1-Development of classification of elements-
In 1803, a chemist named Dalton published a table of relative atomic masses, which are now accepted as atomic masses. This table served as an important foundation for the classification of elements.
2-Division into metals and non-metals-
Initially the elements were classified according to their properties. Base But divided into metal and non-metal. Later, some such elements were discovered, whose properties were similar to both metals and non-metals. We named the elements as metals or metalloids. Later this classification became invalid, as it was not based on the fundamental properties of the elements.
3- Mendeleev’s Periodic Table and Periodic Law
Professor Dmitry Ivanovich Mendeleef, a Russian scientist, studied a relationship between the atomic masses of elements and their physical and chemical catastrophes, a total of 63 elements were known during his time. Mendeleev arranged the then known elements on the basis of their similar atomic masses.
, In other words, Mendeleev arranged the elements on the basis of similarities in the formula of the compounds they formed (for example – oxides, hydrides etc.). It was observed that the right elements should be placed on the periodic table in the order of their increasing atomic masses (at that time called atomic mass).
It was found that recurring iteration or frequency This means that the properties of every eighth element are the same as that of the first element, that is,
The physical and chemical properties of elements are periodic functions of their atomic masses (atomic masses). This periodic table had vertical columns (groups) and horizontal rows (periods).
In Mendeleev’s table, although all the elements were arranged in the order of their increasing atomic masses; Pairs of some elements are placed in the inverse of their atomic masses. For example – cobalt (atomic mass 58.98) and nickel (58. 7) tellurium (127.6) and iodine 126.90).
In the periodic table this inversion is called chemical This was due to the similarities of properties with the elements of the group in which that element was placed. For example, tellurium (Te) was placed before iodine, whereas Te has a higher atomic mass. This was done because the properties of iodine are similar to those of bromine.
He predicted the properties of the elements to be discovered in the future based on the position of the elements in the periodic table to fill in the blanks left in this table, not like the properties of selenium (Se).
4-General features of Mendeleev’s periodic table.
(1) The elements in each period are arranged in the order of their increasing atomic masses.
(2) All the elements of the same group have similar properties.
(3) The negative-electron valency of the elements decreases while moving from left to right in each period, while the positive-electric valency increases.
(4) The atomic mass of an element is its fundamental property.
(5) elements with low atomic mass; For example, H, C, O, N are found relatively in abundance in nature.
(6) of the elements of the blank spaces in the table properties can be told in advance.
(7) In the periodic table, some elements were placed in such places according to which their properties were not there, the atomic masses of these elements were modified and then they got a logical place in the table.
(8) According to the position of any element in the table, its properties can be told like-
The properties of an element in a group are the average properties of the elements above and below that element in that group.
William Prout in 1815 AD imagination that the atoms of all the elements are merely associations of hydrogen atoms and that the atomic weights of all elements are simple multiples of the atomic mass of hydrogen, because the atomic mass of hydrogen is assumed to be one,
Therefore, the atomic weight of 14 of nitrogen means that one atom of nitrogen is made up of 14 atoms of hydrogen and on this basis the atomic mass of an element is a simple multiple of the atomic mass of hydrogen.
This hypothesis of Prout proved to be wrong, because there were some elements whose atomic weights were not simple integers but in decimals, such as the atomic weight of chlorine 35. There are 5.
Read in Hindi: periodic classification of elements