Question

(a) On what basis did Mendelev arrange the elements in his periodic table?
(b) On what basis are they arranged now?

Answer 1

Hint : In the periodic table, the elements are arranged in the increasing order of their atomic number. These 118 elements are divided into 7 periods and 18 groups. Atomic weight describes the average mass of atoms of an element.

Complete Step By Step Answer:
a) Mendelev arranged the elements in his periodic table in the increasing order of their atomic weights. It means that the physical and chemical properties are the periodic functions of atomic weights. At that time, Mendelev didn’t have any idea about the internal structure of an atom i.e subatomic particles protons, neutrons, and electrons.
b) In 1913, Moseley observed the regularities in the properties of elements by their X-ray spectra. From these observations, he concludes that atomic number is the fundamental property that is responsible for the regularities in the property. Therefore, now the elements of the periodic table are arranged in the increasing order of their atomic number. This is called the Modern periodic table. According to this, the physical and chemical properties are the periodic functions of atomic numbers.

Note :
It is important to note that Mendelev arranged the elements in his periodic table in the increasing order of their atomic weights. It means that the physical and chemical properties are the periodic functions of atomic weights. Now, the elements of the periodic table are arranged in the increasing order of their atomic number.