Question

Atomic weight of element X is 39, and that of element Z is 132, atomic weight of their intermediate element Y, as per the Dobereiner Triad, will be:
(A) $ 88.5 $
(B) $ 93.0 $
(C) 171
(D) $ 85.5 $

Answer 1

Hint: The element with atomic weight 39, belongs to the first group of the periodic table and the element with atomic weight 132 also belongs to the first group of the periodic table, therefore the element Y should also belong to the first group of the periodic table. The element would have an average atomic mass of the other two elements.

Complete step by step solution
The Dobereiner’s Law of Triads states that the arithmetic mean of the atomic weights of the first and the third element of a triad will be approximately equal to the atomic weight of the second element. This statement by him was based on the observations of the properties of the alkaline earth metals by him that strontium has properties that were intermediate to that of calcium and barium. By 1829, Dobereiner also found another such group of triads whose physical properties were similarly related.
So, going with the statement of Dobereiner, the atomic weight of the intermediate element Y, will be:
 $ \dfrac{39+132}{2}=88.5 $ .
The atomic weight of potassium is 39 and the atomic weight of Caesium is 132, and hence the intermediate element will be Rubidium, whose atomic weight is $ 88.5 $ .
Hence the correct answer is option A.

Note
The main discrepancy with the Dobereiner’s Law of triads was that there were only three triads that he could find, i.e., at a total of nine elements. Dobereiner’s triads could not classify most of the elements known at that time. Hence the theory was ultimately rejected.