Question

State one example of a Dobereiner’s triad, showing in it that the atomic mass of the middle element is half-way between those of the other two.

Answer 1

Lithium (Li), sodium (Na) and potassium (K) with atomic masses 7, 23, 39 constitute a Dobereiner's triad. The atomic mass of sodium (middle element) is half-way between those of the other two. This can be seen in the illustration below:

Average of the atomic masses of lithium and potassium = $\frac{\text{Atomic mass of lithium + Atomic mass potassium}}{2}=\frac{7+39}{2}=\frac{46}{2}=23$