CHEMISTRY [3 Mark Question Answer]

Atomic number	Element	Electronic configuration	Select an element of the same group
11	Sodium	2, 8, 1	K
15	Phosphorus	2, 8, 5	N
16	Sulphur	2, 8, 6	O
9	Fluorine	2, 7	Cl

Elements, when arranged in the increasing order of their atomic weights, are similar to the eighth and the first note of the musical scale. For example, the eighth element from lithium is sodium. Similarly, the eighth element from sodium is potassium. Thus, lithium and sodium provide any specific place for hydrogen.
This classification did not work with heavier elements.
Newland adjusted two elements Cobalt (Co) and Nickel (Ni) in the same slot.
Fe, which resembles Co and Ni in properties, has been placed far away.

The basis of the earliest attempts for the classification of elements was the chemical and physical properties observed by the scientists. Some reactions with known elements and compounds were compared.
The atomic weights were used to order them. To group them into groups they tried to observe the periodicity of the properties as the atomic weight increases. Law of triads and then the law of octaves was proposed. Doberneiner, Chancourtois, and Newlands proposed these laws and prepared some tables.

It is impossible for a chemist to study all the elements and their compounds. Hence, classification is a must.
Following are the reasons for the classification of elements:

To study elements better

To correlate the properties of the elements with some fundamental properties which are characteristic of all the elements

To reveal relationships between elements

Merits of Mendeleev's classification of elements :

Grouping of elements

Gaps for undiscovered elements: Mendeleev left some gaps in his periodic table for subsequent inclusion of elements not known at that time.

He predicted the properties of the then-unknown elements on the basis of properties of elements lying adjacent to the vacant slots (eka-aluminum and eka-silicon).