Question

What are the limitations of Dobereiner's classification?

Answer 1

Hint: Law of triad was given by Dovereiner. He was the first to consider the idea of trends among properties of elements. He noted that the middle element of each of the triad had an atomic weight about half way between the atomic weight of the other two elements. He also noted that the middle element was having physical and chemical properties in between the other two members.

Complete step by step answer:
Johann Dobereiner, the German chemist proposed the law of triad in 1829. He worked on various elements and formed the triad of three elements that are having chemical and physical properties relatable with his theory.
Elements that are following Dobereiner's triad law are:
1. Li,Ca and Cl with atomic weight 7,40,35.5 respectively.
2. Na,Sr and Br with atomic weight 23,88,80 respectively.
3. K,Ba and I with atomic weight 39,137,127 respectively.
He was able to classify only 9 elements by his triad law but a large number of elements are still there which didn't follow triad law.
He was unable to group the similar elements in his triad. For example iron, manganese, nickel,cobalt,zinc and copper are similar elements but could not be placed in the triad.
It was also possible to group quite dissimilar elements together in the triad on the basis of its atomic weight.
Some elements did not follow the triad law. For example: Cu,Ag ,Au and Zn,Cd, Hg.

Note:
Dobereiner was the first scientist who initiated the study of periodic relationship.He was the first who showed the clear relationship between properties of an element and its atomic weight.