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Dobereiner observed that when chemically similar elements were arranged in a group of three, in increasing atomic weights, the atomic weight of the middle element was close to the average of the atomic weights of the other two elements.
Dobereiner is credited for the earliest attempt of organizing chemical elements.
In 1829, he published his ‘Law of Triads’.
Example - This is a triad of Li, Na and K. The average of the at.wt. of Li and K is equal to the at.wt. of the middle element Na.
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