Question

Law of triads is applicable to which set of elements?

Answer 1

Hint: There were various classifications of the periodic table given by scientists in order to come to the modern periodic table. These were based on the similarity in properties of elements. Law of triads was given by Dobereneir. This consists of three elements with similar properties to be grouped together. But it was applicable to some elements only.

Complete answer:
After the discovery of various chemical elements, scientists tried to arrange the elements in an ordered way. This ordered way depicted the similarity in the properties like mass and chemical reactivity of the elements.
One such classification of elements was given by a German scientist Johann Wolfgang Dobereiner. His classification was known as Dobereiner triads or the law of triads. He arranged the elements in groups of 3 (triads). According to his law, in each triad the atomic weight of the middle element was the average of the atomic weight of 1st and 3rd elements. Also the property of the middle element is in between the properties of the 1st and 3rd elements.
This law was applicable to only four triads, the four triads are, Li, Na, K; Ca, Sr, Ba; Cl, Br, I; S, Se, Te. So, the drawback for the law of triads was that only 4 triads were listed.
Hence, law of triads is applicable to 4 sets of elements as Li, Na, K; Ca, Sr, Ba; Cl, Br, I; S, Se, Te.

Note:
The law of triads was applicable as, for example, the atomic weight of lithium Li = 6, and potassium K = 39, its average, 6 + 39 = 45, its average is $\dfrac{45}{2}=$ 22.5, and the atomic weight of sodium Na is 22.5 approximately. Some other classification of elements includes the Newland law of octaves and Mendeleev’s periodic table. The modern periodic table is a varied form of Henry Moseley’s classification of elements according to the atomic numbers.