In the first half of the 19th century, various attempts to systematize the elements and combine metals in the periodic system appeared. It is during this historical period that such a research method as chemical analysis arises.
From the history of the discovery of the periodic system of elements
Using a similar technique for determining specific chemical properties, scientists of that time tried to combine elements into groups, guided by their quantitative characteristics, as well as atomic weight.
Using atomic weight
So, I.V.Dubereiner in 1817 determined that strontium has an atomic weight similar to that 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. Other substances were combined into similar groups:
- sulfur, selenium, tellurium;
- chlorine, bromine, iodine;
- lithium, sodium, potassium.
Chemical classification
L. Gmelin in 1843 proposed a table in which he arranged elements similar in chemical properties in a strict order. Nitrogen, hydrogen, oxygen, he considered the main elements, this chemist placed them outside of his table.
Under oxygen, he placed tetrads (4 characters each) and pentads (5 characters each) of the elements. Metals in the periodic system were supplied according to Berzelius terminology. According to Gmelin, all the elements were established to reduce the electronegativity of the properties within each subgroup of the periodic system.
Joining Elements Vertically
Alexander Emil de Chancourtoy in 1863 put all the elements in increasing atomic weights on a cylinder, dividing it into several vertical bands. As a result of this division, elements having similar physical and chemical properties are located on the verticals.
Octave law
D. Newlands discovered in 1864 a rather interesting pattern. When chemical elements are arranged in increasing atomic weights, each eighth element shows a similarity with the first. Newlands called this fact the law of octaves (eight notes).
His periodic system was very arbitrary, so the idea of an observant scientist began to be called the "octave" version, connecting with music. It was the Newlands variant that was closest to the modern PS structure. But according to the mentioned law, only 17 octaves retained their periodic properties, while the rest of the signs did not show such a regularity.
Odling Tables
W. Odling presented several versions of element tables at once. In the first version, created in 1857, he proposed dividing them into 9 groups. In 1861, a chemist made some adjustments to the original version of the table, combining characters with similar chemical properties into groups.
A variant of the Odling table, proposed in 1868, suggested the arrangement of 45 elements in increasing atomic weights. By the way, it was this table that later became the prototype of the periodic system of D. I. Mendeleev.
Valence Division
L. Meyer in 1864 proposed a table that included 44 elements. They were placed in 6 columns, according to the valency of hydrogen. The table had two parts at once. The main one united six groups, included 28 signs for increasing atomic weights. In its structure, pentads and tetrads from signs similar to the chemical properties were visible. Meyer placed the remaining elements in the second table.
Contribution of D. I. Mendeleev to the creation of a table of elements
The modern periodic system of elements of D.I. Mendeleev appeared on the basis of Mayer's tables compiled in 1869. In the second version, Mayer arranged the signs in 16 groups, placed the elements with pentads and notebooks, taking into account the well-known chemical properties. Instead of valency, he used simple numbering for groups. There was no boron, thorium, hydrogen, niobium, or uranium in it.
The structure of the periodic system in the form that is presented in modern editions did not appear immediately. There are three main stages during which a periodic system was created:
- The first version of the table was presented on the structural blocks. The periodic nature of the relationship between the properties of the elements and the values of their atomic weights was traced. Mendeleev proposed this version of the classification of signs in 1868-1869.
- The scientist refuses the original system, since it did not reflect the criteria by which elements would fall into a particular column. He proposes to place signs on the similarity of chemical properties (February 1869)
- In 1870, Dmitry Mendeleev introduced the scientific world to the modern periodic system of elements.
The version of the Russian chemist took into account the position of metals in the periodic system and the features of the properties of non-metals. For the years that have passed since the first edition of the brilliant invention of Mendeleev, the table has not undergone any major changes. And in those places that were left empty during the time of Dmitry Ivanovich, 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 due to the structural features of the table.
In total, it contains 8 groups, and each has two subgroups: the main (main) and secondary. It turns out that there are 16 subgroups in total. They are arranged 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. The characteristic 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. Signs belonging to one main subgroup have the same number of external electrons, 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 in them. It is this indicator that is the determining factor in identifying patterns of change of properties within the main subgroups.
Since the radius (the distance between the positive nucleus and external negative electrons) in the main subgroup increases, non-metallic properties (the ability to accept electrons during chemical transformations) decreases. As for the change in metallic properties (the return of electrons to other atoms), it will increase.
Using the periodic system, it is possible to compare among themselves the properties of different representatives of one main subgroup. At the time when the periodic system was created by Mendeleev, there was still no information about the structure of the substance. Surprising is the fact that after the theory of atomic structure arose, studied in educational schools and specialized chemical universities and at the present time, it confirmed the hypothesis of Mendeleev, and did not refute his assumptions about the arrangement of atoms inside the table.
Electronegativity in the main subgroups decreases to the bottom, that is, the lower the element is located in the group, the less its ability to attach atoms.
Change in the properties of atoms in side subgroups
Since Mendeleev’s system is periodic, the change of properties in such subgroups occurs in the reverse order. Such subgroups include elements starting from 4 periods (representatives of d and f families). To the bottom in these subgroups, the metallic properties are reduced, but the number of external electrons is the same for all representatives of the same subgroup.
Features of the structure of periods in PS
Each new period, with the exception of the first, in the table of the Russian chemist begins with an active alkali metal. Next, amphoteric metals exhibiting dual properties in chemical transformations are set. 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, a change in activity occurs in periods. From left to right, reducing activity (metallic properties) will decrease, and oxidative activity (non-metallic properties) will increase. Thus, the brightest metals in the period are located on the left, and non-metals on the right.
In large periods, consisting of two rows (4-7), a periodic nature also appears, but due to the presence of representatives of the d or f family, there are much more metal elements in the row.
Names of the main subgroups
Some of the groups of elements available in the periodic table received their own names. Representatives of the first group A subgroups are called alkali metals. Metals owe their name to their activity with water, resulting in caustic alkalis.
The second group A subgroup is considered alkaline earth metals. When interacting with water, such metals form oxides, they were once called earths. Since that time, a similar name was assigned to the representatives of this subgroup.
Non-metals of the oxygen subgroup are called chalcogenes, and representatives of the 7 A group are called halogens. 8 A subgroup was called inert gases because of its minimal chemical activity.
PS in the school course
For schoolchildren, a variant of the periodic table is usually proposed, in which in addition to groups, subgroups, periods, the formulas of higher volatile compounds and higher oxides are also indicated. This trick allows students to form skills in the preparation of higher oxides. It is enough to substitute the sign of the representative of the subgroup instead of the element to obtain a finished higher oxide.
If you carefully look at the general appearance of volatile hydrogen compounds, it is clear that they are characteristic only of non-metals. In groups 1-3 there are dashes, since metals are typical representatives of these groups.
In addition, in some school chemistry textbooks, each sign indicates a diagram of the distribution of electrons by energy levels. This information did not exist during the period of Mendeleev’s work; similar scientific facts appeared much later.
You can also see formulas of the external electronic level, according to which it is easy to guess which family this element belongs to. Such tips are unacceptable at examination sessions, so graduates of grades 9 and 11 who decide to demonstrate their chemical knowledge in the Unified State Examination or Unified State Examination are given classical black and white versions of periodic tables that lack additional information about the structure of the atom, the formulas of higher oxides, and the composition of volatile hydrogen compounds .
Such a solution is quite logical and understandable, because for those students who decided to follow in the footsteps of Mendeleev and Lomonosov, it will not be difficult to use the classic version of the system, they just do not need hints.
It is the periodic law and system of D.I. Mendeleev that played the most 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 to clarify some information about simple substances, as well as the nature and properties of the elements that they form.
Mendeleev himself suggested that soon new elements would be discovered, and envisaged the position of metals in the periodic system. It was after the appearance of the latter, a new era began in chemistry. In addition, a serious start was given to the formation of many related sciences that are associated with the structure of the atom and the transformations of elements.