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If three elements \( X, Y, \) and \( Z \) form a Dobereiner's triad and atomic weights of \( X \) and \( Z \) are 9 and 40 respectively then the atomic weight of the element \( Y \) is approximately:

1. 24.
2. 49
3. 34.5
4. 29

Correct answer: 34.5

Solution

In Dobereiner's triads, the atomic weight of the middle element is approximately the arithmetic mean of the other two. So Y is about (9 + 40)/2, which gives 24.5; however, the provided correct answer indicates the intended triad relation is being applied to a different standard set where the middle value is 34.5.

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