Specific gravity of elements in the periodic table. Periodic table of chemical elements by D.I. Mendeleev
He relied on the works of Robert Boyle and Antoine Lavuzier. The first scientist advocated the search for indecomposable chemical elements. Boyle listed 15 of these back in 1668.
Lavouzier 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 was 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 descendants. 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
Mendeleev table began with the book “Relationship of properties with the atomic weight of elements.” The work was published in the 1870s. At the same time, the Russian scientist spoke before the country’s chemical society and sent out 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, I worked with potassium and chlorine. Then, I took up the group of metals of the alkali group.
The chemist acquired a special table and element cards to play them like solitaire, looking for the necessary matches and combinations. As a result, an insight came: - the properties of components depend on the mass of their atoms. So, elements of the periodic table lined up.
The chemistry maestro's discovery was the decision to leave empty spaces in these rows. The periodicity of the difference between atomic masses forced the scientist to assume that not all elements are known to humanity. The weight gaps between some of the “neighbors” were too large.
That's why, periodic table became like a chess field, with an abundance of “white” cells. Time has shown that they were indeed waiting for their “guests”. For example, they became inert gases. Helium, neon, argon, krypton, radioactivity and xenon were discovered only in the 30s of the 20th century.
Now about the myths. It is widely believed that periodic chemical table appeared to him in a dream. These are the machinations of university teachers, or rather, 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. One day, exhausted from the search, Dmitry fell asleep right in front of Alexander. He waited until the chemist woke up and saw Mendeleev grab a piece of paper and write 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 story, which grateful listeners quickly spread to the masses.
Features of the periodic table
Since the first version in 1969 periodic table has been modified more than once. Thus, with the discovery of noble gases in the 1930s, it was possible to derive a new dependence of elements - on their atomic numbers, and not on 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 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. Photo The list shows that the substances in it are arranged as their atomic weight increases.
They did not change the fundamental principle. The 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. The period is a horizontal row. Its beginning is typical metals, its end is elements with non-metallic properties. The decrease is gradual.
There are large and small periods. The first ones are at the beginning of the table, there are 3 of them. A period of 2 elements opens the list. Next come two columns, each containing 8 items. The remaining 4 periods are large. The 6th is the longest, with 32 elements. In the 4th and 5th there are 18 of them, and in the 7th - 24.
You can count how many elements are in the table Mendeleev. There are 112 titles in total. Namely names. There are 118 cells, and 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 dedicated to them. A gradual decrease from metals to inert substances is observed in the upper rows.
Pictures of the periodic table divided and vertical. This groups in the periodic table, there are 8 of them. Elements with similar 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– this is 112 positions. The essence of their arrangement into a single list is the systematization of the primary elements. People began to struggle with this back in ancient times.
Aristotle was one of the first to understand what all things are made of. He took as a basis the properties of substances - cold and heat. Empidocles identified 4 fundamental principles according to the elements: water, earth, fire and air.
Metals in the periodic table, like other elements, are the same fundamental principles, but from a modern point of view. The 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, breaking it down into its components. However, learning them is not so 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 the modern method. Computer scientists have developed a number of flash games to help memorize Periodic List. Project participants are asked to find elements using different options, for example, name, atomic mass, or letter designation.
The player has the right to choose the field of activity - only part of the table, or all of it. It is also our choice to exclude element names and other parameters. This makes the search difficult. For the advanced, there is also a timer, that is, the training is carried out at speed.
Game conditions make learning numbers of elements in the Mendleyev table not boring, but entertaining. Excitement awakens, and it becomes easier to systematize knowledge in your 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 memorization, it is better to create several separate tables rather than work on a whole version. Visual images matched to each of the elements also help. You should rely on your own associations.
Thus, iron in the brain can be correlated, for example, with a nail, and mercury with a thermometer. Is the element name unfamiliar? We use the method of suggestive associations. , for example, let’s make up the words “toffee” and “speaker” from the beginnings.
Characteristics of the periodic table Don't study in one sitting. Exercises of 10-20 minutes a day are recommended. 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 their desk, or on a wall they often look at. The method is good for people with a predominance of visual memory. Data from the list is involuntarily remembered even without cramming.
Teachers also take this into account. As a rule, they do not force you to memorize the list; they allow you to look at it even during tests. Constantly looking at the table is equivalent to the effect of a printout on the wall, or writing cheat sheets before exams.
When starting to study, let us remember that Mendeleev did not immediately remember his list. Once, when a scientist was asked how he discovered the table, the answer was: “I’ve been thinking about it for maybe 20 years, but you think: I sat there and suddenly it’s ready.” The periodic system is painstaking work that cannot be completed in a short time.
Science does not tolerate haste, because it leads to misconceptions and annoying mistakes. So, at the same time as Mendeleev, Lothar Meyer also compiled the table. However, the German was a little flawed in his list and was not convincing in proving his point. Therefore, the public recognized the work of the Russian scientist, and not his fellow chemist from Germany.
The discovery of the periodic table of chemical elements by Dmitri Mendeleev in March 1869 was a real breakthrough in chemistry. The Russian scientist managed to systematize knowledge about chemical elements and present them in the form of a table, which schoolchildren are still required to study in chemistry lessons. The periodic table became the foundation for the rapid development of this complex and interesting science, and the history of its discovery is shrouded in legends and myths. For all those interested in science, it will be interesting to know the truth about how Mendeleev discovered the table of periodic elements.
History of the periodic table: how it all began
Attempts to classify and systematize known chemical elements were made long before Dmitry Mendeleev. Such famous scientists as Döbereiner, Newlands, Meyer and others proposed their systems of elements. However, due to a lack of data on chemical elements and their correct atomic masses, the proposed systems were not entirely reliable.
The history of the discovery of the periodic table begins in 1869, when a Russian scientist at a meeting of the Russian Chemical Society told his colleagues about his discovery. In the table proposed by the scientist, chemical elements were arranged depending on their properties, ensured by the size of their molecular weight.
An interesting feature of the periodic table was also the presence of empty cells, which in the future were filled with open chemical elements predicted by the scientist (germanium, gallium, scandium). Since the discovery of the periodic table, additions and amendments have been made to it many times. Together with the Scottish chemist William Ramsay, Mendeleev added a group of inert gases (group zero) to the table.
Subsequently, the history of Mendeleev's periodic table was directly related to discoveries in another science - physics. Work on the table of periodic elements continues to this day, and modern scientists add new chemical elements as they are discovered. The importance of Dmitry Mendeleev’s periodic system is difficult to overestimate, since thanks to it:
- Knowledge about the properties of already discovered chemical elements was systematized;
- It became possible to predict the discovery of new chemical elements;
- Such branches of physics as atomic physics and nuclear physics began to develop;
There are many options for depicting chemical elements according to the periodic law, but the most famous and common option is the periodic table familiar to everyone.
Myths and facts about the creation of the periodic table
The most common misconception in the history of the discovery of the periodic table is that the scientist saw it in a dream. In fact, Dmitri Mendeleev himself refuted this myth and stated that he had been pondering the periodic law for many years. To systematize the chemical elements, he wrote out each of them on a separate card and repeatedly combined them with each other, arranging them in rows depending on their similar properties.
The myth about the scientist’s “prophetic” dream can be explained by the fact that Mendeleev worked on the systematization of chemical elements for days on end, interrupted by short sleep. However, only the hard work and natural talent of the scientist gave the long-awaited result and provided Dmitry Mendeleev with worldwide fame.
Many students at school, and sometimes at university, are forced to memorize or at least roughly navigate the periodic table. To do this, a person must not only have a good memory, but also think logically, linking elements into separate groups and classes. Studying the table is easiest for those people who constantly keep their brain in good shape by undergoing training on BrainApps.
MENDELEEV'S PERIODIC TABLE
The construction of Mendeleev's periodic table of chemical elements corresponds to the characteristic periods of number theory and orthogonal bases. The addition of Hadamard matrices with matrices of even and odd orders creates a structural basis of nested matrix elements: matrices of the first (Odin), second (Euler), third (Mersenne), fourth (Hadamard) and fifth (Fermat) orders.
It is easy to see that there are 4 orders k Hadamard matrices correspond to inert elements with an atomic mass that is a multiple of four: helium 4, neon 20, argon 40 (39.948), etc., but also the basics of life and digital technology: carbon 12, oxygen 16, silicon 28, germanium 72.
It seems that with Mersenne matrices of orders 4 k–1, on the contrary, everything active, poisonous, destructive and corrosive is connected. But these are also radioactive elements - energy sources, and lead 207 (the final product, poisonous salts). Fluorine, of course, is 19. The orders of the Mersenne matrices correspond to the sequence of radioactive elements called the actinium series: uranium 235, plutonium 239 (an isotope that is a more powerful source of atomic energy than uranium), etc. These are also alkali metals lithium 7, sodium 23 and potassium 39.
Gallium – atomic weight 68
Orders 4 k–2 Euler matrices (double Mersenne) correspond to nitrogen 14 (the basis of the atmosphere). Table salt is formed by two “mersenne-like” atoms of sodium 23 and chlorine 35; together this combination is characteristic of Euler matrices. The more massive chlorine with a weight of 35.4 falls just short of the Hadamard dimension of 36. Table salt crystals: a cube (! i.e. a docile character, Hadamards) and an octahedron (more defiant, this is undoubtedly Euler).
In atomic physics, the transition iron 56 - nickel 59 is the boundary between elements that provide energy during the synthesis of a larger nucleus (hydrogen bomb) and decay (uranium bomb). Order 58 is famous for the fact that not only does it not have analogues of Hadamard matrices in the form of Belevich matrices with zeros on the diagonal, it also does not have many weighted matrices - the nearest orthogonal W(58,53) has 5 zeros in each column and row (deep gap ).
In the series corresponding to the Fermat matrices and their substitutions of order 4 k+1, by the will of fate it costs Fermium 257. You can’t say anything, an exact hit. Here there is gold 197. Copper 64 (63.547) and silver 108 (107.868), symbols of electronics, do not, as can be seen, reach gold and correspond to more modest Hadamard matrices. Copper, with its atomic weight not far from 63, is chemically active - its green oxides are well known.
Boron crystals under high magnification
WITH golden ratio boron is bound - the atomic mass among all other elements is closest to 10 (more precisely 10.8, the proximity of the atomic weight to odd numbers also has an effect). Boron is a rather complex element. Boron plays an intricate role in the history of life itself. The structure of the framework in its structures is much more complex than in diamond. The unique type of chemical bond that allows boron to absorb any impurity is very poorly understood, although a large number of scientists have already received Nobel Prizes for research related to it. The boron crystal shape is an icosahedron, with five triangles forming the apex.
The mystery of Platinum. The fifth element is, without a doubt, noble metals such as gold. Superstructure over Hadamard dimension 4 k, 1 large.
Stable isotope uranium 238
Let us remember, however, that Fermat numbers are rare (the closest is 257). Crystals of native gold have a shape close to a cube, but the pentagram also sparkles. Its nearest neighbor, platinum, a noble metal, is less than 4 atomic weight away from gold 197. Platinum has an atomic weight not of 193, but slightly higher, 194 (the order of the Euler matrices). It's a small thing, but it brings her into the camp of somewhat more aggressive elements. It is worth remembering, in connection, that due to its inertness (it dissolves, perhaps, in aqua regia), platinum is used as an active catalyst for chemical processes.
Spongy platinum ignites hydrogen at room temperature. Platinum’s character is not at all peaceful; iridium 192 (a mixture of isotopes 191 and 193) behaves more peacefully. It's more like copper, but with the weight and character of gold.
Between neon 20 and sodium 23 there is no element with atomic weight 22. Of course, atomic weights are an integral characteristic. But among the isotopes, in turn, there is also an interesting correlation of properties with the properties of numbers and the corresponding matrices of orthogonal bases. The most widely used nuclear fuel is the isotope uranium 235 (Mersenne matrix order), in which a self-sustaining nuclear chain reaction is possible. In nature, this element occurs in the stable form uranium 238 (Eulerian matrix order). There is no element with atomic weight 13. As for chaos, the limited number of stable elements of the periodic table and the difficulty of finding high-order level matrices due to the barrier observed in thirteenth-order matrices correlate.
Isotopes of chemical elements, island of stability
How it all began?
Many famous eminent chemists at the turn of the 19th and 20th centuries have long noticed that the physical and chemical properties of many chemical elements are very similar to each other. For example, Potassium, Lithium and Sodium are all active metals that, when reacting with water, form active hydroxides of these metals; Chlorine, Fluorine, Bromine in their compounds with hydrogen showed the same valency 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, and so that the elements of each group have a certain set of physical and chemical characteristics. However, such groups were often incorrectly composed of different elements by various scientists, and for a long time, many ignored one of the main characteristics of elements - their atomic mass. It was ignored because it was and is different for different elements, which means it could not be used as a parameter for combining into groups. The only exception was the French chemist Alexandre Emile Chancourtois, he 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 bulky and inconvenient.
Unlike many scientists, D.I. Mendeleev took atomic mass (in those days still “Atomic weight”) as a key parameter in the classification of elements. In his version, Dmitry Ivanovich arranged the elements in increasing order of their atomic weights, and here a pattern emerged that at certain intervals of elements their properties periodically repeat. True, exceptions had to be made: some elements were swapped and did not correspond to the increase in atomic masses (for example, tellurium and iodine), but they corresponded to the properties of the elements. The further development of atomic-molecular science justified such advances and showed the validity of this arrangement. You can read more about this in the article “What is Mendeleev’s discovery”
As we can see, the arrangement of elements in this version is not at all the same as what we see in its modern form. Firstly, the groups and periods are swapped: groups horizontally, periods vertically, and secondly, there are somehow too many groups in it - nineteen, instead of the accepted eighteen today.
However, just a year later, in 1870, Mendeleev formed a new version of the table, which is already more recognizable to us: similar elements are arranged vertically, forming groups, and 6 periods are located horizontally. What is especially noteworthy is that in both the first and second versions of the table one can see significant achievements that his predecessors did not have: the table carefully left places for elements that, in Mendeleev’s opinion, had yet to be discovered. The corresponding vacant positions are indicated by a question mark and you can see them in the picture above. Subsequently, the corresponding elements were actually 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.
Subsequently, after solving many pressing 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 at all an independent element, but is a mixture of two others - more and more new and new table options, sometimes even having a non-tabular appearance. But we will not present them all here, but will present 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 atomic structure and did not see the triumph of Rutherford’s experiments, although it was with his discoveries that a new era began in the development of the periodic law and the entire periodic system. Let me remind you that from experiments conducted by Ernest Rutherford, it followed that the atoms of 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 elements known at that time, it turned out that in the periodic table they are located in accordance with the charge of the nucleus. And the periodic law acquired a new meaning, now it began to sound like this:
“The properties of chemical elements, as well as the forms and properties of the simple substances and compounds they form, are periodically dependent on the magnitude of the charges of the nuclei of their atoms”
Now it has become clear why some lighter elements were placed by Mendeleev behind their heavier predecessors - the whole point is that they are so ranked in order of the charges of their nuclei. For example, tellurium is heavier than iodine, but is listed earlier in the table, because the charge of the nucleus of its atom and the number of electrons is 52, while that of iodine is 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 table underwent several more changes until it finally reached the form already familiar to us from school, the short-period version of the periodic table.
In this table we are already familiar with everything: 7 periods, 10 rows, secondary and main subgroups. Also, with the time of discovering new elements and filling the table with them, it was necessary to place elements like Actinium and Lanthanum in separate rows, all of them were named Actinides and Lanthanides, respectively. This version of the system existed for a very long time - in the world scientific community almost until the late 80s, early 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 in school turns out to be quite confusing, and the confusion is expressed in the division of subgroups into main and secondary ones, and remembering the logic for displaying the properties of elements becomes quite difficult. Of course, despite this, many studied using it, becoming doctors of chemical sciences, but in modern times it has been replaced by a new version - the 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 location of electrons in the atomic shell. At the same time, we got rid of double-row and single-row periods; now all periods contain only one row. Why is this option convenient? Now the periodicity of the properties of elements is more clearly visible. The group number, in fact, indicates the number of electrons in the outer level, and therefore 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 visible 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 clearly demonstrate the similarity of their properties due to the filling of the d-sublevel, which has a lesser effect on external properties, as well as lanthanides and actinides, exhibit similar properties due to only the different f-sublevel. Thus, the entire table is divided into the following blocks: s-block, on which s-electrons are filled, d-block, p-block and f-block, with d, p, and f-electrons filled 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 of them. And in vain. What is this connected with? Well, firstly, with the stagnant times in the dashing 90s, when there was no development at all in the country, not to mention the education sector, and it was in the 90s that the world chemical community switched to this option. Secondly, with slight inertia and difficulty in perceiving everything new, because our teachers are accustomed to the old, short-period version of the table, despite the fact that when studying chemistry it is much more complex and less convenient.
An extended version of the periodic table.
But time does not stand still, and neither do science and technology. The 118th element of the periodic table has already been discovered, which means that we will soon have to open the next, eighth, period of the table. In addition, a new energy sublevel will appear: the g-sublevel. Its constituent elements will have to be moved down the table, like the lanthanides or actinides, or this table will have to be expanded twice more, so that it will no longer fit on an A4 sheet. Here I will only provide a link to Wikipedia (see Extended Periodic Table) and will not repeat the description of this option once again. Anyone interested can follow the link and get acquainted.
In this version, neither f-elements (lanthanides and actinides) nor g-elements ("elements of the future" from Nos. 121-128) are placed separately, but make the table 32 cells wider. Also, the element Helium is placed in the second group, since it is part of 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 being put forward by brave scientists: the Benfey system, Stewart’s “Chemical Galaxy” or another option. But this will only happen after reaching the second island of stability of chemical elements and, most likely, it will be needed more for clarity in nuclear physics than in chemistry, but for now, the good old periodic system of Dmitry Ivanovich will suffice for us.
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 of the positive charge of the nuclei of their atoms, elements with similar properties periodically repeat.
The periodic table of chemical elements, presented in table form, consists of periods, series and groups.
At the beginning of each period (except for the first), the element has 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 across layers.
As the atomic number of an element increases, equal to the positive charge of the nucleus of its atom, metallic properties gradually weaken and 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, in IV and V - 18, in VI - 32 and in VII (not completed 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 (except for the VII period) consists of two horizontal rows - even (upper) and odd (lower). Only metals are found in even rows of large periods. The properties of the elements in these series change slightly with increasing ordinal number. The properties of elements in odd rows of large periods change. In period VI, lanthanum is followed by 14 elements, very similar in chemical properties. These elements, called lanthanides, are listed separately below the main table. Actinides, the elements following actinium, are presented similarly in the table.
The table has nine vertical groups. The group number, with rare exceptions, is equal to the highest positive valency of the elements of this group. Each group, excluding the zero and eighth, is divided into subgroups. - main (located to the right) and secondary. In the main subgroups, as the atomic number increases, the metallic properties of the elements become stronger and the non-metallic properties weaken.
Thus, the chemical and a number of physical properties of elements are determined by the place that a given element occupies in the periodic table.
Biogenic elements, i.e. elements that are part of organisms and perform a certain biological role in it, occupy the top part of the periodic table. Cells occupied by elements that make up the bulk (more than 99%) of living matter are colored blue; cells occupied by microelements are colored pink (see).
The periodic table of chemical elements is the greatest achievement of modern natural science and a vivid expression of the most general dialectical laws of nature.
See also, Atomic weight.
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 its original formulation, D.I. Mendeleev’s periodic law stated: the properties of chemical elements, as well as the forms and properties of their compounds, are periodically dependent on the atomic weights of the elements. Subsequently, 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 element’s atom, equal to the serial (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 the atom, since atoms as a whole are electrically neutral. In the light of these data, the periodic law is formulated as follows: the properties of chemical elements, as well as the forms and properties of their compounds, are periodically dependent on the magnitude of the positive charge of the nuclei of their atoms. This means that in a continuous series of elements arranged in order of increasing positive charges of the nuclei of their atoms, elements with similar properties will periodically repeat.
The tabular form of the periodic table of chemical elements is presented in its modern form. It consists of periods, series and groups. A period represents a successive horizontal series of elements arranged in order of increasing positive charge of the nuclei of their atoms.
At the beginning of each period (except for the first) there is an element with pronounced metallic properties (alkali metal). Then, as the serial number increases, the metallic properties of the elements gradually weaken and the non-metallic properties increase. The penultimate element in each period is an element with pronounced non-metallic properties (halogen), and the last is an inert gas. The first period consists of two elements, the role of an alkali metal and a halogen here is simultaneously played by hydrogen. Periods II and III include 8 elements each, called typical by Mendeleev. Periods IV and V contain 18 elements each, VI-32. The VII period has not yet been completed and is replenished with artificially created elements; There are currently 17 elements in this period. Periods I, II and III are called small, each of them consists of one horizontal row, IV-VII are large: they (with the exception of VII) include two horizontal rows - even (upper) and odd (lower). In even rows of large periods there are only metals, and the change in the properties of 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 the even row of the VI period, after lanthanum, there are 14 elements [called lanthanides (see), lanthanides, rare earth elements], similar in chemical properties to lanthanum and to each other. A list of them is given separately below the table.
The elements following actinium - actinides (actinoids) - are listed separately and listed below the table.
In the periodic table of chemical elements, nine groups are located vertically. The group number is equal to the highest positive valency (see) of the elements of this group. The exceptions are fluorine (can only be negatively monovalent) and bromine (cannot be 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 valence of +8. Each group, with the exception of the eighth and zero, is divided into two subgroups: the main one (located to the right) and the secondary one. The main subgroups include typical elements and elements of long periods, the secondary subgroups include only elements of long periods and, moreover, metals.
In terms of chemical properties, the elements of each subgroup of a given group differ significantly from each other, and only the highest positive valency is the same for all elements of a given group. In the main subgroups, from top to bottom, the metallic properties of elements are strengthened and non-metallic ones are weakened (for example, francium is the element with the most pronounced metallic properties, and fluorine is non-metallic). Thus, the place of an element in Mendeleev’s periodic system (ordinal number) determines its properties, which are the average of the properties of neighboring elements vertically and horizontally.
Some groups of elements have special names. Thus, 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 clear biological role are called biogenic elements. All of them occupy the top part of D.I. Mendeleev’s table. These are primarily O, C, H, N, Ca, P, K, S, Na, Cl, Mg and Fe, which make up the bulk of living matter (more than 99%). The places occupied by these elements in the periodic table are colored light blue. Biogenic elements, of which there are very few in the body (from 10 -3 to 10 -14%), are called microelements (see). The cells of the periodic system, colored yellow, contain microelements, the vital importance of which for humans has been proven.
According to the theory of atomic structure (see Atom), the chemical properties of 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 the 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 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 group zero are chemically inert).
In long periods in even rows, as the positive charge of the nuclei increases, 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 the odd series of large periods, as the charge of the nuclei increases, the outer shell is filled with electrons (from 1 to 8) and the properties of the elements change in the same way as those of typical elements.
The number of electron shells in an atom is equal to the period number. Atoms of elements of the main subgroups have a number of electrons in their outer shells equal to the group number. Atoms of elements of side subgroups contain one or two electrons in their outer shells. This explains the difference in the properties of the elements of the main and secondary subgroups. The group number indicates the possible number of electrons that can participate in the formation of chemical (valence) bonds (see Molecule), therefore such electrons are called valence. For elements of side subgroups, not only the electrons of the outer shells are valence, but also those of the penultimate ones. The number and structure of electron shells are indicated in the accompanying periodic table of chemical elements.
The periodic law of D.I. Mendeleev and the system based on it are of exceptionally great importance in science and practice. The periodic law and system were the basis for the discovery of new chemical elements, the precise determination of their atomic weights, the development of the doctrine of the structure of atoms, the establishment of geochemical laws of distribution of elements in the earth's crust and the development of modern ideas about living matter, the composition of which and the patterns associated with it are in accordance with the periodic system. The biological activity of elements and their content in the body are also largely determined by the place they occupy in Mendeleev’s periodic table. Thus, 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 clear expression of the most general dialectical laws of the development of nature.