Question

Which will be the third element among the given elements to complete the given Doberiener triad?

(Atomic mass of Be 9; Na 23; Mg 24; Si 28; Cl 35; Ca 40)

Be, ___, Ca

Answer 1

Dobereiner arranged the elements in increasing order of atomic masses. He found that the atomic mass of the middle element was approximately equal to the arithmetic mean (average) of the atomic masses of the other two elements of that triad when they are arranged in their increasing order of atomic mass.

When we take the arithmetic mean of atomic masses of $Be$ and $Ca$ it comes out to be $\frac{40+9}{2}=24.5$, which is approximately equal to 24 which is the atomic weight of magnesium.

Hence, the third element in the triad is magnesium (Mg).