Question

The law of triads is not applicable to:
(A) Os, Ir, Pt
(B) Ca, Sr, Ba
(C) Fe, Co, Ni
(D) Ru, Rh, Pt

Answer 1

When three elements called a triad are arranged in ascending order of their atomic masses then the atomic mass of the middle element is equal to the arithmetic mean of the other two elements present then the metals are going to apply the law mass of the triad.
Atomic weight of middle element = $\dfrac{\text{sum of the atomic weights other two elements}}{\text{2}}$

Complete step by step answer:
We have to calculate the arithmetic mean of the given elements in the given options to know the elements which are not obeying the law of triad.
Coming to the given options, Option A, the given elements are Osmium, Iridium, and Platinum.
The atomic weights of Osmium, Iridium, and Platinum are 190, 192.5, and 195.
Atomic weight of the middle element (192.5)

& =\dfrac{\text{sum of the atomic weights other two elements}}{\text{2}} \\\ & =\dfrac{190+195}{2} \\\ & =192.5 \\\ \end{aligned}$$ Therefore option A is correct, because the three elements in option A are obeying the law of triad. Coming to option B, the given elements are Calcium, Strontium, and Barium. The atomic weights of Calcium, Strontium, and Barium are 40, 88, and 137. Atomic weight of the middle element (88) $$\begin{aligned} & =\dfrac{\text{sum of the atomic weights other two elements}}{\text{2}} \\\ & =\dfrac{40+137}{2} \\\ & =88.5 \\\ \end{aligned}$$ Therefore option B is correct, because the three elements in option B are obeying the law of triad. Coming to option C, the given elements are Iron, Cobalt, and Nickel. The atomic weights of Iron, Cobalt, and Nickel are 56, 59, and 59. Atomic weight of the middle element (59) $$\begin{aligned} & =\dfrac{\text{sum of the atomic weights other two elements}}{\text{2}} \\\ & =\dfrac{56+59}{2} \\\ & =57.5 \\\ \end{aligned}$$ Therefore option C is wrong, because the three elements in option C are not obeying the law of triad. Coming to the option D, the given elements are Ruthenium, Rhodium, and Platinum The atomic weights of Ruthenium, Rhodium, and Platinum are 101, 103 and 195. Atomic weight of the middle element (103) $$\begin{aligned} & =\dfrac{\text{sum of the atomic weights other two elements}}{\text{2}} \\\ & =\dfrac{101+195}{2} \\\ & =148 \\\ \end{aligned}$$ Therefore option D is also wrong, because the three elements in option D are not obeying the law of triad. **So, the correct options which are not applicable for law of the triad are C and D.** **Note:** The Law of Triads was introduced by Johann Dobereiner in 1817. He observed that Strontium had similar chemical properties to Calcium and Barium and that due to the atomic weight of strontium is the average of the atomic weights of calcium and barium. Then it is called the law of triad.