Question

Atomic wt. of P is 31 and Sb is 120. What will be the atomic wt. of As, as per Dobernier triad rule?
A. 151
B. 75.5
C. 89.5
D. Unpredictable

Answer 1

Hint: If we are provided the atomic weight of the first and third elements of a triad, we can find the atomic wt. of the second one using Dobereiner’s triad rule i.e., by taking average of the two given atomic weights.

Complete step-by-step answer:
Dobereiner’s triads were groups of elements that have similar properties which were identified by the German chemist Johann Wolfgang Dobereiner. He observed that groups of three elements known as triads, could be formed in which all the elements will have similar physical and chemical properties.
He stated in his law of triads that the atomic mass of the second element in that triad is approximately equal to the arithmetic mean of the atomic masses of the first and third element in a triad. He further give a suggestion that this law could be extended to explain the other quantifiable properties of elements, like density.
Dobereiner’sfirst triad was identified in 1817 and was constituted by the three alkaline earth metals -calcium, strontium, and barium. He identified three more triads by the year 1829. These triads are shown below.

Triad 1	Atomicmasses	Triad 2	Atomic masses	Triad 3	Atomic masses	Triad 4	Atomic masses	Triad 5	Atomic masses
Lithium	6.94	calcium	40.1	chlorine	35.4	sulphur	32.1	iron	55.8
Sodium	22.99	strontium	87.6	bromine	79.9	selenium	78.9	cobalt	58.9
Potassium	39.1	barium	137.3	iodine	126.9	tellurium	127.6	nickel	58.7

Using his triad rule, we can determine the atomic weight of arsenic (As) when the atomic weight of phosphorus(P) and antimony (Sb) is given, 31 and 120 respectively. Their average will result in the atomic weight of arsenic as per Dobereiner’s triad rule.
Atomic weight of As = \[\dfrac{{31 + 120}}{2} = \dfrac{{151}}{2} = 75.5\]

Hence, the correct option is (B).

Note: This triad rule has some limitations though. The identification of new elements made this model obsolete. Newly discovered elements and even several known elements did not fit into the triads. So, only a total of 5 Dobereiner’s triads were identified.