Question

How did Dobereiner made his periodic table in the form of triad or the form of the table

Answer 1

Dobereiner grouped elements based on similarities.
He arranged the elements according to
Law of Traids which is :
Middle element in the triad had atomic weight that was the average of the other two members.
He first noticed:
Calcium (atomic weight 40), strontium (atomic weight 88), and barium (atomic weight 137) possess similar chemical properties.Dobereiner noticed the atomic weight of strontium(atomic weight:88) fell midway between the weights of calcium(atomic weight:40) and barium(atomic weight:137).
He noticed the same pattern for the alkali metal triad (Li/Na/K) and the halogen triad (Cl/Br/I).