Question

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 approximate:

• A. 24.5
• B. 49
• C. 34.5
• D. 29

Answer 1

The correct option is A 24.5

According to Dobereiner, the atomic weight of the central element of the triad is the arithmetic mean of the atomic weights of the other two members.

Therefore, atomic weight of Y = $\frac{9+40}{2}=24.5g$

Hence, option A is correct.