Rn in the periodic table 5 letters scanword. Periodic system of Mendeleev

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 various fields of 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 the elements along a helix and noted the cyclical repetition chemical properties. Chemist and musician John Alexander Newlands proposed his version of the 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 made a number of very non-standard decisions. So, he guessed to leave a place in the table for yet undiscovered elements, and also changed atomic masses 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 increase in the opposite direction.
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.

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. We remind you that learning a new subject is 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.

He drew on the work of Robert Boyle and Antoine Lavouzier. The first scientist advocated the search for indecomposable chemical elements. 15 of those Boyle listed back in 1668.

Lavuzier added 13 more to them, but a century later. The search dragged on because there was no coherent theory of the connection between the elements. Finally, Dmitry Mendeleev entered the "game". He decided that there is a connection between the atomic mass of substances and their place in the system.

This theory allowed the scientist to discover dozens of elements without discovering them in practice, but in nature. This was placed on the shoulders of posterity. But now it's not about them. Let's dedicate the article to the great Russian scientist and his table.

The history of the creation of the periodic table

periodic table began with the book "Relationship of properties with the atomic weight of the elements." The work was issued in the 1870s. At the same time, the Russian scientist spoke to the chemical society of the country and sent the first version of the table to colleagues from abroad.

Before Mendeleev, 63 elements were discovered by various scientists. Our compatriot began by comparing their properties. First of all, he worked with potassium and chlorine. Then, he took up the group of metals of the alkaline group.

The chemist got a special table and element cards to lay them out like solitaire, looking for the right matches and combinations. As a result, an insight came: - the properties of the components depend on the mass of their atoms. So, elements of the periodic table lined up in ranks.

The discovery of the maestro of chemistry was the decision to leave voids in these ranks. The periodicity of the difference between atomic masses led the scientist to assume that not all elements are known to mankind yet. The gaps in weight between some of the "neighbors" were too large.

That's why, periodic table of Mendeleev became like a chessboard, with an abundance of "white" cells. Time has shown that they really were waiting for their "guests". They, for example, became inert gases. Helium, neon, argon, krypton, radioact and xenon were discovered only in the 30s of the 20th century.

Now about myths. It is widely believed that periodic table of chemistry appeared to him in a dream. These are the intrigues of university teachers, more precisely, one of them - Alexander Inostrantsev. This is a Russian geologist who lectured at the St. Petersburg University of Mining.

Inostrantsev knew Mendeleev and visited him. Once, exhausted by the search, Dmitry fell asleep right in front of Alexander. He waited until the chemist wakes up and saw how Mendeleev grabs a piece of paper and writes down the final version of the table.

In fact, the scientist simply did not have time to do this before Morpheus captured him. However, Inostrantsev wanted to amuse his students. Based on what he saw, the geologist came up with a bike, which grateful listeners quickly spread to the masses.

Features of the periodic table

Since the first version in 1969 ordinal periodic table improved many times. So, with the discovery of noble gases in the 1930s, it was possible to derive a new dependence of the elements - on their serial numbers, and not on the mass, as the author of the system stated.

The concept of "atomic weight" was replaced by "atomic number". It was possible to study the number of protons in the nuclei of atoms. This number is the serial number of the element.

Scientists of the 20th century also studied the electronic structure of atoms. It also affects the periodicity of elements and is reflected in later editions. periodic tables. A photo The list shows that the substances in it are arranged as the atomic weight increases.

The fundamental principle was not changed. Mass increases from left to right. At the same time, the table is not single, but divided into 7 periods. Hence the name of the list. Period is a horizontal row. Its beginning is typical metals, the end is elements with no metallic properties. The decline is gradual.

There are big and small periods. The first ones are at the beginning of the table, there are 3 of them. It opens a list with a period of 2 elements. Following are two columns, in which there are 8 items. The remaining 4 periods are large. The 6th is the longest, it has 32 elements. In the 4th and 5th there are 18 of them, and in the 7th - 24.

Can be counted how many elements in the table Mendeleev. There are 112 titles in total. Names. There are 118 cells, but there are variations of the list with 126 fields. There are still empty cells for undiscovered elements that do not have names.

Not all periods fit on one line. Large periods consist of 2 rows. The amount of metals in them outweighs. Therefore, the bottom lines are completely devoted to them. A gradual decrease from metals to inert substances is observed in the upper rows.

Pictures of periodic table divided vertically. it groups in the periodic table, there are 8 of them. Elements similar in chemical properties are arranged vertically. They are divided into main and secondary subgroups. The latter begin only from the 4th period. The main subgroups also include elements of small periods.

The essence of the periodic table

Names of elements in the periodic table is 112 positions. The essence of their arrangement in a single list is the systematization of primary elements. They began to fight over this even in ancient times.

Aristotle was one of the first to understand what everything that exists was made of. He took as a basis the properties of substances - cold and heat. Empidocles singled out 4 fundamental principles according to the elements: water, earth, fire and air.

Metals in the periodic table, like other elements, are the very fundamental principles, but from a modern point of view. The Russian chemist managed to discover most of the components of our world and to suggest the existence of still unknown primary elements.

It turns out that pronunciation of the periodic table- voicing a certain model of our reality, decomposing it into components. However, learning them is not easy. Let's try to make the task easier by describing a couple of effective methods.

How to learn the periodic table

Let's start with modern method. Computer scientists have developed a number of flash games that help memorize Mendeleev's list. Project participants are offered to find elements by different options, for example, name, atomic mass, letter designation.

The player has the right to choose the field of activity - only part of the table, or all of it. In our will, also, exclude the names of elements, other parameters. This complicates the search. For the advanced, a timer is also provided, that is, training is carried out at speed.

Game conditions do study element numbers in the periodic table not boring, but entertaining. Excitement wakes up, and it becomes easier to systematize knowledge in the head. Those who do not accept computer flash projects offer a more traditional way of memorizing a list.

It is divided into 8 groups, or 18 (according to the 1989 edition). For ease of remembering, it is better to create several separate tables, rather than working on a whole version. Help and visual images matched to each of the elements. Rely on your own associations.

So, iron in the brain can be correlated, for example, with a nail, and mercury with a thermometer. The name of the element is unfamiliar? We use the method of suggestive associations. , for example, we will compose from the beginnings of the words "taffy" and "speaker".

Characteristics of the periodic table don't study in one sitting. Lessons are recommended for 10-20 minutes a day. It is recommended to start by remembering only the basic characteristics: the name of the element, its designation, atomic mass and serial number.

Schoolchildren prefer to hang the periodic table above the desktop, or on the wall, which is often looked at. The method is good for people with a predominance visual memory. Data from the list is involuntarily remembered even without cramming.

This is also taken into account by teachers. As a rule, they do not force you to memorize the list, they allow you to look at it even on the control ones. Constantly looking at the table is tantamount to the effect of printing on the wall, or writing cheat sheets before exams.

Starting the study, let us recall that Mendeleev did not immediately remember his list. Once, when the scientist was asked how he opened the table, the answer was: “I’ve been thinking about it for maybe 20 years, but you think: I sat and, suddenly, it’s ready.” The periodic system is painstaking work that cannot be mastered in a short time.

Science does not tolerate haste, because it leads to delusions and annoying mistakes. So, at the same time as Mendeleev, the table was compiled by Lothar Meyer. However, the German did not finish the list a bit and was not convincing in proving his point of view. Therefore, the public recognized the work of the Russian scientist, and not his fellow chemist from Germany.

The periodic system 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, in a continuous series of elements arranged in order of increasing magnitude positive charge 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 atomic number of the element increases, equal to 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, chemical and series physical properties 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 in it, occupy the upper part of the periodic table. The cells occupied by the elements that make up the bulk (more than 99%) of living matter are colored blue, the cells occupied by microelements are colored pink (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.

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) is an element that plays an important role in the activity of the human thyroid gland.

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 did 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 Valency 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 AT. 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

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 concept 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 in detail the most important properties of ekaaluminum (future gallium), ekabor (scandium) and ekasilicon (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 general character, since these elements were located in little-studied regions 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 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 of electronic configurations of the 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 it graphic image, mainly in the form of tables, but also 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 series of elements is no longer used, since it is devoid of physical sense. The groups, in turn, are subdivided into the main (a) and secondary (b) subgroups. Each subgroup contains elements - chemical analogues. The elements of the 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 is that it contains only 2 gaseous free form elements: 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, there is a distinct weakening of metallic and strengthening non-metallic properties elements. 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 the 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 represent 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 2p subshell is formed, 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 the atoms of the 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 tend not to form compounds. higher degrees oxidation. 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. The chemical nature of elements with Z = 106–110 has not been experimentally evaluated.

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). The reached limit of artificial nuclear fusion is an element with the atomic number 110. The question remains: will it be possible to obtain elements with higher atomic numbers, which ones and how many? It cannot yet be answered with any certainty.

With the help of 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., 4 serial numbers earlier. 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 of various fields of natural science. 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 connections.

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 chemical synthesis substances with predetermined properties, the development of new semiconductor materials, the selection of specific catalysts for various chemical processes, etc. Finally, the periodic system underlies the teaching of chemistry.