In the first half of the 19th century, there were various attempts to systematize the elements and combine metals in the periodic system. It was during this historical period that such a research method as chemical analysis arose.
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 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 quite a lot in common between the properties of barium, strontium and calcium. Based on these observations, the famous chemist compiled the so-called triad of elements. Other substances were combined into similar groups:
- sulfur, selenium, tellurium;
- chlorine, bromine, iodine;
- lithium, sodium, potassium.
Classification by chemical properties
L. Gmelin in 1843 proposed a table in which he arranged similarelements in a strict order according to their chemical properties. Nitrogen, hydrogen, oxygen he considered the main elements, this chemist placed them outside his table.
Under oxygen he placed tetrads (4 signs each) and pentads (5 signs each) of the elements. The metals in the periodic system were placed according to the terminology of Berzelius. As conceived by Gmelin, all elements were determined by decreasing electronegativity properties within each subgroup of the periodic system.
Merge elements vertically
Alexander Emile de Chancourtois in 1863 put all the elements in ascending atomic weights on a cylinder, dividing it into several vertical stripes. As a result of this division, elements with similar physical and chemical properties are located on the verticals.
Law of octaves
D. Newlands discovered in 1864 a rather interesting pattern. When the chemical elements are arranged in ascending order of their atomic weights, every eighth element shows similarities with the first. Newlands called a similar fact the law of octaves (eight notes).
His periodic system was very arbitrary, so the idea of an observant scientist was called the "octave" version, associating it with music. It was the Newlands version that was closest to the modern PS structure. But according to the mentioned law of octaves, only 17 elements retained their periodic properties, while the rest of the signs did not show a similar pattern.
Odling tables
U. Odling presented several variants of tables of elements at once. In the firstversion, created in 1857, he proposed to divide them into 9 groups. In 1861, the chemist made some adjustments to the original version of the table, grouping signs with similar chemical properties.
A variant of Odling's table, proposed in 1868, assumed the arrangement of 45 elements in ascending atomic weights. By the way, it was this table that later became the prototype of the periodic system of D. I. Mendeleev.
Valency division
L. Meyer in 1864 proposed a table that included 44 elements. They were placed in 6 columns, according to hydrogen valency. The table had two parts at once. The main one united six groups, included 28 signs in ascending atomic weights. In its structure, pentads and tetrads were seen from signs similar to chemical properties. Meyer placed the remaining elements in the second table.
The contribution of D. I. Mendeleev to the creation of the 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 into 16 groups, placed the elements in pentads and tetrads, taking into account known chemical properties. And instead of valency, 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 that is presented in modern editions did not appear immediately. Can be distinguishedthree main stages during which the periodic system was created:
- The first version of the table was presented on building 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 abandons the original system, since it did not reflect the criteria by which elements would fall into a certain column. He proposes to place signs according to the similarity of chemical properties (February 1869)
- In 1870, Dmitri Mendeleev introduced the modern periodic system of elements to the scientific world.
The Russian chemist's version took into account both the position of metals in the periodic system and the properties of non-metals. Over the years that have passed since the first edition of Mendeleev's brilliant invention, 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, discovered after his death.
Features of the periodic table
Why is it considered that the described system is periodic? This is due to 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 there are 16 subgroups in total. They are located vertically, that is, from top to bottom.
Besides, the table also has horizontal rows called periods. They also have theiradditional division into small and large. The characteristic of the periodic system implies taking into account the location of the element: its group, subgroup and period.
How properties change in the main subgroups
All main subgroups in the periodic table begin with elements of the second period. For signs belonging to the same main subgroup, the number of outer electrons is the same, but the distance between the last electrons and the positive nucleus varies.
In addition, an increase in the atomic weight (relative atomic mass) of the element occurs in them from above. It is this indicator that is the determining factor in identifying patterns of change in properties within the main subgroups.
Since the radius (the distance between the positive nucleus and the outer 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 (donating electrons to other atoms), it will increase.
Using the periodic system, you can compare the properties of different representatives of the same main subgroup. At the time when Mendeleev created the periodic system, there was still no information about the structure of matter. Surprising is the fact that after the theory of the structure of the atom arose, studied in educational schools and specialized chemical universities and at the present time, it confirmed Mendeleev's hypothesis, and did not refute his assumptions on the arrangement of atoms inside the table.
Electronegativity inthe main subgroups decreases to the bottom, that is, the lower the element is located in the group, the less its ability to attach atoms will be.
Changing the properties of atoms in side subgroups
Since Mendeleev's system is periodic, the change in properties in such subgroups occurs in reverse order. Such subgroups include elements starting from period 4 (representatives of d and f families). To the bottom in these subgroups, metallic properties decrease, but the number of external electrons is the same for all representatives of one 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 are the amphoteric metals, which exhibit 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, there is a change in activity in periods. From left to right, reducing activity (metallic properties) will decrease, oxidizing activity (non-metallic properties) will increase. Thus, the brightest metals in the period are on the left, and non-metals on the right.
In large periods, consisting of two rows (4-7), a periodic character also appears, but due to the presence of representatives of the d or f family, there are much more metallic elements in the row.
Names of main subgroups
Part of the groups of elements present in the periodic table has received its own names. Representatives of the first group A of the subgroup are called alkali metals. Metals owe this name to their activity with water, resulting in the formation of 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. It was from that time that a similar name was assigned to the representatives of this subgroup.
Nonmetals of the oxygen subgroup are called chalcogens, and representatives of the 7 A group are called halogens. 8 A subgroup is called inert gases because of its minimal chemical activity.
PS in the school course
For schoolchildren, a variant of the periodic table is usually offered, in which, in addition to groups, subgroups, periods, the formulas of higher volatile compounds and higher oxides are also indicated. Such a trick allows students to develop skills in compiling higher oxides. It is enough to substitute the sign of the representative of the subgroup instead of the element to get the finished highest oxide.
If you look closely at the general appearance of volatile hydrogen compounds, you can see that they are characteristic only of non-metals. There are dashes in groups 1-3, since metals are typical representatives of these groups.
In addition, in some school chemistry textbooks, each sign indicates the distribution of electrons alongenergy levels. This information did not exist during the period of Mendeleev's work, similar scientific facts appeared much later.
You can also see the formula of the external electronic level, by which it is easy to guess which family this element belongs to. Such tips are unacceptable at examination sessions, therefore, graduates of grades 9 and 11, who decide to demonstrate their chemical knowledge at the OGE or the Unified State Examination, are given classic black and white versions of periodic tables that do not contain 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 schoolchildren 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 simply do not need prompts.
It was the periodic law and the system of D. I. Mendeleev that played the most important role in the further development of the atomic and 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 about the nature and properties of the elements that they form.
Mendeleev himself assumed that new elements would soon be discovered, 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 many related sciences that are related to the structure of the atom andtransformations of elements.
