The sixth element of the periodic table. 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 tasks. 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 charge 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 everything were so simple .. This version of 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 Émile Chancourtois placed the elements along a helix and noted the cyclical repetition of 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 not yet open 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, to order the elements, instead of atomic weight (atomic mass), the concept is used atomic number(the number of protons in the nucleus). 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.

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. Atomic number, as already mentioned, is equal to the number 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. Difference mass number and 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.

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.

So, 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 chemical table Mendeleev 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 in the 1930s noble gases managed to deduce 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 figure is serial number element.

20th century scientists have studied and electronic structure 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 non-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. This is 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 with modern point vision. Russian chemist managed to discover most of the components of our world and 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 make learning 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 more traditional way learning the 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. Visual images matched to each of the elements also help. 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 of 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.

If the periodic table seems difficult for you to understand, you are not alone! Although it can be difficult to understand its principles, knowing how to work with it will help in learning natural sciences. To get started, study the structure of the table and what information can be learned from it about each chemical element. Then you can start exploring the properties of each element. And finally, using the periodic table, you can determine the number of neutrons in an atom of a particular chemical element.

Steps

Part 1

Table structure

The periodic table, or the periodic table of chemical elements, begins at the left upper corner and ends at the end of the last row of the table (lower right corner). The elements in the table are arranged from left to right in ascending order of their atomic number. The atomic number tells you how many protons are in one atom. In addition, as the atomic number increases, so does the atomic mass. Thus, by the location of an element in the periodic table, you can determine its atomic mass.

As you can see, each next element contains one more proton than the element preceding it. This is obvious if you look at atomic numbers. Atomic numbers increase by one as you move from left to right. Since the elements are arranged in groups, some table cells remain empty.

For example, the first row of the table contains hydrogen, which has atomic number 1, and helium, which has atomic number 2. However, they are on opposite ends because they belong to different groups.

Learn about groups that include elements with similar physical and chemical properties. The elements of each group are located in the corresponding vertical column. As a rule, they are indicated by the same color, which helps to identify elements with similar physical and chemical properties and predict their behavior. All elements of a particular group have the same number electrons in the outer shell.
- Hydrogen can be attributed both to the group of alkali metals and to the group of halogens. In some tables it is indicated in both groups.
- In most cases, the groups are numbered from 1 to 18, and the numbers are placed at the top or bottom of the table. Numbers can be given in Roman (eg IA) or Arabic (eg 1A or 1) numerals.
- When moving along the column from top to bottom, they say that you are "browsing the group".
Find out why there are empty cells in the table. Elements are ordered not only according to their atomic number, but also according to groups (elements of the same group have similar physical and chemical properties). This makes it easier to understand how an element behaves. However, as the atomic number increases, elements that fall into the corresponding group are not always found, so there are empty cells in the table.
- For example, the first 3 rows have empty cells, since transition metals are found only from atomic number 21.
- Elements with atomic numbers from 57 to 102 belong to the rare earth elements, and they are usually placed in a separate subgroup in the lower right corner of the table.
Each row of the table represents a period. All elements of the same period have the same number atomic orbitals on which electrons are located in atoms. The number of orbitals corresponds to the period number. The table contains 7 rows, that is, 7 periods.
- For example, the atoms of the elements of the first period have one orbital, and the atoms of the elements of the seventh period have 7 orbitals.
- As a rule, periods are indicated by numbers from 1 to 7 on the left of the table.
- As you move along a line from left to right, you are said to be "scanning through a period".
Learn to distinguish between metals, metalloids and non-metals. You will better understand the properties of an element if you can determine what type it belongs to. For convenience, in most tables, metals, metalloids and non-metals are designated different colors. Metals are on the left, and non-metals are on the right side of the table. Metalloids are located between them.

Part 2
Element designations
1. Each element is designated by one or two Latin letters. As a rule, the element symbol is shown in large letters in the center of the corresponding cell. A symbol is an abbreviated name for an element that is the same in most languages. When experimenting and working with chemical equations element symbols are commonly used, so it's good to remember them.
  - Typically, element symbols are shorthand for them. Latin name, although for some, especially recently discovered elements, they are derived from the common name. For example, helium is denoted by the symbol He, which is close to the common name in most languages. At the same time, iron is designated as Fe, which is an abbreviation of its Latin name.
2. Pay attention to the full name of the element, if it is given in the table. This "name" of the element is used in normal texts. For example, "helium" and "carbon" are the names of the elements. Usually, although not always, full names elements are listed below their chemical symbol.
  - Sometimes the names of the elements are not indicated in the table and only their chemical symbols are given.
3. Find the atomic number. Usually the atomic number of an element is located at the top of the corresponding cell, in the middle or in the corner. It can also appear below the symbol or element name. Elements have atomic numbers from 1 to 118.
  - The atomic number is always an integer.
4. Remember that the atomic number corresponds to the number of protons in an atom. All atoms of an element contain the same number of protons. Unlike electrons, the number of protons in the atoms of an element remains constant. Otherwise, another chemical element would have turned out!

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 Chemical properties elements also have a certain sequence (it is said 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 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- sodium (Na) and potassium (K) as they play important role in the process of human life and are part of the 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 each, and one electron is just missing to the "complete set".

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 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 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

How it all began?

Many well-known eminent chemists at the turn of the XIX-XX centuries have long noticed that the physical and chemical properties of many chemical elements are very similar to each other. So for example Potassium, Lithium and Sodium are all active metals, which, when interacting with water, form active hydroxides of these metals; Chlorine, Fluorine, Bromine in their compounds with hydrogen showed the same valence equal to I and all these compounds are strong acids. From this similarity, the conclusion has long been suggested that all known chemical elements can be combined into groups, moreover, so that the elements of each group have a certain set physical and chemical characteristics. However, such groups were often incorrectly compiled from different elements by various scientists, and for a long time one of the main characteristics of the elements was ignored by many - this is their atomic mass. It was ignored because there was and is a different various elements, which means it could not be used as a parameter for grouping. The only exception was the French chemist Alexander Emile Chancourtua, who tried to arrange all the elements in a three-dimensional model along a helix, but his work was not recognized by the scientific community, and the model turned out to be cumbersome and inconvenient.

Unlike many scientists, D.I. Mendeleev took atomic mass (at that time still "Atomic weight") as a key parameter in the classification of elements. In his version, Dmitry Ivanovich arranged the elements in ascending order of their atomic weights, and here a pattern emerged that at certain intervals of the elements, their properties are periodically repeated. True, exceptions had to be made: some elements were swapped and did not correspond to the increase atomic masses(for example, tellurium and iodine), but they corresponded to the properties of the elements. Further development atomic and molecular theory justified such advances and showed the validity of this arrangement. You can read more about this in the article "What is the discovery of Mendeleev"

As we can see, the layout of the elements in this version is not at all the same as we see in the modern form. Firstly, groups and periods are reversed: groups horizontally, periods vertically, and secondly, there are a bit too many groups in it - nineteen, instead of eighteen accepted today.

However, just a year later, in 1870, Mendeleev formed new version table, which is already more recognizable to us: similar elements are lined up vertically, forming groups, and 6 periods are arranged horizontally. It is especially noteworthy that in both the first and second versions the tables are visible significant achievements that his predecessors did not have: places were carefully left in the table for elements that, according to Mendeleev, had yet to be discovered. The corresponding vacancies are indicated by him with a question mark and you can see them in the picture above. Subsequently, the corresponding elements were indeed discovered: Galium, Germanium, Scandium. Thus, Dmitry Ivanovich not only systematized the elements into groups and periods, but also predicted the discovery of new, not yet known, elements.

Later, after resolving many of the topical mysteries of chemistry of that time - the discovery of new elements, the isolation of a group of noble gases together with the participation of William Ramsay, the establishment of the fact that Didymium is not an independent element at all, but is a mixture of two others - more and more new and new versions of the table, sometimes even having a non-table view at all. But we will not give them all here, but we will give only the final version, which was formed during the life of the great scientist.

Transition from atomic weights to nuclear charge.

Unfortunately, Dmitry Ivanovich did not live to see the planetary theory of the structure of the atom and did not see the triumph of Rutherford's experiments, although it was with his discoveries that the new era in the development of the periodic law and the whole periodic system. Let me remind you that from the experiments conducted by Ernest Rutherford, it followed that the atoms of the elements consist of a positively charged atomic nucleus and negatively charged electrons revolving around the nucleus. After determining the charges of the atomic nuclei of all the elements known at that time, it turned out that in the periodic system they are located in accordance with the charge of the nucleus. BUT periodic law has gained new meaning, now it began to sound like this:

"Properties of chemical elements, as well as the forms and properties formed by them simple substances and connections are in periodic dependence on the magnitude of the charges of the nuclei of their atoms "

Now it became clear why some of the lighter elements were put by Mendeleev behind their heavier predecessors - the whole point is that this is how they stand in the order of the charges of their nucleus. For example, tellurium is heavier than iodine, but it is earlier in the table, because the charge of the nucleus of its atom and the number of electrons is 52, while iodine has 53. You can look at the table and see for yourself.

After the discovery of the structure of the atom and the atomic nucleus, the periodic system underwent several more changes, until, finally, it reached the form already familiar to us from school, the short-period version of the periodic table.

In this table, we already know everything: 7 periods, 10 series, side and main subgroups. Also, with the time of opening new elements and filling the table with them, I had to take out separate rows elements like Actinium and Lanthanum, all of which were respectively named Actinides and Lanthanides. This version of the system existed for a very long time - in the world scientific community practically until the end of the 80s, the beginning of the 90s, and in our country even longer - until the 10s of this century.

A modern version of the periodic table.

However, the option that many of us went through at school actually turns out to be very confusing, and the confusion is expressed in the division of subgroups into main and secondary ones, and remembering the logic of displaying the properties of elements becomes quite difficult. Of course, despite this, many studied it, became doctors chemical sciences, but still in modern times it has been replaced by a new option - a long-period one. I note that this particular option is approved by IUPAC (International Union of Pure and Applied Chemistry). Let's take a look at it.

The eight groups were replaced by eighteen, among which there is no longer any division into main and secondary, and all groups are dictated by the arrangement of electrons in atomic shell. At the same time, they got rid of two-row and single-row periods, now all periods contain only one row. How convenient is this option? Now the periodicity of the properties of elements is viewed more clearly. The group number essentially refers to the number of electrons in the external level, in connection with which all the main subgroups of the old version are located in the first, second and thirteenth to eighteenth groups, and all the "former side" groups are located in the middle of the table. Thus, it is now clearly seen from the table that if this is the first group, then these are alkali metals and no copper or silver for you, and it is clear that all transit metals demonstrate well the similarity of their properties due to the filling of the d-sublevel, which affects to a lesser extent external properties, as well as lanthanides and actinides exhibit similar properties due to only the difference in the f-sublevel. Thus, the whole table is divided into the following blocks: s-block, on which s-electrons are filled, d-block, p-block and f-block, with filling of d, p, and f-electrons, respectively.

Unfortunately, in our country this option has been included in school textbooks only in the last 2-3 years, and even then not in all. And very wrong. What is it connected with? Well, firstly, with stagnant times in the dashing 90s, when there was no development at all in the country, not to mention the education sector, namely in the 90s, the world chemical community switched to this option. Secondly, with a slight inertia and difficulty in perceiving everything new, because our teachers are accustomed to the old, short-term version of the table, despite the fact that it is much more difficult and less convenient when studying chemistry.

Expanded version of the periodic system.

But time does not stand still, science and technology too. The 118th element of the periodic system has already been discovered, which means that the next, eighth, period of the table will soon have to be discovered. In addition, a new energy sublevel will appear: the g-sublevel. The elements of its constituents will have to be moved down the table, like lanthanides or actinides, or this table will be expanded twice more, so that it will no longer fit on an A4 sheet. Here I will give only a link to Wikipedia (see Extended Periodic System) and will not repeat the description of this option once again. Anyone who is interested can follow the link and have a look.

In this version, neither f-elements (lanthanides and actinides) nor g-elements ("elements of the future" from Nos. 121-128) are listed separately, but make the table wider by 32 cells. Also, the element Helium is placed in the second group, since it is included in the s-block.

In general, it is unlikely that future chemists will use this option, most likely the periodic table will be replaced by one of the alternatives that are already put forward by brave scientists: the Benfey system, Stewart's "Chemical Galaxy" or another option. But this will be only after the achievement of the second island of stability of chemical elements and, most likely, more will be needed for clarity in nuclear physics than in chemistry, but for now, the good old periodic system of Dmitry Ivanovich will suffice for us.