Hey hi. I am glad to answer this question as periodic table is my favorite topic. Before Dobereiner there were many chemists who attempted to classify elements on the basis of their properties, nature, character, valency,etc. But none of them became so famous to be learned at present and many other consequences like variable valency and dual character of elements was not explained and shown by their contributions. During period of 1815-29, Dobereiner Arranged elements in increasing order of their ATOMIC WEIGHT. He arranged in a group of three which was known as TRIAD according to which the atomic weight of the middle element was the arithmetic mean(i.e., average) of the remaining two i.e., the first and last element. Only 9 out of 63 elements were able to be arrange according to his rule of triad. They are- 1. Li(at. Wt. 7), Na(at. Wt. 23), K(at. Wt. 39) 2. Ca(at. Wt. 40), Sr(at. Wt. 88), Ba(at. Wt. 137) 3. Cl(at. Wt.35.5), Br(at.wt.80), I(at. Wt. 127) This method was discarded since did not hold true for all elements. He gave rise to earlier curiosities for more knowledge about PERIODIC TABLE. Answered By--- Ayushi Shukla from Lucknow in India.
