Solution:
If the two members of a Döbereiner triad are calcium and strontium, the third member of the triad is Barium.The first of Dobereiner's triads was identified in the year 1817 and was constituted of the alkaline earth metals calcium, strontium and Barium. Various attempts were made to classify the known elements based on their properties and in a different order. Johan Döbereiner, In 1817, classified elements based on their properties. He arranged the elements showing the same properties (triads) in a group called a family. For example, the Alkaline earth metal group includes Calcium (Ca), Strontium (Sr), and Barium (Ba) and shows the same chemical properties because of the same valency.
