Question

what relation about elements did dobereiner want to establish?

Answer 1

when three elements are taken in the order of their increasing atomic masses the then the average mass of first and third element is equal to the second element

Answer 2

The German Chemist J.W. Dobereiner in 1829, grouped certain elements in the groups of three, called Dobereiner's triads or simply as triads. The three elements in a triad hd similr chemicl properties. In these triads, the tomic mass of the middle element is nearly equal to the mean of the first and third elements in each triad. But the idea of classifiction could nt be applied to all the elements..

hope it helps.. ;)