Question

(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 was Dobereiner's triad discarded?

Answer 1

(a) As per Dobereiner's triads, when element are arranged in the order of increasing atomic masses, groups of three elements, having similar chemical properties are obtained. The atomic mass of the middle element of the triad being equal to the arithmetic mean of the atomic masses of the other two elements. Thus,

(atomic mass of A + atomic mass of C)/2 = atomic mass of B

(7+39)/2

46/2=23

Atomic mass of B is 23.

(b) Dobereiner’s law of triads failed for the following reasons :

• All the then known elements could not be arranged in the form of triads.

• For very low mass or for very high mass elements, the law was not holding good.
  Example: F, Cl, Br. Atomic mass of Cl is not an arithmetic mean of atomic masses of F and Br.