Question

The law of triad is applicable to a group of:
This question has multiple correct options
(A) $Cl,Br,I$
(B) $C,N,O$
(C) $Na,K,Rb$
(D) $H,O,N$

Answer 1

Hint: According to the law of triads, in a group of three elements, the average atomic masses of the first and last element would be approximately equal to the atomic mass of middle elements. By calculating the atomic masses of given options, we would be able to find which all groups obey the Law of triads.

Complete step by step answer:
- In the earlier days of chemistry there were several methods used for the classification of elements based on their properties. The law of triads was such a type of classification element and was proposed by a scientist named Dobereiner and it was called Dobereiner's law of triads.
- According to his law of triads, if elements of similar properties are taken three at a time and arranged in the increasing order of their atomic masses then the atomic mass of the middle one would be the average of the other two. Or simply we can say that the atomic weight of the middle element would be almost average of the other two elements.
- Dobereiner also suggested that the law of triads can also be extended for other quantifiable properties such as density of elements.

-Let's look at each of the given options and which of the elements in the options obey the law of triads.
(i) $Cl,Br,I$
By the definition of law, the atomic weight of bromine, must be approximately equal to the average of atomic masses of chlorine and iodine.
$\dfrac{35.5+127}{2}=81.25$
This value is approximately equal to atomic mass of bromine which has the value 79.9. Hence these groups obey the law of triads.

(ii) $C,N,O$
By law, the atomic mass of nitrogen should be equal to the average of atomic masses of carbon and oxygen.
$\dfrac{12+16}{2}=14$
 Nitrogen's atomic mass is 14 and thus this group also obeys law of triads.

(iii) $Na,K,Rb$
 $\dfrac{23+85.5}{2}=54.25$
The average atomic masses of sodium and Rubidium is 54.25 whereas the atomic mass of potassium is 39. Thus, this group doesn’t obey the law of triads.

(iv) $H,O,N$
$\dfrac{1+14}{2}=7.5$
The average of atomic masses of hydrogen and nitrogen is 7.5 whereas the atomic mass of oxygen is 15. Therefore, the law of triads is not applicable in this group.
So, the correct answer is “Option A and B”.

Note: It should be noted that the law of triads has a lot of shortcomings and hence its an outdated theory. Also do not confuse the law of triads with the law of octaves in which the elements with similar chemical and physical properties occur after each interval of seven elements when they are arranged according to the increasing atomic weight.