia is the Latin name for Russia), europium and americium. Here are the elements named after cities: hafnium (in honor of Copenhagen), lutetium (in the old days, Paris was called Lutetium), berkelium (in honor of the city of Berkeley in the USA), yttrium, terbium, erbium, ytterbium (the names of these elements come from Ytterby - small town in Sweden, where the mineral containing these elements was first discovered), dubnium (Fig. 23).

Rice. 23.
Etymology of the name of element No. 105 of the Periodic Table of D. I. Mendeleev

Finally, the names of the elements immortalized the names of great scientists: curium, fermium, einsteinium, mendelevium (Fig. 24), lawrencium.

Rice. 24.
Etymology of the name of element No. 101 of the Periodic Table of D. I. Mendeleev

Each chemical element is assigned in the periodic table, in the common "house" of all elements, its own "apartment" - a cell with a strictly defined number. The deeper meaning of this number will be revealed to you in the further study of chemistry. The number of storeys of these "apartments" is also strictly distributed - the periods in which the elements "live". Like the ordinal number of an element (the number of the "apartment"), the number of the period ("floor") conceals the most important information about the structure of the atoms of chemical elements. Horizontally - "number of storeys" - The periodic table is divided into seven periods:

• The 1st period includes two elements: hydrogen H and helium He;
• The 2nd period begins with lithium Li and ends with neon Ne (8 elements);
• The third period begins with sodium Na and ends with argon Ar (8 elements).

The first three periods, each consisting of one row, are called small periods.

Periods 4, 5 and 6 each include two rows of elements, they are called large periods; The 4th and 5th periods each contain 18 elements, the 6th - 32 elements.

The 7th period - unfinished, so far only consists of one row.

Pay attention to the "basement floors" of the Periodic Table - there "live" 14 twin elements, some similar in their properties to lanthanum La, others to anemones Ac, which represent them on the upper "floors" of the table: in the 6th and 7th -th periods.

Vertically, chemical elements "living" in similar "apartments" are located one under the other in vertical columns - groups, of which there are eight in DI Mendeleev's table.

Each group consists of two subgroups - main and secondary. The subgroup, which includes elements of both small and large periods, is called the main subgroup or group A. The subgroup, which includes elements of only large periods, is called the secondary subgroup or group B. , sodium, potassium, rubidium and francium are a subgroup of lithium Li; a secondary subgroup of this group (group IB) is formed by copper, silver and gold - this is the copper subgroup Cu.

In addition to the form of DI Mendeleev's table, which is called short-period (it is shown on the flyleaf of the textbook), there are many other forms, for example, the long-period version.

Just as a child can construct a huge number of different objects from the elements of the Lego game (see Fig. 10), so nature and man have created a variety of substances around us from chemical elements. Another model is even clearer: just as 33 letters of the Russian alphabet form various combinations, tens of thousands of words, and 114 chemical elements in various combinations create over 20 million different substances.

Try to learn the patterns of the formation of words - chemical formulas, and then the world of substances will open before you in all its colorful diversity.

But for this, first, learn the letters - the symbols of chemical elements (Table 1).

Table 1
The names of some chemical elements

Key words and phrases

1. Periodic table of chemical elements (table) D.I. Mendeleev.
2. The periods are large and small.
3. Groups and subgroups - main (A group) and secondary (B group).
4. Symbols of chemical elements.

Work with computer

1. Please refer to the electronic attachment. Study the material in the lesson and complete the suggested assignments.
2. Search the Internet for email addresses that can serve additional sources, revealing the content of the keywords and phrases of the paragraph. Offer to help the teacher prepare a new lesson by reporting on the keywords and phrases in the next paragraph.

Questions and tasks

1. Using dictionaries (etymological, encyclopedic and chemical terms), name the most important properties that are reflected in the names of chemical elements: bromine Br, nitrogen N, fluorine F.
2. Explain how the names of the chemical elements titanium and vanadium reflect the influence of ancient Greek myths.
3. Why is the Latin name for gold Aurum (aurum), and silver - Argentum (Argentum)?
4. Tell the story of the discovery of any (of your choice) chemical element and explain the etymology of its name.
5. Write down the "coordinates", that is, the position in the Periodic Table of D. I. Mendeleev (element number, period number and its type - large or small, group number and subgroup - main or secondary), for the following chemical elements: calcium, zinc , antimony, tantalum, europium.
6. Divide the chemical elements listed in Table 1 into three groups according to the "pronunciation of the chemical symbol". Can this activity help you memorize chemical symbols and pronunciation of element symbols?

The periodic table of chemical elements is a classification of chemical elements created by D.I.Mendeleev on the basis of the periodic law discovered by him in 1869.

D. I. Mendeleev

According to the modern formulation of this law, elements with similar properties are periodically repeated in a continuous series of elements arranged in ascending order of the positive charge of the nuclei of their atoms.

The periodic table of chemical elements, presented in the form of a table, consists of periods, rows and groups.

At the beginning of each period (except for the first) there is an element with pronounced metallic properties (alkali metal).

Legend to the color table: 1 - chemical sign of the element; 2 - name; 3 - atomic mass(atomic weight); 4 - serial number; 5 - distribution of electrons over layers.

As the ordinal number of the element, equal to the value of the positive charge of the nucleus of its atom, increases, the metallic properties gradually weaken and the non-metallic properties increase. The penultimate element in each period is an element with pronounced non-metallic properties (), and the last is an inert gas. In period I there are 2 elements, in II and III - 8 elements each, in IV and V - 18 each, in VI - 32 and in VII (unfinished period) - 17 elements.

The first three periods are called small periods, each of them consists of one horizontal row; the rest - in large periods, each of which (excluding the VII period) consists of two horizontal rows - even (upper) and odd (lower). Only metals are in the even rows of large periods. The properties of elements in these rows change little with increasing serial number. The properties of elements in odd rows of large periods change. In the VI period, lanthanum was followed by 14 elements, very similar in chemical properties. These elements, called lanthanides, are listed separately below the main table. Actinides, which are elements following actinium, are presented in the table in a similar way.

There are nine vertical groups in the table. The group number, with rare exceptions, is equal to the highest positive valency of the elements of this group. Each group, excluding the zero and eighth, is subdivided into subgroups. - main (located to the right) and secondary. In the main subgroups, with an increase in the serial number, the metallic properties of the elements increase and the non-metallic properties of the elements weaken.

Thus, the chemical and a number of physical properties of elements are determined by the place that a given element occupies in the periodic table.

Biogenic elements, that is, elements that make up organisms and perform a certain biological role in it, occupy upper part periodic tables. The cells occupied by the elements that make up the bulk (more than 99%) of living matter are colored blue, in pink color- cells occupied by microelements (see).

The periodic table of chemical elements is the greatest achievement of modern natural science and a vivid expression of the most general dialectical laws of nature.

See also, Atomic Weight.

Periodic table of chemical elements - natural classification chemical elements, created by D.I.Mendeleev on the basis of the periodic law discovered by him in 1869.

In the original formulation, the periodic law of D. I. Mendeleev stated: the properties of chemical elements, as well as the forms and properties of their compounds, are periodically dependent on the value of the atomic weights of the elements. Later, with the development of the theory of the structure of the atom, it was shown that a more accurate characteristic of each element is not the atomic weight (see), but the value of the positive charge of the nucleus of the atom of the element, equal to the ordinal (atomic) number of this element in the periodic system of D.I.Mendeleev ... The number of positive charges in the nucleus of an atom is equal to the number of electrons surrounding the nucleus of an atom, since atoms as a whole are electrically neutral. In the light of these data, the periodic law is formulated as follows: the properties of chemical elements, as well as the forms and properties of their compounds, are periodically dependent on the magnitude of the positive charge of the nuclei of their atoms. This means that in a continuous series of elements, arranged in the order of increasing positive charges of the nuclei of their atoms, elements with similar properties will be periodically repeated.

The tabular form of the periodic table of chemical elements is presented in its modern form... It consists of periods, rows and groups. The period is a sequential horizontal row of elements arranged in ascending order of the positive charge of the nuclei of their atoms.

At the beginning of each period (except for the first) there is an element with pronounced metallic properties (alkali metal). Then, as the serial number increases, the metallic properties gradually weaken and the non-metallic properties of the elements increase. The penultimate element in each period is an element with pronounced non-metallic properties (halogen), and the last is an inert gas. The first period consists of two elements, the role of an alkali metal and a halogen here is simultaneously played by hydrogen. The II and III periods each include 8 elements, named by Mendeleev as typical. IV and V periods have 18 elements each, VI-32. The VII period has not yet been completed and is being replenished with artificially created elements; there are currently 17 elements in this period. I, II and III periods are called small, each of them consists of one horizontal row, IV-VII are large: they (with the exception of VII) include two horizontal rows - even (upper) and odd (lower). In even rows of large periods, only metals are found, and the change in the properties of elements in a row from left to right is weakly expressed.

In odd series of large periods, the properties of the elements in the series change in the same way as the properties of typical elements. In the even row of the VI period, after lanthanum, there are 14 elements [called lanthanides (see), lanthanides, rare earth elements], similar in chemical properties to lanthanum and to each other. A list of them is given separately under the table.

The elements following actinium - actinides (actinides) are separately written out and listed under the table.

There are nine groups along the vertical lines in the periodic table of chemical elements. The group number is equal to the highest positive valency (see) of the elements of this group. The exceptions are fluorine (it happens only negatively monovalent) and bromine (it is not heptavalent); in addition, copper, silver, gold can exhibit a valence of more than +1 (Cu-1 and 2, Ag and Au-1 and 3), and of the elements of group VIII, only osmium and ruthenium have a valency of +8. Each group, with the exception of the eighth and zero, is divided into two subgroups: the main (located to the right) and the secondary. The main subgroups include typical elements and elements of large periods, in secondary ones - only elements of large periods and, moreover, metals.

In terms of chemical properties, the elements of each subgroup of a given group differ significantly from each other, and only the highest positive valence is the same for all elements of a given group. In the main subgroups, from top to bottom, the metallic properties of the elements are enhanced and the non-metallic properties are weakened (for example, francium is an element with the most pronounced metallic properties, and fluorine is non-metallic). Thus, the place of an element in the periodic system of Mendeleev (serial number) determines its properties, which are the average of the properties of neighboring elements vertically and horizontally.

Some groups of elements have special names. So, the elements of the main subgroups of group I are called alkali metals, group II - alkaline earth metals, group VII - halogens, elements located behind uranium - transuranic. Elements that are part of organisms, take part in metabolic processes and have a pronounced biological role, are called biogenic elements. All of them occupy the upper part of D.I.Mendeleev's table. These are primarily O, C, H, N, Ca, P, K, S, Na, Cl, Mg and Fe, which make up the bulk of living matter (more than 99%). The places occupied by these elements in the periodic table are colored light blue. Biogenic elements, which are very few in the body (from 10 -3 to 10 -14%), are called microelements (see). In the cells of the periodic system, stained with yellow placed trace elements, vital essential which has been proven for humans.

According to the theory of the structure of atoms (see Atom) Chemical properties elements depend mainly on the number of electrons in the outer electron shell. The periodic change in the properties of elements with an increase in the positive charge of atomic nuclei is explained by the periodic repetition of the structure of the outer electron shell (energy level) of atoms.

In small periods, with an increase in the positive charge of the nucleus, the number of electrons on the outer shell increases from 1 to 2 in period I and from 1 to 8 in periods II and III. Hence, the change in the properties of elements in the period from alkali metal to inert gas. The outer electron shell, containing 8 electrons, is complete and energetically stable (the elements of the zero group are chemically inert).

In large periods in even rows, with an increase in the positive charge of the nuclei, the number of electrons on the outer shell remains constant (1 or 2) and the second shell is filled with electrons outside. Hence the slow change in the properties of elements in even rows. In odd series of large periods, with an increase in the nuclear charge, the outer shell is filled with electrons (from 1 to 8) and the properties of the elements change in the same way as for typical elements.

The number of electron shells in an atom is equal to the number of the period. The atoms of the elements of the main subgroups have on the outer shells the number of electrons equal to the group number. Atoms of elements of secondary subgroups contain one or two electrons on the outer shells. This explains the difference in the properties of the elements of the main and secondary subgroups. The group number indicates the possible number of electrons that can participate in the formation of chemical (valence) bonds (see Molecule), therefore such electrons are called valence. For elements of side subgroups, valence is not only the electrons of the outer shells, but also of the penultimate ones. The number and structure of electron shells is indicated in the attached periodic table of chemical elements.

D. I. Mendeleev's periodic law and the system based on it have exclusively great importance in science and practice. The periodic law and system were the basis for the discovery of new chemical elements, the precise determination of their atomic weights, the development of the theory of the structure of atoms, the establishment of geochemical laws of the distribution of elements in earth crust and the development of modern ideas about living matter, the composition of which and the laws associated with it are in accordance with the periodic system. The biological activity of elements and their content in the body are also largely determined by the place they occupy in the periodic system of Mendeleev. So, with an increase in the serial number in a number of groups, the toxicity of elements increases and their content in the body decreases. The periodic law is a vivid expression of the most general dialectical laws of the development of nature.