In 1817, a German chemist Johann Wolfgang Dobereiner arranged the elements with similar properties in a group.
Dobereiner Triad is based on three elements group. He was the first person who started grouping of elements on the basis of atomic weight.
The three elements of the triad were arranged in such a manner that the atomic mass of the middle element was approximately the average of the atomic masses of the other two elements.
For example:- Atomic weight of Li=7, Na=23, K=39 the average atomic weight of 1st and 3rd element results in the atomic weight of 2nd element.