John Dobereiner proposed the idea of similarity in the properties of elements which can be placed together.
He proposed the idea of classification so that we can study the properties of a group of elements together
According to him, three elements can be grouped based on their similarities in properties. This group of three elements was termed as "Triads"
For example :
Li, Na, K
Now the significance of the triad was that the atomic weight of middle element is average of atomic weights of the side atoms
So atomic weight of Na = 23 is average of atomic weights of Lithium and Potassium
7 + 39/2 = 46/2= 23
The other triads were
Ca Sr Ba
Cl Br I
S Se Te
