Periodic system: classification of chemical elements

In the first half of the 19th century, various attempts were made to systematize the elements and combine metals in the periodic system. It is in this historical period that a method of investigation emerges, such as chemical analysis.

From the history of the discovery of the periodic table of elements

Using a similar technique for determining specific chemical properties, scientists of that time tried to group together elements based on their quantitative characteristics, as well as atomic weight.

Use of atomic weight

Thus, IV Dubereiner in 1817 determined that in strontium the atomic weight is similar to the corresponding indices of barium and calcium. He also managed to find out that there is a lot in common between the properties of barium, strontium and calcium. Based on these observations, the famous chemist made up the so-called triad of elements. In the similar groups, other substances were combined:

• Sulfur, selenium, tellurium;
• Chlorine, bromine, iodine;
• Lithium, sodium, potassium.

Classification by chemical properties

L. Gmelin in 1843 proposed a table in which he placed similar in chemical properties elements in a strict order. Nitrogen, hydrogen, oxygen, he considered the main elements, their given chemist placed outside his table.

Under oxygen, tetrads were arranged (4 signs each) and pentads (5 characters each). Metals in the periodic table were put in the terminology of Berzelius. According to Gmelin's plan, all elements were established by reducing the electronegativity of properties within each subgroup of the periodic system.

Merge elements vertically

Alexander Emil de Chancourtua in 1863 all the elements put in ascending atomic weights on the cylinder, dividing it into several vertical strips. As a result of this division on the verticals, elements with similar physical and chemical properties are located.

The law of octaves

D. Newlands discovered in 1864 a rather interesting pattern. With the arrangement of chemical elements in increasing atomic weights, each eighth element shows a similarity to the first. A similar fact Newlands called the law octaves (eight notes).

His system of periodic was very conditional, so the idea of the observant scientist began to be called an "octave" version, associating it with music. It was the Newlands variant that was closest to the modern structure of the PS. But according to the above-mentioned octave law, only 17 elements retained their periodic properties, while the remaining signs showed no such regularity.

Odling tables

U. Odling presented several variants of the element tables at once. In the first version, created in 1857, he proposed to divide them into 9 groups. In 1861, the chemist made some corrections to the original version of the table, combining in groups signs bearing similar chemical properties.

A variant of Odling table, proposed in 1868, assumed the arrangement of 45 elements in increasing atomic weights. By the way, it was this table that later became the prototype of DI Mendeleyev's periodic system.

Division by valence

L. Meyer in 1864 proposed a table, which included 44 elements. They were placed in 6 columns, according to the hydrogen valence. In the table there were just two parts. The main group consisted of six groups, including 28 signs of increasing atomic weights. In its structure pentads and tetrads from chemical signs similar to chemical properties were looked through. The rest of the elements Meyer placed in the second table.

DI Mendeleyev's contribution to the creation of a table of elements

The modern periodic system of DI Mendeleyev's elements appeared on the basis of the tables of Mayer, compiled in 1869. In the second version, Mayer arranged the signs for 16 groups, placed the elements with pentads and tetrads, given the known chemical properties. Instead of valence, he used a simple numbering for groups. There was no boron, thorium, hydrogen, niobium, uranium in it.

The structure of the periodic system in the form presented in modern editions did not appear immediately. There are three main stages during which a periodic system was created:

1. The first version of the table was presented on the building blocks. The periodic character of the relationship between the properties of the elements and the values of their atomic weights was traced. This version of the classification of Mendeleev signs was proposed in 1868-1869.
2. The scientist refuses the original system, since it did not reflect the criteria by which the elements would fall into a certain column. He suggests placing signs by the similarity of chemical properties (February 1869)
3. In 1870, Dmitry Mendeleev was presented to the scientific world of the modern periodic system of elements.

The version of the Russian chemist took into account both the position of metals in the periodic system, and the features of the properties of nonmetals. For those years that have passed since the first edition of the genius invention of Mendeleyev, the table has not undergone any major changes. And in those places that were left empty during Dmitry Ivanovich's time, new elements appeared that were discovered after his death.

Features of the periodic table

Why is it considered that the described system is periodic? This is explained by the peculiarities of the structure of the table.

In total, it contains 8 groups, and each has two subgroups: the main (main) and secondary. It turns out that all the subgroups 16. They are located vertically, that is, from top to bottom.

In addition, there are horizontal rows in the table, called periods. They also have their additional division into small and large. Characterization of the periodic system involves taking into account the location of the element: its group, subgroup and period.

How properties change in the main subgroups

All the main subgroups in the periodic table begin with elements of the second period. For signs belonging to one main subgroup, the number of external electrons is the same, but the distance between the last electrons and the positive nucleus changes.

In addition, an increase in the atomic weight (relative atomic mass) of the element occurs from above. It is this indicator is the determining factor in determining the pattern of change of properties within the main subgroups.

Since the radius (the distance between the positive nucleus and the external negative electrons) in the main subgroup increases, the nonmetallic properties (the ability to receive electrons in the course of chemical transformations) decrease. As for the change in the metallic properties (the recoil of electrons to other atoms), it will increase.

Using a periodic system, one can compare the properties of different representatives of one main subgroup. At a time when Mendeleev was creating a periodic system, there was still no information on the structure of matter. Surprising is the fact that after the theory of the structure of the atom, which has been studied in educational schools and profile chemical universities, has now been confirmed, Mendeleyev's hypothesis was confirmed, rather than disproving his assumptions about the arrangement of atoms within the table.

Electronegativity in the main subgroups to the bottom decreases, that is, the lower the element is in the group, the more its ability to attach atoms will be smaller.

Change in the properties of atoms in the secondary subgroups

Since Mendeleev's system is periodic, the change in properties in such subgroups occurs in reverse sequence. Such subgroups include elements beginning with the 4th period (representatives of d and f families). To the bottom in these subgroups the metal properties decrease, but the number of external electrons is the same for all representatives of one subgroup.

Peculiarities of the period structure in the PS

Each new period, with the exception of the first, in the table of the Russian chemist begins with an active alkali metal. Further, amphoteric metals are exhibited , exhibiting dual properties in chemical transformations. Then there are several elements with non-metallic properties. The period ends with an inert gas (non-metal, practical, not showing chemical activity).

Given that the system is periodic, in periods there is a change in activity. From left to right, reducing activity (metal properties) will decrease, oxidative activity (non-metallic properties) will increase. Thus, the brightest metals in the period are on the left, and the non-metals on the right.

In large periods, consisting of two rows (4-7), the periodical character also appears, but because of the presence of representatives of the d or f family, the metallic elements in the series are much larger.

Names of main subgroups

Part of the groups of elements available in the periodic table has its own names. Representatives of the first group A subgroups are called alkali metals. To a similar name, metals owe their activity to water, resulting in the formation of caustic alkalis.

The second group A is considered to be a subgroup of alkaline earth metals. When interacting with water, such metals form oxides, they were once called lands. It was from that time that the name was assigned to representatives of this subgroup.

Non-metals of the oxygen subgroup are called chalcogenes, and the 7A groups are called halogens. 8 A subgroup was called inert gases because of its minimal chemical activity.

PS in the school year

For schoolchildren, a variant of the periodic table is usually proposed, in which, in addition to groups, subgroups, periods, formulas for higher volatile compounds and higher oxides are also indicated. This kind of cunning allows the students to build skills for the formation of higher oxides. It suffices to replace the element with the sign of the representative of the subgroup to obtain the finished higher oxide.

If you look closely at the general view of volatile hydrogen compounds, it can be seen that they are characteristic only of nonmetals. In 1-3 groups there are dashes, as typical representatives of these groups are metals.

In addition, in some school textbooks of chemistry, each sign is indicated by a scheme for the distribution of electrons over energy levels. This information did not exist during Mendeleev's work, such scientific facts appeared much later.

You can see the formulas of the external electronic level, by which it is easy to guess which family this item belongs to. Such hints are unacceptable in examination sessions, therefore, graduates of grades 9 and 11 who have decided to demonstrate their chemical knowledge on OGE or EGE give classic black-and-white variants of periodic tables in which there are no additional information about the structure of the atom, the formulas of higher oxides, the composition of volatile hydrogen compounds .

Such a decision is quite logical and understandable, because for those students who decided to follow the footsteps of Mendeleev and Lomonosov, it will not be difficult to use the classical version of the system, they simply do not need hints.

It was the periodic law and the system of DI Mendeleev that played an important role in the further development of atomic-molecular theory. After the creation of the system, scientists began to pay more attention to the study of the composition of the element. The table helped clarify some information about simple substances, as well as the nature and properties of the elements that they form.

Mendeleyev himself assumed that soon new elements would open, and provided for the position of metals in the periodic system. It was after the appearance of the latter that a new era began in chemistry. In addition, a serious start was given to the formation of a multitude of related sciences that are related to the structure of the atom and the transformations of the elements.