Question

Analyse the elements given below and identify ‘X’ if the given elements are forming Dobereiner's triad.

Li (6.9), Na (X), K (39.0)

• A. 15.3
• B. 23.0
• C. 19.0
• D. 45.9

Answer 1

The correct option is B 23.0

In Dobereiner's triads when three elements in a triad are written in the increasing order of atomic masses, the atomic mass of the middle element is roughly the average of the atomic masses of the other two elements. Applying this property for the given triad we have:

Li (6.9), Na (X), K (39.0)
X = $\frac{(6.9+39.0)}{2}$ $\approx$ 23.0
or X= 23