The structure of the periodic table briefly. The structure of the periodic system of Mendeleev

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Periodic system chemical elements- This 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, in a continuous series of elements, arranged in order of increasing magnitude of the positive charge of the nuclei of their atoms, elements with similar properties are periodically repeated.

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

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

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

As the ordinal number of the element increases, equal to the value of the positive charge of the nucleus of its atom, 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 elements each, in VI - 32 and in VII (incomplete 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). In even rows of large periods are only metals. The properties of the elements in these rows change slightly with increasing serial number. The properties of elements in odd series of large periods change. In period VI, lanthanum is followed by 14 elements that are very similar in chemical properties. These elements, called lanthanides, are listed separately under the main table. Actinides, the elements following actinium, are similarly presented in the table.

The table has nine vertical groups. The group number, with rare exceptions, is equal to the highest positive valence of the elements of this group. Each group, excluding zero and eighth, is divided into subgroups. - main (located to the right) and side. In the main subgroups, with an increase in the serial number, the metallic properties of the elements are enhanced and the non-metallic properties of the elements are weakened.

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

Biogenic elements, i.e., elements that make up organisms and perform a certain biological role, occupy upper part periodic tables. Cells occupied by elements that make up the bulk (more than 99%) of living matter are stained blue. pink color- cells occupied by trace elements (see).

The Periodic Table of Chemical Elements is the biggest achievement modern natural science and a vivid expression of the most general dialectical laws of nature.

See also , Atomic weight.

The periodic system of chemical elements is a natural classification of 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 in a periodic dependence on the magnitude of the atomic weights of the elements. Later, with the development of the doctrine 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 on 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 in a periodic dependence on the positive charge of the nuclei of their atoms. This means that in a continuous series of elements, arranged in ascending order of the positive charges of the nuclei of their atoms, elements with similar properties will be periodically repeated.

The tabular form of the periodic system of chemical elements is presented in its modern form. It consists of periods, series and groups. A period represents 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 (with the exception of the first) there is an element with pronounced metallic properties (alkali metal). Then, as the serial number increases, the metallic properties of the elements 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. Period I consists of two elements, the role of an alkali metal and a halogen is simultaneously performed by hydrogen. II and III periods include 8 elements each, called Mendeleev typical. IV and V periods have 18 elements each, VI-32. VII period is not yet completed and is 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 - 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 the elements in the 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 an even number of the VI period after lanthanum 14 elements follow [called lanthanides (see), lanthanides, rare earth elements], similar in chemical properties to lanthanum and to each other. Their list is given separately under the table.

Separately, the elements following the actinium-actinides (actinides) are written out and given under the table.

There are nine vertical groups 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 does not happen heptavalent); in addition, copper, silver, gold can exhibit a valency greater 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, the secondary - only elements of large periods and, moreover, metals.

In terms of chemical properties, the elements of each subgroup of this group differ significantly from each other, and only the highest positive valency is the same for all elements of this group. In the main subgroups, from top to bottom, the metallic properties of elements increase and non-metallic ones weaken (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 - transuranium. 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 the table of D. I. Mendeleev. This is 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 in 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 in yellow, trace elements are placed, the vital importance of which for humans has been proven.

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 in 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 the elements in the period from an alkali metal to an inert gas. The outer electron shell, containing 8 electrons, is complete and energetically stable (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 in the outer shell remains constant (1 or 2) and the second outer shell is filled with electrons. Hence the slow change in the properties of elements in even rows. In odd series of long periods, with an increase in the charge of the nuclei, 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 period number. The atoms of the elements of the main subgroups have a number of electrons on their outer shells equal to the group number. The atoms of the elements of the 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 secondary subgroups, not only the electrons of the outer shells, but also the penultimate ones, are valence. The number and structure of electron shells are indicated in the attached periodic table of chemical elements.

The periodic law of D. I. Mendeleev and the system based on it have exclusively great importance in science and practice. The periodic law and the system were the basis for the discovery of new chemical elements, exact definition their atomic weights, the development of the doctrine of the structure of atoms, the establishment of geochemical laws for the distribution of elements in earth's crust and development contemporary 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 the 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.

The periodic system is an ordered set of chemical elements, their natural classification, which is a graphical (tabular) expression of the periodic law of chemical elements. Its structure, in many respects similar to the modern one, was developed by D. I. Mendeleev on the basis of the periodic law in 1869–1871.

The prototype of the periodic system was the “Experiment of a system of elements based on their atomic weight and chemical similarity”, compiled by D. I. Mendeleev on March 1, 1869. For two and a half years, the scientist continuously improved the “Experience of the System”, introduced the idea of groups, series and periods of elements. As a result, the structure of the periodic system acquired in many respects modern outlines.

Important for its evolution was the concept of the place of an element in the system, determined by the numbers of the group and period. Based on this concept, Mendeleev came to the conclusion that it is necessary to change the atomic masses of some elements: uranium, indium, cerium and its satellites. It was the first practical use periodic system. Mendeleev was also the first to predict the existence and properties of several unknown elements. The scientist described the most important properties ekaaluminum (the future gallium), ekabora (scandium) and ekasilicium (germanium). In addition, he predicted the existence of analogues of manganese (future technetium and rhenium), tellurium (polonium), iodine (astatine), cesium (francium), barium (radium), tantalum (protactinium). The scientist's predictions regarding these elements were of a general nature, since these elements were located in little-studied areas of the periodic system.

The first versions of the periodic system in many respects represented only an empirical generalization. After all, the physical meaning of the periodic law was not clear, there was no explanation of the reasons for the periodic change in the properties of elements depending on the increase in atomic masses. As a result, many problems remained unresolved. Are there limits to the periodic system? Is it possible to determine the exact number of existing elements? The structure of the sixth period remained unclear - what is the exact amount of rare earth elements? It was not known whether there are still elements between hydrogen and lithium, what is the structure of the first period. Therefore, right up to the physical substantiation of the periodic law and the development of the theory of the periodic system, serious difficulties arose more than once. Unexpected was the discovery in 1894-1898. five inert gases that seemed to have no place in the periodic table. This difficulty was eliminated thanks to the idea of including an independent zero group in the structure of the periodic system. Mass discovery of radioelements at the turn of the 19th and 20th centuries. (by 1910 their number was about 40) led to a sharp contradiction between the need to place them in the periodic system and its existing structure. For them, there were only 7 vacancies in the sixth and seventh periods. This problem was solved as a result of the establishment of shift rules and the discovery of isotopes.

One of the main reasons for the impossibility of explaining the physical meaning of the periodic law and the structure of the periodic system was that it was not known how the atom was arranged (see Atom). The most important milestone in the development of the periodic system was the creation of the atomic model by E. Rutherford (1911). On its basis, the Dutch scientist A. Van den Broek (1913) suggested that the ordinal number of an element in the periodic system is numerically equal to the charge of the nucleus of its atom (Z). This was experimentally confirmed by the English scientist G. Moseley (1913). The periodic law received a physical justification: the periodicity of changes in the properties of elements began to be considered depending on Z - the charge of the nucleus of an atom of an element, and not on atomic mass (see Periodic law of chemical elements).

As a result, the structure of the periodic system has been significantly strengthened. The lower bound of the system has been determined. This is hydrogen, the element with the minimum Z = 1. It has become possible to accurately estimate the number of elements between hydrogen and uranium. "Gaps" in the periodic system were identified, corresponding to unknown elements with Z = 43, 61, 72, 75, 85, 87. However, questions about the exact number of rare earth elements remained unclear and, most importantly, the reasons for the periodic change in the properties of elements were not revealed. depending on Z.

Based on the established structure of the periodic system and the results of the study of atomic spectra, the Danish scientist N. Bohr in 1918–1921. developed ideas about the sequence of construction of electron shells and subshells in atoms. The scientist came to the conclusion that similar types electronic configurations outer shells of atoms are periodically repeated. Thus, it was shown that the periodicity of changes in the properties of chemical elements is explained by the existence of periodicity in the construction of electron shells and subshells of atoms.

The periodic system covers more than 100 elements. Of these, all transuranium elements (Z = 93–110), as well as elements with Z = 43 (technetium), 61 (promethium), 85 (astatine), 87 (francium) were obtained artificially. Over the entire history of the existence of the periodic system, it has been proposed very a large number of(>500) variants of its graphic representation, mainly in the form of tables, as well as in the form of various geometric shapes(spatial and planar), analytical curves (spirals, etc.), etc. The short, semi-long, long and ladder forms of tables are most widely used. Currently, the short form is preferred.

The fundamental principle of building the periodic system is its division into groups and periods. Mendeleev's concept of rows of elements is not currently used, since it is devoid of physical meaning. The groups, in turn, are subdivided into the main (a) and secondary (b) subgroups. Each subgroup contains elements - chemical analogues. The elements of a- and b-subgroups in most groups also show a certain similarity among themselves, mainly in higher oxidation states, which, as a rule, are equal to the group number. A period is a set of elements that begins with an alkali metal and ends with an inert gas (a special case is the first period). Each period contains a strictly defined number of elements. The periodic system consists of eight groups and seven periods, and the seventh period has not yet been completed.

Peculiarity first period lies in the fact that it contains only 2 gaseous elements in the free form: hydrogen and helium. The place of hydrogen in the system is ambiguous. Since it exhibits properties in common with alkali metals and halogens, it is placed either in the 1a- or Vlla-subgroup, or both at the same time, enclosing the symbol in brackets in one of the subgroups. Helium is the first representative of the VIIIa‑subgroup. For a long time, helium and all inert gases were separated into an independent zero group. This provision required revision after the synthesis chemical compounds krypton, xenon and radon. As a result, inert gases and elements of the former group VIII (iron, cobalt, nickel and platinum metals) were combined into one group.

Second period contains 8 elements. It begins with the alkali metal lithium, whose only oxidation state is +1. Next comes beryllium (metal, oxidation state +2). Boron already exhibits a weakly expressed metallic character and is a non-metal (oxidation state +3). Next to the boron, carbon is a typical non-metal that exhibits both +4 and −4 oxidation states. Nitrogen, oxygen, fluorine and neon are all non-metals, with nitrogen having the highest oxidation state of +5 corresponding to the group number. Oxygen and fluorine are among the most active non-metals. The inert gas neon completes the period.

Third period (sodium - argon) also contains 8 elements. The nature of the change in their properties is largely similar to that observed for the elements of the second period. But there is also its own specificity. So, magnesium, unlike beryllium, is more metallic, as well as aluminum compared to boron. Silicon, phosphorus, sulfur, chlorine, argon are all typical non-metals. And all of them, except for argon, exhibit the highest oxidation states equal to the group number.

As we can see, in both periods, as Z increases, a distinct weakening of the metallic and strengthening of the non-metallic properties of the elements is observed. D. I. Mendeleev called the elements of the second and third periods (in his words, small ones) typical. The elements of small periods are among the most common in nature. Carbon, nitrogen and oxygen (along with hydrogen) are organogens, that is, the main elements of organic matter.

All elements of the first - third periods are placed in a‑subgroups.

Fourth period (potassium - krypton) contains 18 elements. According to Mendeleev, this is the first big period. After the alkali metal potassium and the alkaline earth metal calcium, a series of elements follows, consisting of 10 so-called transition metals (scandium - zinc). All of them are included in b‑subgroups. Most transition metals exhibit higher oxidation states equal to the group number, except for iron, cobalt, and nickel. Elements from gallium to krypton belong to the a-subgroups. A number of chemical compounds are known for krypton.

Fifth period (rubidium - xenon) in its construction is similar to the fourth. It also contains an insert of 10 transition metals (yttrium - cadmium). The elements of this period have their own characteristics. In the triad ruthenium - rhodium - palladium, compounds are known for ruthenium where it exhibits an oxidation state of +8. All elements of the a‑subgroups exhibit the highest oxidation states equal to the group number. The features of the change in the properties of the elements of the fourth and fifth periods as Z grows are more complex in comparison with the second and third periods.

Sixth period (cesium - radon) includes 32 elements. In this period, in addition to 10 transition metals (lanthanum, hafnium - mercury), there is also a set of 14 lanthanides - from cerium to lutetium. The elements from cerium to lutetium are chemically very similar, and for this reason they have long been included in the family of rare earth elements. In the short form of the periodic system, the lanthanide series is included in the lanthanum cell and the decoding of this series is given at the bottom of the table (see Lanthanides).

What is the specificity of the elements of the sixth period? In the triad osmium - iridium - platinum, the oxidation state of +8 is known for osmium. Astatine has a fairly pronounced metallic character. Radon is the most reactive of all inert gases. Unfortunately, due to the fact that it is highly radioactive, its chemistry has been little studied (see Radioactive Elements).

Seventh period starts with france. Like the sixth, it should also contain 32 elements, but 24 of them are known so far. Francium and radium, respectively, are elements of subgroups Ia and IIa, actinium belongs to subgroup IIIb. Next comes the actinide family, which includes elements from thorium to lawrencium and is arranged similarly to the lanthanides. The decoding of this row of elements is also given at the bottom of the table.

Now let's see how the properties of chemical elements change in subgroups periodic system. The main pattern of this change is the strengthening of the metallic nature of the elements as Z increases. This pattern is especially pronounced in IIIa–VIIa subgroups. For metals of Ia–IIIa‑subgroups, an increase in chemical activity is observed. In the elements of IVa–VIIa‑subgroups, as Z increases, a weakening of the chemical activity of the elements is observed. For elements of b‑subgroups, the nature of the change in chemical activity is more complex.

The theory of the periodic system was developed by N. Bohr and other scientists in the 1920s. 20th century and is based on a real scheme for the formation of electronic configurations of atoms (see Atom). According to this theory, as Z increases, the filling of electron shells and subshells in the atoms of elements included in the periods of the periodic system occurs in the following sequence:

Period numbers
1	2	3	4	5	6	7
1s	2s2p	3s3p	4s3d4p	5s4d5p	6s4f5d6p	7s5f6d7p

Based on the theory of the periodic system, the following definition of a period can be given: a period is a collection of elements that begins with an element with a value of n equal to the period number and l = 0 (s-elements) and ends with an element with the same value of n and l = 1 (p- elements) (see Atom). The exception is the first period, which contains only 1s elements. From the theory of the periodic system, the numbers of elements in periods follow: 2, 8, 8, 18, 18, 32 ...

In the table, the symbols of elements of each type (s-, p-, d- and f-elements) are shown on a specific color background: s-elements - on red, p-elements - on orange, d-elements - on blue, f-elements - on green. Each cell contains the serial numbers and atomic masses of the elements, as well as the electronic configurations of the outer electron shells.

It follows from the theory of the periodic system that elements with n equal to the period number and l = 0 and 1 belong to the a-subgroups. The b-subgroups include those elements in whose atoms the shells that previously remained incomplete are completed. That is why the first, second and third periods do not contain elements of b‑subgroups.

The structure of the periodic system of elements is closely related to the structure of atoms of chemical elements. As Z increases, similar types of configuration of the outer electron shells are periodically repeated. Namely, they determine the main features of the chemical behavior of elements. These features manifest themselves differently for the elements of the a-subgroups (s- and p-elements), for the elements of the b-subgroups (transitional d-elements) and the elements of the f-families - lanthanides and actinides. A special case is represented by the elements of the first period - hydrogen and helium. Hydrogen is highly reactive because its only 1s electron is easily split off. At the same time, the configuration of helium (1s 2) is very stable, which makes it chemically inactive.

For elements of a-subgroups, the outer electron shells of atoms are filled (with n equal to the period number), so the properties of these elements change noticeably as Z increases. Thus, in the second period, lithium (configuration 2s) is an active metal that easily loses a single valence electron ; beryllium (2s 2) is also a metal, but less active due to the fact that its outer electrons are more firmly bound to the nucleus. Further, boron (2s 2 p) has a weakly pronounced metallic character, and all subsequent elements of the second period, in which the construction of a 2p subshell occurs, are already nonmetals. The eight-electron configuration of the outer electron shell of neon (2s 2 p 6) - an inert gas - is very strong.

The chemical properties of the elements of the second period are explained by the desire of their atoms to acquire the electronic configuration of the nearest inert gas (the helium configuration for elements from lithium to carbon or the neon configuration for elements from carbon to fluorine). This is why, for example, oxygen cannot exhibit a higher oxidation state equal to the group number: after all, it is easier for it to achieve the neon configuration by acquiring additional electrons. The same nature of the change in properties is manifested in the elements of the third period and in the s- and p-elements of all subsequent periods. At the same time, the weakening of the strength of the bond between the outer electrons and the nucleus in a-subgroups as Z increases manifests itself in the properties of the corresponding elements. So, for s-elements, there is a noticeable increase in chemical activity as Z increases, and for p-elements, an increase in metallic properties.

In atoms of transitional d-elements, previously unfinished shells are completed with the value of the main quantum number n, one less than the period number. With some exceptions, the configuration of the outer electron shells of transition element atoms is ns 2 . Therefore, all d-elements are metals, and that is why the changes in the properties of d-elements as Z increases are not as sharp as is observed in s- and p-elements. In higher oxidation states, d-elements show a certain similarity with p-elements of the corresponding groups of the periodic system.

Features of the properties of the elements of triads (VIIIb‑subgroup) are explained by the fact that the b‑subshells are close to completion. This is why iron, cobalt, nickel and platinum metals, as a rule, are not inclined to give compounds of higher oxidation states. The only exceptions are ruthenium and osmium, which give the oxides RuO 4 and OsO 4 . For elements of Ib- and IIb-subgroups, the d-subshell actually turns out to be complete. Therefore, they exhibit oxidation states equal to the group number.

In the atoms of lanthanides and actinides (all of them are metals), the completion of previously incomplete electron shells occurs with the value of the main quantum number n two units less than the period number. In the atoms of these elements, the configuration of the outer electron shell (ns 2) remains unchanged, and the third outside N shell is filled with 4f electrons. That's why the lanthanides are so similar.

For actinides, the situation is more complicated. In atoms of elements with Z = 90–95, electrons 6d and 5f can take part in chemical interactions. Therefore, actinides have many more oxidation states. For example, for neptunium, plutonium and americium, compounds are known where these elements act in the heptavalent state. Only elements starting from curium (Z = 96) become stable in the trivalent state, but even here there are some peculiarities. Thus, the properties of the actinides differ significantly from those of the lanthanides, and therefore both families cannot be considered similar.

The actinide family ends with an element with Z = 103 (lawrencium). An evaluation of the chemical properties of kurchatovium (Z = 104) and nilsborium (Z = 105) shows that these elements should be analogues of hafnium and tantalum, respectively. Therefore, scientists believe that after the family of actinides in atoms, the systematic filling of the 6d subshell begins. Grade chemical nature elements with Z = 106–110 has not been experimentally carried out.

The finite number of elements that the periodic system covers is unknown. The problem of its upper limit is, perhaps, the main riddle of the periodic system. The heaviest element found in nature is plutonium (Z = 94). Reached the limit of artificial nuclear fusion- element with serial number 110. The question remains: will it be possible to obtain elements with higher serial numbers, which ones and how many? It cannot yet be answered with any certainty.

Using the most complex calculations performed on electronic computers, scientists tried to determine the structure of atoms and evaluate the most important properties of "superelements", up to huge serial numbers (Z = 172 and even Z = 184). The results obtained were quite unexpected. For example, in an atom of an element with Z = 121, the appearance of an 8p electron is expected; this is after the formation of the 8s subshell was completed in the atoms with Z = 119 and 120. But the appearance of p-electrons after s-electrons is observed only in atoms of elements of the second and third periods. Calculations also show that in the elements of the hypothetical eighth period, the filling of the electron shells and sub-shells of atoms occurs in a very complex and peculiar sequence. Therefore, to evaluate the properties of the corresponding elements is a very difficult problem. It would seem that the eighth period should contain 50 elements (Z = 119–168), but, according to calculations, it should end at the element with Z = 164, i.e., by 4 sequence numbers before. And the "exotic" ninth period, it turns out, should consist of 8 elements. Here is his "electronic" record: 9s 2 8p 4 9p 2. In other words, it would contain only 8 elements, like the second and third periods.

It is difficult to say how true the calculations made with the help of a computer would be. However, if they were confirmed, then it would be necessary to seriously revise the patterns underlying the periodic system of elements and its structure.

The periodic system has played and continues to play a huge role in the development various areas natural sciences. It was the most important achievement of atomic and molecular science, contributed to the emergence modern concept"chemical element" and clarifying the concepts of simple substances and compounds.

The laws revealed by the periodic system had a significant impact on the development of the theory of the structure of atoms, the discovery of isotopes, and the emergence of ideas about nuclear periodicity. A strictly scientific statement of the problem of forecasting in chemistry is connected with the periodic system. This manifested itself in the prediction of the existence and properties of unknown elements and new features of the chemical behavior of elements already discovered. Now the periodic system is the foundation of chemistry, primarily inorganic, significantly helping to solve the problem of chemical synthesis of substances with predetermined properties, the development of new semiconductor materials, the selection of specific catalysts for various chemical processes etc. And finally, the periodic system underlies the teaching of chemistry.

How to use the periodic table? For an uninitiated person, reading the periodic table is the same as looking at the ancient runes of elves for a dwarf. And the periodic table, by the way, if used correctly, can tell a lot about the world. In addition to serving you in the exam, it is also simply indispensable in solving problems. huge amount chemical and physical problems. But how to read it? Fortunately, today everyone can learn this art. In this article we will tell you how to understand the periodic table.

The periodic system of chemical elements (Mendeleev's table) is a classification of chemical elements that establishes the dependence of various properties of elements on the charge of the atomic nucleus.

History of the creation of the Table

Dmitri Ivanovich Mendeleev was not a simple chemist, if someone thinks so. He was a chemist, physicist, geologist, metrologist, ecologist, economist, oilman, aeronaut, instrument maker and teacher. During his life, the scientist managed to conduct a lot of fundamental research in the most different areas knowledge. For example, it is widely believed that it was Mendeleev who calculated the ideal strength of vodka - 40 degrees. We do not know how Mendeleev treated vodka, but it is known for sure that his dissertation on the topic “Discourse on the combination of alcohol with water” had nothing to do with vodka and considered alcohol concentrations from 70 degrees. With all the merits of the scientist, the discovery of the periodic law of chemical elements - one of the fundamental laws of nature, brought him the widest fame.

There is a legend according to which the scientist dreamed of the periodic system, after which he only had to finalize the idea that had appeared. But if it were that easy... This version about the creation of the periodic table, apparently, is nothing more than a legend. When asked how the table was opened, Dmitry Ivanovich himself answered: “ I’ve been thinking about it for maybe twenty years, and you think: I sat and suddenly ... it’s ready. ”

In the middle of the nineteenth century, attempts to streamline the known chemical elements (63 elements were known) were simultaneously undertaken by several scientists. For example, in 1862 Alexandre Emile Chancourtois placed elements along a helix and marked cyclic repetition chemical properties. Chemist and musician John Alexander Newlands proposed his own version periodic table in 1866. An interesting fact is that in the arrangement of the elements the scientist tried to discover some mystical musical harmony. Among other attempts was the attempt of Mendeleev, which was crowned with success.

In 1869, the first scheme of the table was published, and the day of March 1, 1869 is considered the day of the discovery of the periodic law. The essence of Mendeleev's discovery was that the properties of elements with increasing atomic mass do not change monotonously, but periodically. The first version of the table contained only 63 elements, but Mendeleev undertook a number of very non-standard solutions. So, he guessed to leave a place in the table for yet undiscovered elements, and also changed the atomic masses of some elements. The fundamental correctness of the law derived by Mendeleev was confirmed very soon, after the discovery of gallium, scandium and germanium, the existence of which was predicted by scientists.

Modern view of the periodic table

Below is the table itself.

Today, instead of atomic weight (atomic mass), the concept of atomic number (the number of protons in the nucleus) is used to order elements. The table contains 120 elements, which are arranged from left to right in ascending order of atomic number (number of protons)

The columns of the table are so-called groups, and the rows are periods. There are 18 groups and 8 periods in the table.

The metallic properties of elements decrease when moving along the period from left to right, and in reverse direction- increase.
The dimensions of atoms decrease as they move from left to right along the periods.
When moving from top to bottom in the group, the reducing metallic properties increase.
Oxidizing and non-metallic properties increase along the period from left to right. I am.

What do we learn about the element from the table? For example, let's take the third element in the table - lithium, and consider it in detail.

First of all, we see the symbol of the element itself and its name under it. In the upper left corner is the atomic number of the element, in the order in which the element is located in the table. The atomic number, as already mentioned, is equal to the number of protons in the nucleus. The number of positive protons is usually equal to the number of negative electrons in an atom (with the exception of isotopes).

The atomic mass is indicated under the atomic number (in this version of the table). If we round the atomic mass to the nearest integer, we get the so-called mass number. The difference between the mass number and the atomic number gives the number of neutrons in the nucleus. Thus, the number of neutrons in a helium nucleus is two, and in lithium - four.

So our course "Mendeleev's Table for Dummies" has ended. In conclusion, we invite you to watch a thematic video, and we hope that the question of how to use the periodic table of Mendeleev has become clearer to you. Reminder to study new item always more effective not alone, but with the help of an experienced mentor. That is why, you should never forget about those who will gladly share their knowledge and experience with you.

The properties of chemical elements allow them to be combined into appropriate groups. On this principle, a periodic system was created, which changed the idea of existing substances and made it possible to assume the existence of new, previously unknown elements.

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Periodic system of Mendeleev

The Periodic Table of Chemical Elements was compiled by D. I. Mendeleev in the second half of the 19th century. What is it, and why is it needed? It combines all the chemical elements in order of increasing atomic weight, and all of them are arranged so that their properties change in a periodic manner.

Mendeleev's periodic system brought into a single system all the existing elements that were previously considered simply separate substances.

On the basis of its study, new chemical substances. The significance of this discovery for science cannot be overestimated., it was far ahead of its time and gave impetus to the development of chemistry for many decades.

There are three most common table options, which are conventionally referred to as "short", "long" and "extra long". ». The main table is considered to be a long table, it approved officially. The difference between them is the layout of the elements and the length of the periods.

What is a period

The system contains 7 periods. They are represented graphically as horizontal lines. In this case, the period can have one or two lines, called rows. Each subsequent element differs from the previous one by increasing the nuclear charge (the number of electrons) by one.

Put simply, a period is a horizontal row in the periodic table. Each of them begins with a metal and ends with an inert gas. Actually, this creates periodicity - the properties of elements change within one period, repeating again in the next. The first, second and third periods are incomplete, they are called small and contain 2, 8 and 8 elements, respectively. The rest are complete, they have 18 elements each.

What is a group

Group is a vertical column, containing elements with the same electronic structure or, more simply, with the same higher . The officially approved long table contains 18 groups that start with alkali metals and end with inert gases.

Each group has its own name, which makes it easier to find or classify elements. The metallic properties are enhanced regardless of the element in the direction from top to bottom. This is due to an increase in the number of atomic orbits - the more there are, the weaker the electronic bonds, which makes the crystal lattice more pronounced.

Metals in the periodic table

Metals in the table Mendeleev have a predominant number, their list is quite extensive. They are characterized common features, according to their properties, they are heterogeneous and are divided into groups. Some of them have little in common with metals in the physical sense, while others can exist only for fractions of a second and are absolutely not found in nature (at least on the planet), because they were created, more precisely, calculated and confirmed in laboratory conditions, artificially. Each group has its own characteristics, the name is quite noticeably different from the others. This difference is especially pronounced in the first group.

The position of the metals

What is the position of metals in the periodic table? Elements are arranged by increasing atomic mass, or the number of electrons and protons. Their properties change periodically, so there is no neat one-to-one placement in the table. How to determine metals, and is it possible to do this according to the periodic table? In order to simplify the question, special reception: conventionally, a diagonal line is drawn at the junctions of the elements from Bor to Polonius (or to Astatus). Those on the left are metals, those on the right are non-metals. It would be very simple and great, but there are exceptions - Germanium and Antimony.

Such a “method” is a kind of cheat sheet, it was invented only to simplify the memorization process. For a more accurate representation, remember that the list of non-metals is only 22 elements, therefore, answering the question of how many metals are contained in the periodic table

In the figure, you can clearly see which elements are non-metals and how they are arranged in the table by groups and periods.

General physical properties

There are common physical properties metals. These include:

Plastic.
characteristic brilliance.
Electrical conductivity.
High thermal conductivity.
Everything except mercury is in a solid state.

It should be understood that the properties of metals vary greatly with respect to their chemical or physical essence. Some of them bear little resemblance to metals in the ordinary sense of the term. For example, mercury occupies a special position. She is at normal conditions is in a liquid state crystal lattice, to the presence of which other metals owe their properties. The properties of the latter in this case are conditional; mercury is related to them to a greater extent by chemical characteristics.

Interesting! Elements of the first group, alkali metals, do not occur in their pure form, being in the composition of various compounds.

The softest metal that exists in nature - cesium - belongs to this group. He, like other alkaline similar substances, has little in common with more typical metals. Some sources claim that in fact, the softest metal is potassium, which is difficult to dispute or confirm, since neither one nor the other element exists on its own - being released as a result of a chemical reaction, they quickly oxidize or react.

The second group of metals - alkaline earth - is much closer to the main groups. The name "alkaline earth" comes from ancient times, when oxides were called "earths" because they have a loose crumbly structure. More or less familiar (in the everyday sense) properties are possessed by metals starting from the 3rd group. As the group number increases, the amount of metals decreases.

In nature, there are a lot of repeating sequences:

seasons;
Times of Day;
days of the week…

In the middle of the 19th century, D.I. Mendeleev noticed that the chemical properties of elements also have a certain sequence (they say that this idea came to him in a dream). The result of the miraculous dreams of the scientist was the Periodic Table of Chemical Elements, in which D.I. Mendeleev arranged the chemical elements in order of increasing atomic mass. In the modern table, the chemical elements are arranged in ascending order of the atomic number of the element (the number of protons in the nucleus of an atom).

The atomic number is shown above the symbol of a chemical element, below the symbol is its atomic mass (the sum of protons and neutrons). Note that the atomic mass of some elements is a non-integer! Remember isotopes! Atomic mass is the weighted average of all the isotopes of an element that occur naturally under natural conditions.

Below the table are the lanthanides and actinides.

Metals, non-metals, metalloids

They are located in the Periodic Table to the left of the stepped diagonal line that starts with Boron (B) and ends with polonium (Po) (the exceptions are germanium (Ge) and antimony (Sb). It is easy to see that metals occupy most Periodic table. Basic properties of metals: solid (except mercury); glitter; good electrical and thermal conductors; plastic; malleable; donate electrons easily.

The elements to the right of the stepped diagonal B-Po are called non-metals. The properties of non-metals are directly opposite to the properties of metals: poor conductors of heat and electricity; fragile; non-forged; non-plastic; usually accept electrons.

Metalloids

Between metals and non-metals are semimetals(metalloids). They are characterized by the properties of both metals and non-metals. Semimetals have found their main industrial application in the production of semiconductors, without which no modern microcircuit or microprocessor is inconceivable.

Periods and groups

As mentioned above, the periodic table consists of seven periods. In each period, the atomic numbers of the elements increase from left to right.

The properties of elements in periods change sequentially: so sodium (Na) and magnesium (Mg), which are at the beginning of the third period, give up electrons (Na gives up one electron: 1s 2 2s 2 2p 6 3s 1; Mg gives up two electrons: 1s 2 2s 2 2p 6 3s 2). But chlorine (Cl), located at the end of the period, takes one element: 1s 2 2s 2 2p 6 3s 2 3p 5.

In groups, on the contrary, all elements have the same properties. For example, in the IA(1) group, all elements from lithium (Li) to francium (Fr) donate one electron. And all elements of group VIIA(17) take one element.

Some groups are so important that they have been given special names. These groups are discussed below.

Group IA(1). The atoms of the elements of this group have only one electron in the outer electron layer, so they easily donate one electron.

The most important alkali metals are sodium (Na) and potassium (K), since they play an important role in the process of human life and are part of salts.

Electronic configurations:

Li- 1s 2 2s 1 ;
Na- 1s 2 2s 2 2p 6 3s 1 ;
K- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1

Group IIA(2). The atoms of the elements of this group have two electrons in the outer electron layer, which also give up during chemical reactions. Most important element- calcium (Ca) - the basis of bones and teeth.

Electronic configurations:

Be- 1s 2 2s 2 ;
mg- 1s 2 2s 2 2p 6 3s 2 ;
Ca- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2

Group VIIA(17). Atoms of the elements of this group usually receive one electron each, because. on the outer electronic layer there are five elements and up to " complete set Just one electron is missing.

The most famous elements of this group are: chlorine (Cl) - is part of salt and bleach; iodine (I) - an element that plays an important role in the activity thyroid gland person.

Electronic configuration:

F- 1s 2 2s 2 2p 5 ;
Cl- 1s 2 2s 2 2p 6 3s 2 3p 5 ;
Br- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 5

Group VIII(18). Atoms of the elements of this group have a fully "staffed" outer electron layer. Therefore, they "do not need" to accept electrons. And they don't want to give them away. Hence - the elements of this group are very "reluctant" to enter into chemical reactions. For a long time it was believed that they do not react at all (hence the name "inert", i.e. "inactive"). But chemist Neil Barlett discovered that some of these gases, under certain conditions, can still react with other elements.

Electronic configurations:

Ne- 1s 2 2s 2 2p 6 ;
Ar- 1s 2 2s 2 2p 6 3s 2 3p 6 ;
kr- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6

Valence elements in groups

It is easy to see that within each group, the elements are similar to each other in their valence electrons (electrons of s and p orbitals located on the outer energy level).

Alkali metals have 1 valence electron each:

Li- 1s 2 2s 1 ;
Na- 1s 2 2s 2 2p 6 3s 1 ;
K- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 1

Alkaline earth metals have 2 valence electrons:

Be- 1s 2 2s 2 ;
mg- 1s 2 2s 2 2p 6 3s 2 ;
Ca- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2

Halogens have 7 valence electrons:

F- 1s 2 2s 2 2p 5 ;
Cl- 1s 2 2s 2 2p 6 3s 2 3p 5 ;
Br- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 5

Inert gases have 8 valence electrons:

Ne- 1s 2 2s 2 2p 6 ;
Ar- 1s 2 2s 2 2p 6 3s 2 3p 6 ;
kr- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6

For more information, see the article Valence and the Table of electronic configurations of atoms of chemical elements by periods.

Let us now turn our attention to the elements located in groups with symbols V. They are located in the center of the periodic table and are called transition metals.

A distinctive feature of these elements is the presence of electrons in atoms that fill d-orbitals:

sc- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 1 ;
Ti- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 2

Separate from the main table are located lanthanides and actinides are the so-called internal transition metals. In the atoms of these elements, electrons fill f-orbitals:

Ce- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 4d 10 5s 2 5p 6 4f 1 5d 1 6s 2 ;
Th- 1s 2 2s 2 2p 6 3s 2 3p 6 4s 2 3d 10 4p 6 4d 10 5s 2 5p 6 4f 14 5d 10 6s 2 6p 6 6d 2 7s 2