Question

State Mendeleev’s periodic law. Write two achievements of Mendeleev’s periodic table.

Answer 1

Periodic table was discovered after several attempts were made by Newland, Mosley and Mendeleev. Each scientist has discovered the elements based on the different properties and they arranged the elements according to the different periodic properties.

Complete answer: Periodic table is the representation of a total of $118$ elements arranged in the increasing order of atomic numbers arranged in the vertical columns and horizontal rows.
These vertical columns are called groups and the horizontal rows are called periods.
There is a total of $7$ periods and $18$ groups in the periodic table.
These $118$ elements are arranged according to the increasing order of atomic numbers.
But, earlier different arrangements of the periodic table were done by different scientists.
Mosley arranged the periodic table in the increasing order of atomic numbers.
Mendeleev arranged the periodic table based on the increasing atomic masses of chemical elements.
Thus, Mendeleev's periodic law can be stated as the physical and chemical properties of the elements are the periodic functions of their atomic weights or atomic masses.
Achievements of Mendeleev’s periodic table:
1.He left the gaps for some elements that had to be discovered, and named those elements as the preceding elements in the group by adding the prefix eka-. For example, gallium was
named Eka-Aluminum.
2.His attempt makes the discovery of new elements.
3.The atomic masses were corrected for some chemical elements.

Note:
The periodic table was constructed by different scientists based on different properties, these properties must be considered though they look similar.
The atomic number and atomic masses must be identified clearly, for some elements atomic mass is not exactly double to the atomic number, but may be higher than the double of atomic number.