Question

Briefly explain the first classification of elements and Mendeleev’s periodic table.

Answer 1

Hint :The arrangement of elements in a manner that all elements with same or similar properties are grouped together is done in a periodic table. But initially doing this was not the easy task as at that time many elements were not known and some particles were not discovered.

Complete Step By Step Answer:
The first attempt was Dobereiner’s Triads: As the name suggests triad i.e. three elements in a group, the atomic mass of the middle element was the arithmetic mean of corner elements.

Ex:	Li	Na	K	Mean of atomic masses of first and third elements
Atomic mass	7	23	39	$ \dfrac{{7 + 39}}{2} = 23 $

The concept was limited to a limited number of elements and hence rejected.
Newland’s Law of Octave: He arranged the known elements in increasing order of their atomic masses and he found that every eight element resembles the property of the first element.
Li Be B C N O F Na
Li and Na have similar properties.
The law fails when heavier elements are arranged (Ca).
Mendeleev’s periodic table: Basis of his table was that atomic mass is the fundamental property of the elements and the properties are periodic functions of mass.
His table contained $ 63 $ elements arranged in $ 13 $ horizontal rows (series) and $ 9 $ vertical columns (groups) all arranged in increasing order of atomic weights.
Use of this table: Atomic weight of elements were determined and corrected. For Discovery of new elements gaps were left in the periodic table.
Defects in his table: Position of hydrogen was not clear, variable valencies of elements were not explained, lanthanoids and actinoids were not separated.

Note :
Mendeleev also predicted the properties of some elements before their discovery which later found to be correct, he took the formulae of oxides and hydrides of elements to further classify the elements on this basis.