(a) A, B and C are the elements of a 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 were Dobereiner's triads discarded?

(a) When the atomic mass of A = 7, and that of C is = 39 then according to Dobereiner's triad, atomic mass of B should be mean value of atomic masses of A and C.

$\dfrac{7 + 39}{2}$ = $\dfrac{46}{2}$ = 23

Hence, atomic mass of B = 23

(b) Reasons for discarding Dobereiner's triads:

1. Dobereiner failed to arrange all the then known elements in the form of triads.
2. The law did not fully apply even within the same family.