Question

(a) A, B and C are the elements of Dobereiner's triad. If the atomic mass of A is 7 and that of C is 39, what should be the atomic mass of B?

(b) Why was the Dobereiner triad discarded?

Answer 1

(a) As per the Dobereiner triad rule, If we arrange elements in a triad form in increasing order of their atomic mass then the atomic mass of the middle element will be the average of two side elements.

Mass of element A: 7

Mass of element C: 39

So, according to Dobereiner's triad rule:

Atomic wt. of element B: $\frac{\mathrm{Atomic}\mathrm{wt}.\mathrm{of}\mathrm{A}+\mathrm{Atomic}\mathrm{wt}.\mathrm{of}\mathrm{C}}{2}$

= $\frac{7+39}{2}$

= $\frac{46}{2}$

= $23$

So, the atomic wt. of element C is 23 ( average of atomic wt. of A and Atomic wt. of B)

(b) Dobereiner triad rule was discarded because of the following reasons:

• Dobereiner failed to arrange all the known elements in the form of the triad.
• The law was not fully applied even within the same family eg. Fluorine(19), chlorine(35.5), and bromine(80), it is observed that the average of Fluorine(19) and bromine(Br) is 49.5 which is not the atomic mass of chlorine.
• After Ca this law was unable to apply to elements.